Proxy-Driven Server for Flexible Files Version 2

Proxy-Driven Server for Flexible Files Version 2 Hammerspace

loghyr@gmail.com

General Network File System Version 4 Internet-Draft Parallel NFS (pNFS) with the Flexible Files Version 2 layout type supports client-side erasure coding and per-chunk repair between clients and data servers. This document extends that architecture with a proxy server role: a registered peer of the metadata server that polls the metadata server for work assignments and carries them out -- moving a file from one layout to another, reconstructing a whole file from surviving shards, or translating between encodings for clients that cannot participate in the file's native encoding (including NFSv3 clients). All proxy-server-to-metadata-server coordination is fore-channel: the metadata server returns work assignments inline in the response to a proxy-server-initiated PROXY_PROGRESS poll, and the proxy server reports completion via a fore-channel PROXY_DONE. No callback operations are required for the proxy server protocol. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Network File System Version 4 Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at . Note to Readers This is an individual submission and does not reflect Working Group consensus. The "About This Document" section above has the current discussion venue, latest rendering, and source location.

Introduction The Flexible Files Version 2 layout type () introduces client-side erasure coding for pNFS and a per-chunk repair protocol (CB_CHUNK_REPAIR) that lets the metadata server direct an active client to reconstruct individual damaged chunks. That mechanism is sufficient for repairs whose scope is a handful of chunks in a file that has at least one live client. Three classes of work are outside the per-chunk repair model. The first is whole-file repair: the case in which enough data servers have failed that per-chunk reconstruction would require visiting every chunk, or in which no live client is available to drive the repair at all. The second is layout transitions: a file must move from one layout geometry to another for policy reasons (migrating to a new encoding type, or re-mirroring), for maintenance reasons (evacuating a data server ahead of decommission), or for environmental reasons (moving between transport-security profiles or between filehandle backends). The third is encoding translation: a client that cannot participate in the file's native encoding -- including every NFSv3 client, and any legacy or minimal NFSv4 client that does not implement the file's encoding type -- still needs to read and write the file. This document specifies a proxy server role to address those three cases with a single mechanism: the proxy server opens a session to the metadata server and registers its capabilities via PROXY_REGISTRATION; the proxy server then polls the metadata server using PROXY_PROGRESS for work assignments (move, repair), which the metadata server returns inline in the poll response; the proxy server carries out each assignment and signals completion via PROXY_DONE (or abort via PROXY_CANCEL). All of the proxy-server-to-metadata-server coordination is fore-channel; the proxy server does not require a back-channel callback program. A client reaches a proxied file either by contacting the proxy server directly, as an ordinary NFS server, or through a pNFS layout whose data server is the proxy server. While a migration is in progress every client routes its I/O through the proxy server; for encoding translation, only a client that cannot encode the file does. The flexible file v1 layout () provides no standardized mechanism for migrating a file's layout while the file remains in use. Without such primitives, migration is left to implementation-specific machinery and cannot be performed safely across implementations. This design codifies that mechanism, closing what is today the single biggest interop gap between pNFS and other parallel filesystems that already expose migration primitives.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Definitions The following terms are used with meanings defined in :

data server, metadata server
layout, mirror, mirror set, shard, stripe
chunk (the unit of client-driven encoded write/read), and the chunk state machine (PENDING / FINALIZED / COMMITTED)
encoding type, k (number of data shards), m (number of parity shards)
ffv2_layout4, ffv2_mirror4, ffv2_data_server4, ffv2_encoding_type4, FFV2_DS_FLAGS_PROXY -- the layout XDR types and flags
CHUNK_READ, CHUNK_WRITE, CHUNK_FINALIZE, CHUNK_COMMIT -- the chunk-level data-path operations
TRUST_STATEID / REVOKE_STATEID / BULK_REVOKE_STATEID -- the metadata-server-to-data-server control-plane fencing ops

Local terms defined in this document:

Proxy server (PS):: A host registered with the metadata server via PROXY_REGISTRATION () that accepts and drives file-level migration, repair, and encoding-translation assignments on behalf of the metadata server. The proxy server is a pNFS client of the metadata server (it drives OPEN + LAYOUTGET like any other pNFS client), and it appears as a data server in the layouts the metadata server issues to end clients while the proxy operation is active.
Registered proxy server:: A proxy server whose PROXY_REGISTRATION has been accepted by the metadata server and whose lease has not expired.
Proxy operation:: A metadata-server-orchestrated activity carried out by a proxy server on a single file: a migration (PROXY_OP_MOVE), a repair (PROXY_OP_REPAIR), or an encoding translation. The first two are delivered to the proxy server as work; either is referred to as an "assignment" once the metadata server has committed it to a specific proxy server and minted a proxy_stateid for it (). An encoding translation has no proxy_op_kind4 value and is never assigned: the metadata server routes an encoding-ignorant client to the proxy server by the layout it hands out, and the file's own state does not change ().
Migration:: A PROXY_OP_MOVE proxy operation that shifts a file from one mirror set (L1, the source) to another (L2, the destination).
Assignment:: A proxy_assignment4 the metadata server has minted and is delivering (or has delivered) to a specific proxy server; named by its proxy_stateid.
L1, L2, L3:: The three mirror-set roles the metadata server tracks while a proxy operation is in flight on a file. L1 is the source layout (the file's layout before the proxy operation started), L2 is the destination layout (the file's layout after PROXY_DONE succeeds), and L3 is the composite layout naming the proxy server as a data server, issued to end clients while the proxy operation is in flight and to the proxy server via OPEN(CLAIM_PROXY). Defined precisely in .
Sidecar:: Per-migration reclaim state a metadata server MAY retain across its own reboot to permit a reconnecting proxy server to resume in-flight work. Sidecar contents are implementation-defined; the wire mechanism does not require the metadata server to retain any sidecar.
Autopilot:: The metadata server's internal assignment-selection policy engine. The wire protocol does not constrain how the autopilot chooses proxies or when it issues assignments; the autopilot is named in this document only to disambiguate metadata-server-initiated actions from proxy-server-initiated ones.

Relation to Sibling Flexible File Version 2 Documents defines CB_CHUNK_REPAIR and the per-chunk repair model. This document is the companion whole-file and per-client mechanism. Per-chunk and whole-file operations are mutually exclusive for a given file at a given time; coexistence rules are in .

Scope This section draws the boundary between the wire-level mechanism defined here and the much larger space of useful behaviors a proxy implementation might support. Drawing the boundary tightly keeps the protocol small enough to specify and implement in a single revision; everything beyond the boundary is either future work or implementation latitude, and both categories are called out below so later readers know which is which.

In Scope This document defines a new protocol role, a new session, and a small set of operations that flow on that session. Around that core it specifies the layout conventions a client observes while a proxy operation is active, the credential-forwarding rules a translating proxy must follow, and the recovery semantics for the three actor failures that matter during an operation (proxy server, metadata server, data server). The new role is the proxy server, distinct from the metadata server and data server roles defined in . A proxy server registers with a metadata server and, on receipt of a directive from that metadata server, performs a move, a repair, or an ongoing encoding translation on behalf of a client. The proxy server is opaque to most clients and is visible to the metadata server through a dedicated NFSv4.1+ session that the proxy server itself opens. All proxy-server-to-metadata-server coordination is fore-channel: the proxy server issues ops to the metadata server, and the metadata server returns work assignments inline in its responses. No callback channel is required for the proxy server protocol. The fore-channel protocol is deliberately small. PROXY_REGISTRATION () lets the proxy server declare the encoding set it supports and its lease. PROXY_PROGRESS () is the proxy server's heartbeat and poll: the proxy server sends it within its registration lease and the metadata server replies with zero or more new work assignments inline. PROXY_DONE () commits or rolls back a migration when the proxy server finishes it; PROXY_CANCEL () lets the proxy server abort early. All four ops are fore-channel proxy-server-to-metadata-server. Around the operation set, the document specifies the layout conventions a client sees during a proxy operation and how a client discovers the proxy server in the first place. Encoding translation for encoding-ignorant clients, including NFSv3 clients, is in scope, and is the one case that stretches the move/repair vocabulary. The same proxy machinery that handles move and repair also provides the persistent per-client translation that lets a client incapable of participating in a file's native encoding still read and write the file. Unlike move and repair, which are transient transitions on the file, encoding translation is an ongoing routing arrangement that persists as long as the encoding-ignorant client is active. Credential-forwarding rules for a proxy that translates on behalf of a client are defined in the Security Considerations section. The proxy is a translator, not an authority: authorization decisions MUST remain with the metadata server, using the client's forwarded credentials. Getting this boundary wrong turns the proxy server into a privilege-elevation vehicle, so the rules are stated normatively and enforced at the metadata server rather than policed on the wire. Finally, the document defines recovery semantics for the three actor failures that matter during a proxy operation -- proxy server failure, metadata server failure, and data server failure -- each with its own fencing and re-registration rules so that a mid-operation crash does not leave a file in an unrecoverable state.

Out of Scope The items below are deliberately deferred. They split into three groups: features whose absence was an explicit design decision (delta journaling, partial-range moves), orchestration that belongs to a layer above a single proxy (multi-proxy pipelines, automated load balancing), and proxy-internal behavior that does not appear on the wire and therefore needs no standardization. Nothing on this list is precluded by the current design; each is a reasonable future extension.

Journaling and partial moves:: Move assignments in this revision are always whole-file. The proxy server performs a client-side mirroring write (see the Client-Side Mirroring section of ) to all mirrors -- source D, destination G, and any other mirrors in the file's mirror set -- while reading source bytes from any mirror in the source set; the two-layout state on the metadata server keeps client traffic on L1 throughout, with an atomic swap to L2 at PROXY_DONE time ().
Orchestration beyond a single proxy:: Multi-proxy pipelines (staged moves for very large files) and automated load balancing or predictive selection across registered proxies are out of scope. A metadata server in this revision selects a single proxy server per operation; load distribution across many proxies, when it matters, is expected to be handled by the metadata server's selection policy and requires no new wire protocol.
Server-side copy as an alternative path:: Integration with server-side copy () as an alternative to proxy-server-driven moves for single-file moves within one namespace is adjacent work. The two mechanisms are complementary (server-side copy is a client-directed intra-server operation; the proxy-server-driven move is a metadata-server-directed inter-server operation), and their intersection -- for example, using server-side copy under the hood of a proxy server move assignment -- is better specified in its own extension rather than bolted into this document.
Proxy-internal features that do not appear on the wire:: A proxy MAY implement content-integrity and error-correction layers, encryption and compression pass-through, log-structured write staging, and sector-alignment normalization. These are useful motivating scenarios for the move/repair vocabulary but do not require new protocol machinery beyond what the PROXY_PROGRESS / PROXY_DONE / PROXY_CANCEL fore-channel set already provides, and so they are left to implementation rather than standardized here.

Use Cases Seven motivating scenarios converge on the same mechanism. Six of them are transient transitions on the file itself: the file is changing state (being ingested, re-coded, evacuated, reconstructed, migrated between transport-security profiles, or migrated between filehandle backends), and the transition is what the proxy server drives to completion. The seventh is qualitatively different: the file is not changing, but specific clients -- those that cannot participate in the file's native encoding -- are routed through a proxy server persistently, for as long as they are active. The distinction matters for how the metadata server schedules work. Transient transitions have a terminal state; the metadata server expects each one to complete (via PROXY_DONE, or to fail via PROXY_DONE(FAIL) / PROXY_CANCEL) and then to retire the associated layout. The persistent routing case has no terminal state for the file as a whole; the proxy server stays in the layouts of encoding-ignorant clients as long as those clients are open. In every case, a registered proxy server becomes the source of truth for a file's data during the operation, and clients are redirected to route I/O through that proxy server rather than directly to the original layout's data servers. The individual scenarios are described below.

Administrative Ingest An administrator rsyncs a file from an external source into the cluster as a single-copy file. Server policy requires the file to be mirrored or erasure coded. The metadata server queues a PROXY_OP_MOVE assignment for the file; the next PROXY_PROGRESS poll from a registered proxy whose encoding set covers the destination layout returns the assignment in its response. The proxy populates the destination from the source, while any client that opens the file during the move sees a layout that routes I/O through the proxy. The source "layout" may not even be a flexible file layout; it could be a non-pNFS NFS mount that the proxy reads as an NFSv4.2 client. Throughout the move the proxy presents the file to pNFS clients as if the move had not started, while populating the destination in the background.

Policy-Driven Layout Transition A server-objective or policy change ("files older than 30 days must be erasure coded", "high-access-rate files must have additional mirrors") requires transforming a file's layout without user visibility. The transformation is purely a layout change; the file contents are unchanged except at the shard level. The PROXY_OP_MOVE assignment carries the new layout's geometry and encoding type via the destination deviceid in proxy_assignment4; the proxy reshapes the file's shards to match. Because the transformation type (encode / decode / transcode) is entirely specified by the (source, destination) deviceid pair plus the file's recorded layout type, the assignment does not need a separate transformation-class field.

Data Server Maintenance / Evacuation A data server is scheduled for maintenance (hardware replacement, software upgrade, decommission). All files whose layouts reference that data server must be evacuated to replacement data servers before it is taken offline. The metadata server queues a PROXY_OP_MOVE assignment per file (source = the outgoing data server, target = a replacement); registered proxies pick the assignments up via PROXY_PROGRESS polls. Evacuation can be large-scale (thousands or millions of files); running per-client per-chunk repair over every file would be prohibitively expensive, but a single registered proxy can drive many concurrent migrations subject to the per-proxy server in-flight cap ().

Whole-File Repair Multiple data servers have failed such that per-chunk repair cannot reconstruct the file in place. The metadata server constructs a new layout backed by replacement data servers and queues a PROXY_OP_REPAIR assignment. The next registered-proxy server PROXY_PROGRESS poll receives the assignment; the proxy drives reconstruction from whatever surviving shards remain. If fewer than k shards survive across the mirror set, the proxy reports terminal failure via PROXY_DONE with pd_status set to NFS4ERR_PAYLOAD_LOST, matching the per-chunk repair semantics in the Selecting the Repair Actor section of .

TLS Coverage Transition Transport Layer Security (TLS) protects the RPC transport as specified by RPC-over-TLS . A file whose layout currently points at data servers that do not support it needs to be migrated to TLS-capable data servers, or vice versa (an inventory change, a policy change mandating transport security, onboarding a new storage class whose data servers are TLS-only). A PROXY_OP_MOVE assignment applies: the destination data servers have the required transport security profile, the source data servers are retired. A client that arrives mid-transition is routed through the proxy and does not directly see the heterogeneous data server set. The proxy establishes its own RPC connections to source and destination data servers, potentially with different transport security profiles (non-TLS to source, mutual TLS to destination, or any other combination). The proxy's per-data server security is independent of the client's security to the proxy.

Filehandle / Storage-Backend Transition A data server changes the filehandles it issues for a file; this happens when the data server's underlying storage is migrated (e.g., from one backend object store to another) and the old filehandles become unresolvable on the new backend. Without a proxy, every client holding a layout has to be individually recalled and re-issued. With a proxy, the metadata server points all clients at the proxy (keeping their existing stateids and filehandles intact), the proxy reconciles old-to-new filehandles internally, and clients are recalled only at the end. This same mechanism covers several related situations: an NFSv3-to-NFSv4.2 data server protocol upgrade where the data server filehandles change as a side effect of migrating from to semantics; a data-server-side format change that invalidates existing filehandles (for example, a transition from a local POSIX store to an object store); and backend-opaque filehandle migration where the data server's filehandle structure is internally versioned and old clients hold stale versions.

Encoding Translation for Encoding-Ignorant Clients The coding-type registry defined in the IANA Considerations of is expected to grow. Not every client is required to implement every registered encoding; a minimal client, a legacy client, or an NFSv3 client typically cannot participate in erasure-coded files at all. Per the encoding-negotiation rules in , such a client either retries with a different supported_types hint, falls back to metadata-server-terminated I/O, or (this case) is routed through a proxy that translates on its behalf. Unlike the move, repair, evacuation, and transition use cases above, encoding translation is persistent per client. The file itself is not changing state. What changes is the layout the metadata server hands to an encoding-ignorant client: that client gets a single-data server layout naming a translating proxy server, with a coding_type the client does support (typically FFV2_ENCODING_MIRRORED, or for NFSv3 clients just a flat NFSv3 data path). The proxy encodes and decodes on the fly against the real data servers; the client sees a flat file. The same file may be accessed directly by encoding-aware clients (with a normal layout naming the real data servers) and through the proxy by encoding-ignorant clients (with a proxy layout) simultaneously. The metadata server issues a different layout per request; only the encoding-ignorant case routes through the proxy server. For NFSv4-family clients (NFSv4.0, NFSv4.1, or NFSv4.2 encoding-ignorant), the routing arrangement is on-wire: the metadata server returns a proxy-fronted layout in LAYOUTGET, and the client's existing mount continues to work; the metadata server, not the client, decides that this client's next layout points at the proxy server. NFSv3 client routing, by contrast, is NOT dynamic on the wire. An NFSv3 client cannot receive a pNFS layout, cannot observe the flexible file v2 layout proxy indication, and has no protocol path by which the metadata server can redirect an already-mounted NFSv3 client through the proxy server. NFSv3 routing is therefore an out-of-band front-door export deployment, described here so implementers do not read the NFSv3 flow diagrams as implying on-wire redirection:

The NFSv3 client mounts the proxy server's NFSv3 export from the start. The proxy server is the client's server for MOUNT and for every subsequent NFSv3 op; the metadata server is invisible to the NFSv3 client.
Filehandle authority belongs to the proxy server. The proxy server mints its own NFSv3 filehandles (opaque to the client per ) and maintains an internal mapping from each NFSv3 filehandle to the (metadata-server file, current layout) pair. The NFSv3 filehandle bytes are not derived from and do not need to match the metadata-server-issued nfs_fh4.
Export namespace authority belongs to the proxy server. The proxy server owns the exports it serves to NFSv3 clients; a given proxy server MAY re-export a subset of the metadata server's namespace, and a deployment MAY run multiple proxy servers each re-exporting different subsets. The metadata server is not required to be an NFSv3 server.
Because the routing is fixed at mount time, no on-wire discovery contract exists between the metadata server and the NFSv3 client: the deployment operator directs NFSv3 clients at the proxy server via the normal MOUNT protocol () and DNS or administrative means. A metadata server that decides to retire or migrate a proxy server MUST coordinate that change with the operator; there is no protocol path by which the metadata server can cause a mounted NFSv3 client to fail over automatically.

The NFSv3 read/write flows described below are therefore what the proxy server does when it receives an NFSv3 op from a client that was mounted at the proxy server's endpoint from the start; they are not what the metadata server does to redirect an existing NFSv3 client.

Mechanism A translating proxy runs two sides that meet internally. On its client-facing side it speaks the protocol the encoding-ignorant client can speak: for an NFSv3 client that is an NFSv3 server that re-exports the metadata server's namespace; for a legacy NFSv4.2 client that understands only some encodings, it is an NFSv4.2 data-server interface presenting FFV2_ENCODING_MIRRORED (or an equivalent encoding the client supports). On its metadata-server-facing side it is an NFSv4.2 client to the metadata server plus whatever data server protocol the metadata server's real data servers speak. The proxy translates each client-facing op into the corresponding metadata server or data server op, applies the encoding transformation between the two, and returns results. For an NFSv3 client, a read flows:

Client: NFSv3 READ against the proxy.
Proxy: if it does not hold a layout for the file, issues LAYOUTGET on the metadata server with the client's forwarded credentials (see Security Considerations).
Proxy: issues CHUNK_READ (or v3 READ if the data server is NFSv3) against the real data servers, decodes the shards back to plaintext.
Proxy: returns the plaintext bytes in the NFSv3 READ reply.

A write flows:

Client: NFSv3 WRITE with stable_how and a byte range.
Proxy: encodes the bytes per the file's encoding, issues CHUNK_WRITE / CHUNK_FINALIZE / CHUNK_COMMIT against the real data servers. If the client requested FILE_SYNC or DATA_SYNC, the proxy MUST NOT reply until every mirror in the target set has committed the range at (at least) the requested stability; if the back-end cannot provide that stability, the proxy fails the WRITE per (returning NFS3ERR_IO) rather than replying success at a weaker stability.
Proxy: for a successful WRITE, returns NFSv3 WRITE ok reporting the stability actually achieved (which MUST be >= the requested stable_how; per a server may return a stronger stability than requested but MUST NOT return a weaker one). For an UNSTABLE client request the proxy MAY reply as soon as it has accepted the data; a subsequent client COMMIT drives the range to durability across every mirror in the target set (see the M2 durability rule above).

Stateid binding on the translated path Encoding translation crosses two RPC boundaries -- client <-> proxy server and proxy server <-> real data server(s) -- and each leg carries its own stateid, minted by the server on that leg. On the client <-> proxy server leg, the client uses the stateids the metadata server issued to it: the open, lock, and layout stateids for the proxy layout it received from LAYOUTGET. The proxy server is that leg's data server for I/O purposes, so the stateids the client carries are exactly what any flexible file v2 layout client carries against any data server; the client does not present (and does not know about) any proxy-side stateid. On the proxy server <-> real data server leg, the proxy server uses the layout stateid it acquired from its own OPEN(CLAIM_PROXY) + LAYOUTGET against pa_file_fh (see ). That layout is the L3 composite covering both source and destination data servers, and its stateid governs every CHUNK_READ, CHUNK_WRITE, CHUNK_FINALIZE, and CHUNK_COMMIT the proxy issues on the back end. The client's stateid is not relayed to the real data servers, and the proxy_stateid4 -- which lives in the proxy server <-> metadata server plane () -- is never presented to a data server. No TRUST_STATEID relay of the client's stateid to the back-end data servers is required, because the proxy server operates on the back end under its own layout stateid, not by impersonating the client at the stateid layer. Client-identity forwarding happens at the credential layer (see ), independently of the stateid each leg uses to name the file's I/O state.

Why the same PROXY_REGISTRATION machinery The registered-proxy server mechanism gives the metadata server the information it needs for translation-proxy selection: pra_encodings enumerates the encodings the proxy server can translate between, the metadata server <-> proxy server session carries the fore-channel control-plane traffic, and the lease bounds the relationship. No new op is required for the translation case -- the existing PROXY_REGISTRATION covers it. PROXY_OP_MOVE and PROXY_OP_REPAIR assignments are not used for pure translation (the file is not moving or being repaired); the proxy server simply serves the encoding-ignorant client's I/O requests against the unchanged source layout.

Design Model

Roles This document introduces a third role alongside the pNFS metadata server and data server:

Proxy server:: A persistent, registered peer of the metadata server that carries out whole-file operations on the metadata server's behalf -- moving file content between layouts, reconstructing files whose source layout has been damaged, and translating encodings on behalf of clients that cannot participate in the file's native encoding. A proxy server is a distinct role from a data server; a given server MAY implement both, and typically does, but the protocol does not require that. The metadata server sees the proxy server through a dedicated session whose direction is defined in .

The existing roles are unchanged:

Metadata server:: As defined in : the coordinator for each file, and the authority that issues layouts, manages stateids, and selects repair participants.
Data server:: As defined in : serves the CHUNK data path to pNFS clients.

Only one of the three pairs carries new ops in this document. The metadata server <-> proxy server pair gains the new proxy-server-to-metadata-server regular ops for registration, progress, and terminal reporting (). No new callback ops are introduced; the metadata server pulls work assignments to the proxy server in the PROXY_PROGRESS reply on the fore-channel, and the proxy server reports completion or cancellation back via PROXY_DONE / PROXY_CANCEL on the same session, so no callback program is required for this protocol. The Client <-> proxy server pair gains no new ops: clients reach a proxy server through the normal pNFS data path, seeing it as the data server named in a single-data server layout (). The metadata server <-> data server pair is also unchanged; the tight-coupling control session in carries over as defined there.

Layout Model

Single-Layout Model This design uses a single layout naming the proxy server as the sole data server rather than two linked layouts. Three considerations drive that choice. pNFS clients already handle single layouts cleanly, so no new layout-linkage mechanism needs to be invented or implemented on the client. The client's view of the file -- "the file's data server is the proxy server" -- is the truth during the operation, so exposing the source and destination data servers directly would invite confusion about which entry to address rather than clarify. And late-arriving clients see the proxy layout from the start, without any separate setup path to join an operation already in progress. The alternative -- a source layout plus a destination layout linked by a redirector record -- was considered and rejected on those three grounds. Routing all client I/O through the proxy server has a cost deployments must weigh. For the duration of a migration the proxy server is a data-path single point of failure for the file: the client sees one data server, the proxy server, and the usual flexible file mitigation -- client-side mirroring across several data servers -- is unavailable to it. A file under migration therefore has lower availability than a normally-mirrored file until PROXY_DONE. The proxy server is also a throughput funnel: all client read and write traffic for the file, plus the proxy server's own copy traffic, passes through one proxy server, adding a latency hop and a bandwidth bottleneck. These costs are inherent to the single-layout choice; they argue against migrating very large files in a single proxy operation, and motivate the multi-proxy and partial-range extensions listed as out of scope ().

Two-Layout State on the Metadata Server Side For each file F whose mirror on a draining data server D is being migrated, the metadata server keeps three logical layout records. Only L3 backs a client-facing layout; L1 and L2 are metadata-server-internal bookkeeping for the duration of the migration.

L1:

the pre-migration mirror set, including D. This record is metadata-server-internal: it preserves what the file's layout was before the migration and is handed to no client while the migration is active.

L2:

the post-migration mirror set: L1 with D replaced by the target G. Also metadata-server-internal; it becomes the file's layout after the PROXY_DONE swap.

L3:

the composite the proxy server works from. It backs the layout every client is served during the migration: that client-facing layout names the proxy server as its data server, and the proxy server does the real I/O against L3's two mirror entries:

M1 (read source): the L1 mirror set. The proxy server reads source bytes from any mirror in M1.
M2 (write target): the L1 mirror set PLUS G. The proxy server writes via client-side mirroring to every mirror in M2.

During the migration every client of F -- whichever front door it used -- is served a layout naming the proxy server; the proxy server is the sole writer to M1 and M2. No client addresses D or G directly. Because the proxy server is the sole writer, the M2 durability rule applies at the stability the client requested: for a FILE_SYNC client write, or for a client-issued COMMIT, the proxy server MUST NOT acknowledge until every mirror in M2 is durable at that byte range; for an UNSTABLE client write, the proxy server MAY acknowledge as soon as it has accepted the data and MUST NOT report a stability higher than what the back-end mirrors have actually achieved. A subsequent COMMIT against that byte range MUST NOT return success until every mirror in M2 has committed it. The invariant is that a proxy server crash cannot leave a byte range durable-per-client (FILE_SYNC-ack'd or COMMIT-ack'd) but applied to only part of the target mirror set; UNSTABLE writes acquire durability at COMMIT time as they would against any NFS server. D's presence in M2 (alongside G) is intentional: the proxy server keeps D a current mirror until the PROXY_DONE swap, so a cancelled migration can fall back to L1 with no data loss. The proxy server writes both D and G, and because the proxy server is the only writer they converge to the same byte image with no inter-writer race. Either endpoint MAY be an NFSv3 data server. The proxy server speaks to each mirror in that mirror's own protocol: NFSv3 semantics to an NFSv3 data server, CHUNK semantics to an NFSv4.2 one. A migration may therefore bridge in either direction, or in neither. An NFSv3 mirror holds the file under FFV2_ENCODING_PASSTHROUGH, since the chunked encodings require NFSv4.2's CHUNK operations, so a migration that changes the protocol of a mirror is also an encoding transition. The asymmetric-protocol bridging is the proxy server's responsibility and is not visible to the client.

Pinned definitions

L1.mirrors = the file's pre-migration mirror set, includes D
L2.mirrors = L1.mirrors without D, union {G}
L3.M1 = L1.mirrors (proxy server's read-source set)
L3.M2 = L1.mirrors union {G} (proxy server's client-side mirroring write-target set)

Session Between Metadata Server and Proxy Server The proxy server opens an NFSv4.1+ session to the metadata server as a normal client. All proxy-server-to-metadata-server coordination flows on the fore-channel of that session: PROXY_REGISTRATION establishes the relationship, PROXY_PROGRESS heartbeats and pulls work assignments, PROXY_DONE / PROXY_CANCEL report terminal state. No callback program is required for the proxy server protocol -- the session's back-channel is unused by this draft (the proxy server may still establish one for unrelated NFSv4.1 callbacks if it wishes, but no proxy-server-protocol op rides on it). The session direction is intentionally opposite to the metadata server -> data server tight-coupling control session in : that session is opened by the metadata server to carry metadata-server-originated stateid management to a data server. The metadata server <-> proxy server session is opened by the proxy server because registration is a proxy-server-initiated act -- the proxy server is saying "here I am, with these capabilities." Without a proxy-server-to-metadata-server direction the capability-advertisement would have to be inferred from session-setup flags alone, which is inadequate for the range of capabilities a proxy server can usefully advertise (encoding set and -- as the DEVICEID_REGISTRATION open question anticipates -- fault-zone coordinates and other deployment attributes). A consequence of this direction choice is that a server that implements both the data server and proxy server roles toward the same metadata server runs two sessions between the same pair of hosts: the metadata server opens the data server tight-coupling session toward the box, and the box's proxy server opens the proxy server session toward the metadata server. That is two EXCHANGE_ID exchanges, two CREATE_SESSION exchanges, and two TCP connections. In deployments that use RPCSEC_GSS () or RPC-over-TLS () on the proxy server session -- which the credential-forwarding rules in recommend for any proxy server that translates on behalf of clients -- reserved-port trust is not in use and the doubled connection has no security cost. In a strict AUTH_SYS-only deployment the second outbound reserved port is a real but typically negligible cost, because a storage box's outbound NFS traffic is usually limited to one connection per metadata server it is registered with.

Flow Summary The proxy server opens a session to the metadata server and issues PROXY_REGISTRATION, declaring its supported encodings; the metadata server records the registration and returns a registration id with a granted lease. The proxy server then polls the metadata server via PROXY_PROGRESS at lease/2 cadence (or as the metadata server's ppr_lease_remaining_sec hint directs). When the metadata server decides to move or repair a file, it selects a registered proxy server whose capabilities match the operation and queues an assignment for that proxy server; the next PROXY_PROGRESS reply delivers the assignment in its ppr_assignments<> array. The proxy server picks the work up by issuing OPEN + LAYOUTGET on the assignment's pa_file_fh, drives the data-movement phase to completion, and reports terminal status by issuing LAYOUTRETURN + PROXY_DONE in a single fore-channel compound on the same session. The metadata server may at any time retract an assignment that the proxy server has not yet acknowledged via OPEN+LAYOUTGET by including a PROXY_OP_CANCEL_PRIOR assignment for the same (pa_file_fh, pa_target_deviceid) pair in a subsequent PROXY_PROGRESS reply; the proxy server may cancel work it has already started via the fore-channel PROXY_CANCEL (). Clients interact with the proxy server through the normal layout path. During a proxy operation the metadata server hands out a single-data server layout naming the proxy server; clients route CHUNK I/O to that data server. Clients that arrive mid-operation see the proxy layout from the start and need no additional signalling; clients that held an older (non-proxy) layout are recalled via CB_LAYOUTRECALL and reacquire.

Message Sequence: Policy-Driven Move The simplest flow -- a quiesced whole-file move for a policy transition. Shown as a wire-level message sequence between the three protocol actors; clients are elided because in the quiesced case they are recalled before the proxy server work starts.

Message sequence for a policy-driven move | (PS opens session to MDS) | <--- session est. ------------- | | | | ---- PROXY_REGISTRATION ------> | (advertise encodings) | <--- reg_id, granted_lease ---- | | | | ---- PROXY_PROGRESS ----------> | (heartbeat poll) | <--- NFS4_OK, ppr_assignments | (zero or more entries; one | includes MOVE assignment | delivers the MOVE work) | | | ---- PUTFH(pa_file_fh) -------> | (PS picks up the work) | ---- OPEN(CLAIM_PROXY, | | pa_stateid) ----------> | | ---- LAYOUTGET (L3 composite) > | | | | [PS drives move: reads source | | DSes, encodes per destination | | encoding, writes destination | | DSes via L3 fan-out] | | | | ---- PROXY_PROGRESS ----------> | (heartbeat; lease renewal, | <--- NFS4_OK, ppr_lease_rem.... | no new assignment needed) | | | ... | | | | ---- SEQUENCE | | PUTFH(pa_file_fh) | | LAYOUTRETURN(L3_stid) | | PROXY_DONE(pa_stateid, OK)>| (terminal: PROXY_DONE | | carries the minted | | proxy_stateid, not the | | L3 layout stateid; | | commit L1 -> L2) | <--- NFS4_OK ------------------ | | | | | --- CB_LAYOUTRECALL ---> | | (to clients holding L3; | | L1 was drained before | | PROXY_ACTIVE was | | entered) | | | | <-- LAYOUTRETURN ------ | | (from each L3 client) | | | | (MDS retires source DSes; | | next LAYOUTGET on this | | file returns L2) v v ]]>

Message Sequence: Whole-File Repair Same shape as a move, but the assignment in PROXY_PROGRESS carries pa_kind = PROXY_OP_REPAIR and the source layout is degraded. Terminal outcomes:

NFS4_OK in pd_status: the proxy server reconstructed the file; the metadata server proceeds as in .
NFS4ERR_PAYLOAD_LOST in pd_status: fewer than k shards survived across the mirror set; the metadata server marks the affected byte ranges lost and rolls back to L1. No CB_LAYOUTRECALL is issued because there is no valid destination layout to issue.

Message Sequence: metadata-server-initiated Cancellation The metadata server may decide to retract an assignment. Two cases:

Assignment not yet acknowledged by the proxy server:: The metadata server includes a PROXY_OP_CANCEL_PRIOR assignment in the next PROXY_PROGRESS reply, naming the same (pa_file_fh, pa_target_deviceid) pair as the prior PROXY_OP_MOVE or PROXY_OP_REPAIR assignment. The proxy server, which has not yet issued OPEN(CLAIM_PROXY) for the file, simply drops the prior assignment from its in-flight queue.
Assignment acknowledged and in flight:: The metadata server internally aborts the migration and discards the in-flight record; the proxy server's eventual PROXY_DONE returns NFS4ERR_BAD_STATEID (the L3 layout stateid no longer resolves to a record), and the proxy server abandons the work and releases its OPEN. The metadata server may also let the proxy server's registration lease expire as a coarser cancellation.

The proxy-server-initiated cancellation case uses the fore-channel PROXY_CANCEL ().

New NFSv4.2 Operations This document defines four new NFSv4.2 operations that a proxy server issues to the metadata server on the fore-channel of the proxy server -> metadata server session defined in . PROXY_REGISTRATION (96) is issued once at session setup and on renewal. PROXY_PROGRESS (97) is issued by the proxy server as a heartbeat-and-poll: the proxy server renews its registration lease and requests new work; the metadata server replies inline with zero or more new work assignments. Per-migration terminal reporting is not carried on PROXY_PROGRESS. PROXY_DONE (98) commits or rolls back an individual migration when the proxy server finishes it; PROXY_CANCEL (99) lets the proxy server abort early. None of these operations is sent by pNFS clients.

Proxy server operation numbers metadata server) /// */ /// /// const OP_PROXY_REGISTRATION = 96; /// const OP_PROXY_PROGRESS = 97; /// const OP_PROXY_DONE = 98; /// const OP_PROXY_CANCEL = 99; ]]> Opcodes 96 through 99 continue the control-plane range opened by , which allocates 78 through 95, and carry the proxy-server-to-metadata-server operations. These values MUST NOT overlap any value allocated by that document or by any other document in the flexible file v2 layout family. This document also defines one new EXCHGID4_FLAG value used at session establishment to identify a proxy-server session ():

Proxy-server EXCHGID4 flag The value sits outside the existing MASK_PNFS block (0x00070000 in ) and adjacent to EXCHGID4_FLAG_USE_ERASURE_DS (0x00100000, ), so that the erasure-data-server bit and the proxy-server bit do not collide. The registration is requested in . The following amendment blocks extend the nfs_argop4 and nfs_resop4 dispatch unions from with the new ops. A consumer that combines this document's extracted XDR with the XDR applies the amendments at the unions' extension point.

nfs_argop4 amendment block

nfs_resop4 amendment block

proxy_stateid4: A New Stateid Type This document introduces proxy_stateid4, a new server-issued stateid type used as the canonical handle for an in-flight proxy migration. The wire shape reuses the standard NFSv4 stateid4 from ; no new XDR type is added:

proxy_stateid4 wire shape

Value Space The proxy_stateid value space is disjoint from the open, lock, layout, and delegation stateid value spaces defined in . Disjointness is enforced by context, not by an in-band tag. A proxy_stateid4 appears in exactly four places: the PROXY_DONE and PROXY_CANCEL arguments, the pa_stateid field of each proxy_assignment4 returned in a PROXY_PROGRESS reply (), and the open_claim_proxy4 operand of an OPEN(CLAIM_PROXY) () -- where it is carried inside the open claim, not in a stateid argument slot. An implementation MUST NOT use an open, lock, layout, or delegation stateid lookup table to resolve a proxy_stateid. Conversely, a leaked proxy_stateid presented in the stateid argument of an ordinary operation (e.g., READ, WRITE, SETATTR, CLOSE) MUST be rejected with NFS4ERR_BAD_STATEID.

Metadata Server Minting The metadata server mints a fresh proxy_stateid each time it accepts a work assignment for delivery to a proxy server, and includes it as the pa_stateid field of the proxy_assignment4 carried in the next PROXY_PROGRESS reply (). The metadata server guarantees that no two proxy_stateids in the same (server_state, boot_seq) are equal. An implementation MAY embed the metadata server boot_seq in the high-order bytes of other[12] to enable cheap NFS4ERR_STALE_STATEID detection across reboots. other[12] is opaque on the wire, so how a metadata server structures it is a local matter: no receiver parses it, and nothing in this document depends on its internal layout.

Lifetime A proxy_stateid is valid from the instant the metadata server mints it until either:

The proxy server issues PROXY_DONE(proxy_stateid, ...) or PROXY_CANCEL(proxy_stateid) and the metadata server acknowledges it. On acknowledgment the proxy_stateid is retired; subsequent references return NFS4ERR_BAD_STATEID.
The proxy server's registration lease expires (), at which point all proxy_stateids minted for that proxy server are abandoned. Subsequent references return NFS4ERR_BAD_STATEID (or NFS4ERR_STALE_CLIENTID if the registration itself has been purged).
The metadata server reboots. Subsequent references to a proxy_stateid minted in a prior boot return NFS4ERR_STALE_STATEID.

Renewal Semantics Renewal is implicit in the registration lease: any PROXY_PROGRESS issued within the current lease renews all outstanding proxy_stateids the metadata server has minted for this proxy server. There is no per-assignment renewal operation, and neither the PROXY_PROGRESS arguments nor its results carry a per-assignment seqid bump. The seqid field of proxy_stateid4 is set by the metadata server at minting time (typically 1) and does not change over the assignment's lifetime; the proxy server MUST echo the minted value verbatim on PROXY_DONE and PROXY_CANCEL. The metadata server MUST validate exact seqid equality on those operations and MUST reject a mismatch with NFS4ERR_OLD_STATEID (see PROXY_DONE authorization step 6 in , which PROXY_CANCEL imports). The seqid slot does not otherwise bump over the assignment's lifetime; the exact-equality rule reduces to "the caller presents the minted value unchanged" and is retained for consistency with the general NFSv4 stateid shape in and for a common failure-mode signal when a proxy server confuses one assignment's stateid with another's.

Authorization Possession of a proxy_stateid is not sufficient to drive PROXY_DONE or PROXY_CANCEL on the corresponding migration. The metadata server additionally validates that the calling session's registered-proxy server identity owns that migration (see the "Authorization" subsection of for the full normative rule). Without this check, a proxy server that learned another proxy server's proxy_stateid (through a packet capture, a leaked log, or any other channel) could drive its PROXY_DONE / PROXY_CANCEL on a migration it does not own.

Operation 96: PROXY_REGISTRATION - Register as Proxy Server

ARGUMENTS

XDR for PROXY_REGISTRATION4args ; /// uint32_t pra_lease; /// uint32_t pra_flags; /// }; ]]>

RESULTS

XDR for PROXY_REGISTRATION4res

DESCRIPTION A proxy server calls PROXY_REGISTRATION on the fore-channel of its session to the metadata server () to declare its capabilities. The metadata server records the registration and MAY select that proxy server for subsequent PROXY_OP_MOVE or PROXY_OP_REPAIR work assignments delivered inline in the response to PROXY_PROGRESS. The pra_encodings field lists the ffv2_encoding_type4 values the proxy server supports. The proxy server MUST be able to encode, decode, and transcode between any pair of values in this list. Because the transformation class of a PROXY_OP_MOVE assignment is inherent in the (source, destination) layout pair, this encoding-set membership is all the capability information the metadata server needs to match. An empty list results in NFS4ERR_INVAL in this revision. The pra_lease field is the lease duration the proxy server requests in seconds. The metadata server MAY grant a shorter one, returned in prr_granted_lease. The proxy server MUST renew before the granted lease expires; on expiry the metadata server drops the registration and any in-flight migration record owned by this proxy server is abandoned (rolled back to L1 per ). The pra_flags field is reserved for future use. In this revision the proxy server MUST set pra_flags to 0, and a metadata server that receives a PROXY_REGISTRATION with any bit of pra_flags set MUST reject it with NFS4ERR_INVAL. The "reject, don't ignore" rule follows the NFSv4 extension model in . specifies that when a flag bit is used that is not known in the specified minor version, NFS4ERR_INVAL is returned; then explains that this same error is how a requester determines whether the responder understands the bit. Silently ignoring an unknown bit would break that discovery contract: a proxy server that sets a future capability bit against a metadata server that pre-dates the bit could not tell whether the metadata server honored the capability or simply dropped it. A future revision of this specification (or a successor document that updates it) MAY define new bit values in pra_flags, following the extension rules of . A proxy server that understands a newly defined bit MAY set it when registering with a metadata server that supports it; on NFS4ERR_INVAL the proxy server MAY retry with the bit cleared, treating the response as the signal that the metadata server does not recognize the bit. The pra_registration_id field carries the registration identity the proxy server presents on this call. On a first-time registration the proxy server MUST set pra_registration_id = PROXY_REGISTRATION_ID_NEW (0); the metadata server assigns a fresh non-zero registration ID and returns it in prr_registration_id. On renewal (and on any call intended to re-associate with an existing registration across reconnect), the proxy server MUST set pra_registration_id to the non-zero value the metadata server previously returned in prr_registration_id; the metadata server MUST match this against its live registration table. The metadata server distinguishes four cases:

Fresh registration:: pra_registration_id = PROXY_REGISTRATION_ID_NEW. The metadata server MUST assign a new non-zero registration ID, cryptographically bound (see below) to the caller's authenticated identity, and return it in prr_registration_id.
Renewal:: pra_registration_id matches a live registration under the caller's authenticated identity. The metadata server MUST refresh the granted lease and return the same prr_registration_id.
Unknown or stale registration ID:: pra_registration_id is non-zero but matches no live registration. The metadata server MUST reject with NFS4ERR_STALE_CLIENTID. This is the outcome a reconnecting proxy server observes when the metadata server has rebooted without persisting its registration table (see ); the proxy server SHOULD retry immediately with PROXY_REGISTRATION_ID_NEW (0) to obtain a fresh registration. The registration table is not required to survive a metadata-server reboot: the wire mechanism does not oblige the metadata server to persist (prr_registration_id, bound principal, granted capabilities, lease) across its own restart, and a metadata server that persists none of it is conformant. In-flight migration records tied to the stale prr_registration_id are lost per the drop rules in ; the fresh registration returned by the retry has a new prr_registration_id and no prior migration ownership.
Identity mismatch:: pra_registration_id matches a live registration, but the caller's authenticated identity differs from the identity bound to it. The metadata server MUST reject with NFS4ERR_PERM. This is the principal binding that prevents a malicious registrant from hijacking another proxy server's slot by guessing its registration ID.

The metadata server MUST assign prr_registration_id values that are unpredictable to any party other than the assigning metadata server and the recipient proxy server (a random or otherwise cryptographically unpredictable 64-bit value), so that a passive observer cannot guess a valid ID to present. Replay protection follows from the session's replay cache (); replay of a captured PROXY_REGISTRATION beyond the replay-cache window MUST fail because a matched ID under a differently authenticated principal is rejected as above. The value 0 is reserved as PROXY_REGISTRATION_ID_NEW and MUST NOT be assigned by the metadata server; a registration ID of 0 in pra_registration_id means "first registration" and never "renew". Registration conveys capabilities only; the proxy server's network endpoint is conveyed through the same deviceinfo channel as any other data server's address. When the metadata server selects a proxy server for an operation, the layout issued to clients includes a ffv2_data_server4 entry pointing at the proxy server's existing deviceinfo. PROXY_REGISTRATION is issued on the fore-channel of the metadata server <-> proxy server session. That session is opened by the proxy server, not by the metadata server; it is distinct from the metadata server -> data server tight-coupling control session defined by even when the same host acts as both data server and proxy server. The proxy server MUST present EXCHGID4_FLAG_USE_PROXY_SERVER on its EXCHANGE_ID so that the metadata server can distinguish a proxy-server session from both regular pNFS-client sessions and regular non-pNFS-client sessions (). A metadata server that receives PROXY_REGISTRATION on a session whose owning client did not present EXCHGID4_FLAG_USE_PROXY_SERVER MUST reject it with NFS4ERR_PERM. The proxy server MUST NOT present EXCHGID4_FLAG_USE_NON_PNFS on the same EXCHANGE_ID: it is a pNFS client for the purposes of (it drives LAYOUTGET as part of the CLAIM_PROXY pickup sequence, ), distinguished from ordinary pNFS clients only by its proxy-server role. Before recording the registration, the metadata server MUST authorize the caller as a registered proxy server for this metadata server. How that authorization is established -- a deployment allowlist, a per-metadata server provisioning step, integration with a directory service, or any other mechanism -- is implementation. The metadata server MUST reject an unauthorized PROXY_REGISTRATION with NFS4ERR_PERM. The authorization MUST be applied to a cryptographically authenticated identity, per the metadata server <-> proxy server transport-security requirements in . AUTH_SYS is never sufficient for PROXY_REGISTRATION; the metadata server MUST reject it. Registration conveys capabilities only; a metadata server may hold multiple concurrent registrations from distinct authorized identities (see ), and the arrival of a new authorized registration does not displace any prior one and does not invalidate any client's cached filehandles. To guard against a misbehaving proxy server re-issuing PROXY_REGISTRATION under the same identity while its prior registration still holds a valid lease -- squatting on its own slot -- the metadata server MUST refuse the duplicate: the metadata server returns NFS4ERR_DELAY and SHOULD log the conflict. A renewal -- distinguished by the proxy server presenting in pra_registration_id the same non-zero value it received in prr_registration_id on the prior registration -- is not squatting and the metadata server MUST accept it (refreshing the granted lease). A call whose pra_registration_id is PROXY_REGISTRATION_ID_NEW against an identity that already holds a live registration is squatting and MUST be refused. Registration revocation before lease expiry is not a dedicated operation in this revision. A metadata server that needs to revoke a proxy server before its lease expires MUST cease delivering work assignments to that proxy server in PROXY_PROGRESS replies; MUST return NFS4ERR_STALE_CLIENTID on subsequent PROXY_PROGRESS or PROXY_REGISTRATION-renewal from the revoked proxy server; and MUST handle the proxy server's in-flight migration records as if the lease had expired (see the lease-expiry paragraph above): the records are abandoned and the affected layouts revert to the pre-migration state. The revoked proxy server, on its next PROXY_PROGRESS, sees NFS4ERR_STALE_CLIENTID and may either re-register (if the deployment policy allows) or shut down.

Operation 97: PROXY_PROGRESS - Heartbeat and Receive Work Assignments

ARGUMENTS

XDR for PROXY_PROGRESS4args

RESULTS

XDR for PROXY_PROGRESS4res ; /// }; /// /// struct PROXY_PROGRESS4resok { /// uint32_t ppr_lease_remaining_sec; /// proxy_assignment4 /// ppr_assignments; /// }; /// /// union PROXY_PROGRESS4res switch (nfsstat4 ppr_status) { /// case NFS4_OK: /// PROXY_PROGRESS4resok ppr_resok4; /// default: /// void; /// }; ]]>

DESCRIPTION A registered proxy server calls PROXY_PROGRESS on the fore-channel of its session to the metadata server for two purposes:

Heartbeat: extend the proxy server's registration lease. The metadata server responds with ppr_lease_remaining_sec so the proxy server can size its next poll interval.
Receive work assignments: pick up zero or more units of work the metadata server has queued for this proxy server. Each assignment is a proxy_assignment4 describing one migration or repair the metadata server wants this proxy server to drive.

Per , ppa_flags is a reserved-for-future-use flag word; the metadata server MUST reject any non-zero bit with NFS4ERR_INVAL. The slot allows future revisions to add proxy-server-side appetite signaling (e.g., "do not give me more assignments right now") without an XDR break. The metadata server returns work assignments inline in ppr_assignments<PROXY_MAX_ASSIGNMENTS_PER_OP>. A single PROXY_PROGRESS reply MUST NOT carry more than PROXY_MAX_ASSIGNMENTS_PER_OP (64) assignments. The metadata server MUST additionally verify, before constructing the reply, that the encoded PROXY_PROGRESS4resok fits within the session's negotiated ca_maxresponsesize (); if the next assignment on its queue would overflow the response, the metadata server MUST short-return (omit that assignment and any that follow it from this reply) and MUST include the omitted assignments in a subsequent PROXY_PROGRESS reply. Short-return is not itself an error; the delivery rules above ensure the omitted assignments are re-offered on the next call. A proxy server that does not want new work simply ignores the assignments past its in-flight cap; the metadata server does not retract assignments once acknowledged (see below), other than via an explicit PROXY_OP_CANCEL_PRIOR assignment in a later PROXY_PROGRESS reply. Each assignment names the work type (pa_kind, described below), a single file (pa_file_fh), the source and target data servers the migration moves data between (pa_source_deviceid / pa_target_deviceid), and an opaque descriptor (pa_descriptor<PROXY_MAX_DESCRIPTOR_BYTES>, bounded at 4096 bytes) reserved for future extensions (for example, a precomputed source-layout descriptor so the proxy server can contact the source data servers without a second LAYOUTGET). The pa_stateid field carries the proxy_stateid4 () the metadata server has minted for this migration; the proxy server presents it in the OPEN(CLAIM_PROXY) that binds it to the file () and references it as the handle in the eventual PROXY_DONE / PROXY_CANCEL. The pa_kind discriminates the work type:

PROXY_OP_MOVE:: drain or migrate the file's data between the named data servers. pa_stateid is the proxy_stateid the proxy server will reference in PROXY_DONE / PROXY_CANCEL.
PROXY_OP_REPAIR:: reconstruct a missing or corrupt mirror on pa_target_deviceid from the surviving mirrors. pa_stateid is the proxy_stateid the proxy server will reference in PROXY_DONE / PROXY_CANCEL.
PROXY_OP_CANCEL_PRIOR:: the metadata server rescinds an assignment it delivered in a prior PROXY_PROGRESS reply, before the proxy server acknowledged it via OPEN(CLAIM_PROXY) (see the delivery-and-acknowledgment paragraph below). pa_stateid is the proxy_stateid of the assignment being rescinded; the proxy server MUST drop any in-progress work tagged with this proxy_stateid and MUST NOT issue PROXY_DONE / PROXY_CANCEL for it (the metadata server has already cleaned up the in-flight migration record on its side and retired the proxy_stateid).

For each PROXY_OP_MOVE or PROXY_OP_REPAIR assignment, the proxy server begins by issuing a normal NFSv4 OPEN+LAYOUTGET against pa_file_fh (the L3 composite layout), performs the data movement pa_kind calls for, and reports terminal status via PROXY_DONE(pa_stateid, ...) () or PROXY_CANCEL(pa_stateid) (). Delivery of a PROXY_OP_MOVE or PROXY_OP_REPAIR assignment is at-least-once. There is no explicit acknowledgment: the proxy server's OPEN(CLAIM_PROXY) on the assignment's pa_file_fh serves as one. After minting a proxy_stateid for an assignment, the metadata server MUST repeat that proxy_assignment4, unchanged in every field, in every subsequent PROXY_PROGRESS reply to the assigned proxy server. Re-delivery stops when any of the following occurs:

The proxy server presents that proxy_stateid in an OPEN(CLAIM_PROXY) against pa_file_fh. The metadata server MUST then cease re-delivery and treat the assignment as acknowledged (ASSIGNED to PROXY_ACTIVE, ).
The metadata server rescinds the assignment via PROXY_OP_CANCEL_PRIOR.
The proxy server's registration lease expires, dropping the migration record ().

Repetition is harmless because both halves are idempotent: every re-delivery carries the same proxy_stateid, and OPEN(CLAIM_PROXY) is idempotent by (pa_file_fh, proxy_stateid). A lost PROXY_PROGRESS reply, a PROXY_PROGRESS in flight while the metadata server re-delivers, and a proxy server that has already picked the assignment up therefore produce neither duplicate work nor spurious state; the proxy server ignores any re-delivery of a proxy_stateid it has already opened. pa_file_fh is an nfs_fh4 minted by the metadata server and presented to the proxy server for use against the same metadata server. Per , NFSv4 filehandles are server-private opaque tokens; the receiving server treats the byte string as opaque, validates it only by attempting the lookup, and returns NFS4ERR_STALE or NFS4ERR_BADHANDLE if the bytes do not resolve. The proxy server MUST NOT inspect, mutate, or shape-check pa_file_fh; it forwards the filehandle verbatim in PUTFH on the same metadata server that issued it, and the existing PUTFH semantics apply unchanged. The ppr_lease_remaining_sec field is the metadata server's acknowledgment of this PROXY_PROGRESS as a registration lease renewal. It is the number of seconds remaining until the proxy server's registration would expire absent further PROXY_PROGRESS. It is an UPPER bound on the safe next-poll interval, not a lower bound: the proxy server MUST issue its next PROXY_PROGRESS strictly before this many seconds have elapsed, minus a safety margin for network round-trip time and local clock skew. A proxy server that lets its next poll slip past ppr_lease_remaining_sec will find its registration already expired; the metadata server will have discarded its in-flight migration records, and every subsequent PROXY_PROGRESS or PROXY_DONE / PROXY_CANCEL will fail with NFS4ERR_STALE_CLIENTID (see the lease-expiry paragraph in ). Polling cadence: the proxy server SHOULD poll at ppr_lease_remaining_sec / 2 in steady state to leave one whole renewal interval of headroom. Adaptive backoff on consecutive empty replies is permitted only within the safe budget: the proxy server MAY extend its polling interval up to a bound B < ppr_lease_remaining_sec - S where S is the safety margin above, and MUST reset the cadence on any non-empty reply. Under no adaptive backoff rule may a next poll be scheduled at or after ppr_lease_remaining_sec from the current reply. The metadata server MAY shrink the effective cadence by returning a smaller ppr_lease_remaining_sec on subsequent replies, driving a busy proxy server to poll more often. A metadata server that returns ppr_lease_remaining_sec = 0 signals immediate expiry; the proxy server MUST re-register before issuing any further work-bearing operation. The metadata-server-initiated cancellation case (the metadata server abandons an in-flight assignment before the proxy server has driven it to terminal state) is signaled via the PROXY_OP_CANCEL_PRIOR assignment kind described above. There is no separate cancel callback; the proxy-server-initiated cancel is handled by the fore-channel PROXY_CANCEL ().

New Fore-Channel Operations: PROXY_DONE and PROXY_CANCEL The proxy-server-to-metadata-server protocol uses two new fore-channel operations in addition to the extended PROXY_PROGRESS:

PROXY_DONE (op 98):: proxy server reports terminal success or failure on a specific in-flight migration. The metadata server uses the pd_status to atomically commit (success: swap the file's active layout from L1 to L2) or roll back (failure: keep L1, drop L2/G).
PROXY_CANCEL (op 99):: proxy server aborts a work item it was assigned but cannot complete (e.g., source data server becomes unreachable, proxy server resource exhaustion). The metadata server treats this as PROXY_DONE with a fail-equivalent status: rolls back to L1, drops L2/G, frees the assignment for re-assignment by a later PROXY_PROGRESS poll.

Both operations identify the affected migration by its proxy_stateid (see ), which the metadata server minted at assignment time and delivered in the ppr_assignments result of PROXY_PROGRESS. The metadata server keys its in-flight migration record on the proxy_stateid; the record is authorized to the proxy server by the prr_registration_id returned at PROXY_REGISTRATION (see PROXY_DONE authorization, ), not by the per-EXCHANGE_ID clientid4. The proxy server does present the proxy_stateid to OPEN(CLAIM_PROXY) and to the LAYOUTGET that follows (see ), but the migration record itself is keyed on the proxy_stateid, not on the layout stateid subsequently returned by LAYOUTGET.

Operation 98: PROXY_DONE - Commit or Roll Back a Proxy Operation

ARGUMENTS

PROXY_DONE arguments

RESULTS

PROXY_DONE results

DESCRIPTION PROXY_DONE signals the terminal outcome of a migration the proxy server was assigned via PROXY_PROGRESS. pd_stateid is the proxy_stateid the metadata server minted when it delivered the corresponding proxy_assignment4 (). A pd_status of NFS4_OK directs the metadata server to commit the migration (swap the file's active layout from the pre-migration shape L1 to the post-migration shape L2); any other value directs the metadata server to roll back (keep L1, discard L2 and the proxy-server-only composite L3). The proxy server compounds PROXY_DONE after the data-movement phase completes (or fails): SEQUENCE PUTFH(pa_file_fh) LAYOUTRETURN(L3_stateid) PROXY_DONE(pd_stateid, status) LAYOUTRETURN runs FIRST per , releasing the proxy server's reference to the L3 layout cleanly via the standard mechanism. PROXY_DONE then operates on the in-flight migration record keyed by the proxy_stateid; the record is the single source of truth for migration state, so PROXY_DONE remains valid even though L3 has just been returned. The proxy server MAY issue PROXY_DONE in a subsequent compound, but the single-compound shape is RECOMMENDED to keep the recovery window short.

Authorization The metadata server MUST validate, in this priority order, returning the first failure encountered:

The calling session belongs to a registered proxy server. Otherwise: NFS4ERR_PERM.
pd_stateid.other was minted in the current (server_state, boot_seq) tuple. A proxy_stateid minted in a prior boot returns NFS4ERR_STALE_STATEID.
pd_stateid.other identifies a proxy operation currently in flight at the metadata server. Otherwise: NFS4ERR_BAD_STATEID.
The proxy operation identified by pd_stateid is owned by the calling session's registered-proxy server identity. The identity captured at PROXY_REGISTRATION time -- the prr_registration_id (a non-zero 64-bit value), plus the matched GSS principal / mTLS fingerprint that identity was bound to at registration -- is the authorization principal, not the per-EXCHANGE_ID clientid4. This makes PROXY_DONE / PROXY_CANCEL tolerant of proxy server reconnect: a proxy server that drops its session and reconnects with a fresh EXCHANGE_ID but the same prr_registration_id retains authority over its in-flight migrations, provided the metadata server still holds that registration (see step 2 for the metadata-server-restart case). Mismatch returns NFS4ERR_PERM.
The current filehandle (set by the preceding PUTFH) is the pa_file_fh of the proxy operation identified by pd_stateid. Otherwise: NFS4ERR_BAD_STATEID.
pd_stateid.seqid matches the seqid the metadata server minted at assignment time for this proxy_stateid (see Renewal Semantics: the seqid does not bump over the assignment's lifetime in this revision). Otherwise: NFS4ERR_OLD_STATEID.

If all validations succeed, the metadata server atomically:

For a pd_status of NFS4_OK: commits the migration -- promotes L2 to be the file's layout (D dropped, G promoted), and enters the L3 drain phase. Client-held L1 layouts were already recalled and returned before the migration entered the active phase (a precondition of L3 issuance, see ); during migration, clients hold L3. The metadata server therefore MUST issue CB_LAYOUTRECALL on L3 to every client that received it, MUST retain enough L3 bookkeeping to accept the resulting client LAYOUTRETURN fan-out, and MUST defer both the final retirement of the L3 bookkeeping and the final removal of the decommissioned mirror D until every L3 holder has returned its layout. New reads and writes issued after PROXY_DONE commit are served against L2 (the new active layout); L3 is drained, not left in force. See for the full mechanics.
For any other pd_status: reverts the migration -- restores L1 as the file's active layout; L2 and L3 are discarded. Client-held L3 layouts (issued during migration) MUST be recalled and drained before L3 bookkeeping is retired -- clients received L3 during the active phase and MUST be given the opportunity to complete or abort the associated I/O through the abort sequence defined in . Once L3 is drained, external clients transparently see the pre-migration L1; no CB_LAYOUTRECALL of L1 is needed because L1 was recalled before entering the active phase and is being reissued fresh on next LAYOUTGET. The proxy server owns cleanup of any half-written data it placed on the target G.

In both cases the metadata server retires the proxy operation; pd_stateid is thereafter invalid. Atomicity is critical: external client traffic must transition cleanly across this op; either the per-instance deltas commit fully or they do not commit at all.

Operation 99: PROXY_CANCEL - Abort a Proxy Operation

ARGUMENTS

PROXY_CANCEL arguments

RESULTS

PROXY_CANCEL results

DESCRIPTION PROXY_CANCEL discards an assigned-but-unfinished migration. The proxy server uses it when it knows it cannot complete the assignment (the proxy server is being shut down gracefully, the source data server is unreachable, the destination data server rejected the writes, etc.) and wants to release the work item back to the metadata server without computing a specific failure status. pc_stateid is the proxy_stateid the metadata server minted when it delivered the corresponding proxy_assignment4. Compound shape: SEQUENCE PUTFH(pa_file_fh) LAYOUTRETURN(L3_stateid) PROXY_CANCEL(pc_stateid) LAYOUTRETURN runs first (standard release of the L3 layout); PROXY_CANCEL then operates on the in-flight migration record only.

Authorization The same priority-ordered validation as PROXY_DONE () applies, with pc_stateid substituted for pd_stateid. In particular, the registered-proxy server identity that owns the proxy operation identified by pc_stateid MUST match the caller's, or the metadata server returns NFS4ERR_PERM; a proxy server cannot cancel another proxy server's migration.

Side effects and the abort sequence PROXY_CANCEL, PROXY_DONE with a failing pd_status, and implicit teardown from registration expiry all revert the migration. Reverting from any of these paths, if it discards L3 without fencing delayed I/O, can leave acknowledged writes in the abandoned G or acknowledged writes from a delayed L3 holder landing after the revert; both produce acknowledged-but-unreachable or divergent data. All three paths MUST therefore apply the same abort sequence before the metadata server discards L2/L3 and before L1 is reissued to subsequent LAYOUTGETs:

Stop new L3 issuance. The metadata server MUST refuse any further LAYOUTGET that would return L3 for this file; subsequent client LAYOUTGETs receive L1 (or NFS4ERR_LAYOUTUNAVAILABLE if L1 recall is still in flight).
Recall or revoke client-held L3. For every client to which L3 was issued, the metadata server MUST issue CB_LAYOUTRECALL(L3) and MUST retain L3 bookkeeping until the resulting LAYOUTRETURNs arrive. If a client fails to return within the layout-recall grace period, the metadata server MUST revoke the client's L3 layout stateid via the ordinary NFSv4.1 layout revocation mechanism ().
Fence the proxy server's backend stateids. In every coupling mode, the metadata server MUST fence any stateid the proxy server was using against the source or target data servers, so that a delayed write from the (now-terminated) proxy operation cannot land after the revert. In trusted-stateid tight coupling (, sec-TRUST_STATEID) the metadata server MUST issue REVOKE_STATEID (or the bulk equivalent) against every data server that carried the proxy server's stateid. In loose coupling the metadata server MUST revoke the proxy server's own NFSv4.1 stateids at the source and target data servers via the data-server-native mechanism.
Drain in-flight I/O. After steps 1-3, the metadata server MUST wait for any in-flight compound the proxy server has already issued against a fenced data server to fail (either at the data server, which rejects with NFS4ERR_STALE_STATEID / NFS4ERR_ADMIN_REVOKED, or by transport timeout). The metadata server MAY use its session's own back-channel state to accelerate this drain, but MUST NOT declare the abort complete before the data servers have had at least one lease period to observe the fence.
Retire L2/L3 bookkeeping. Only after steps 1-4 have each completed MAY the metadata server discard the L2 and L3 layouts, drop G (or leave it for administrative cleanup), retire the proxy operation, invalidate the proxy stateid (pc_stateid for PROXY_CANCEL, pd_stateid for a failing PROXY_DONE), and update operator-facing telemetry. L1 MAY then be reissued to ordinary LAYOUTGET traffic.

The abort sequence applies verbatim to registration expiry: when the metadata server abandons an in-flight migration because the owning proxy server's registration lease has expired (), it MUST run the five-step sequence before dropping the migration record. The distinction between PROXY_DONE(FAIL) and PROXY_CANCEL is purely intent / accounting: PROXY_DONE(FAIL) records that the proxy server attempted the migration and ran into a recoverable error; PROXY_CANCEL records that the proxy server abandoned the assignment without attempting it (or while attempting, decided not to report a specific failure cause). A metadata server implementation MAY surface the distinction in operator telemetry but MUST NOT make any behavioral distinction on the wire. In particular, the abort sequence above is identical for both.

Multi-Proxy Server Assignment Fan-out When multiple proxy servers are registered against the same metadata server, the metadata server coordinates assignment fan-out under one hard invariant: at any time, at most one migration MUST be in flight for a given (pa_file_fh, pa_target_deviceid) pair. The metadata server MUST NOT assign a migration whose (pa_file_fh, pa_target_deviceid) matches that of an in-flight migration. How the metadata server chooses among eligible proxy servers -- by load, locality, fault domain, capability match, or any combination -- is an implementation matter. The protocol constrains only the outcome: the proxy server that receives the assignment is registered at delivery time, and the (pa_file_fh, pa_target_deviceid) invariant holds.

Per-Proxy-Server In-Flight Cap The number of concurrently in-flight assignments a proxy server will accept -- its in-flight cap -- is an implementation-side self-imposed limit and is not negotiated on the wire in this revision. The metadata server SHOULD NOT attempt to discover it and SHOULD NOT queue more assignments to a single proxy server than that proxy server can plausibly carry; concretely, once a metadata server has queued enough assignments to a given proxy server to fill any reasonable in-flight budget, it SHOULD prefer other eligible proxy servers before continuing to queue against the same one. A proxy server that receives more assignments than it wishes to work on SHOULD silently ignore the excess (per ): the assignments will be re-delivered on subsequent PROXY_PROGRESS replies (see the delivery-and-acknowledgment paragraph in ) and the metadata server MAY reassign them to another eligible proxy server if the original does not acknowledge within a policy-defined window. Interaction with the ASSIGNED state: a file in ASSIGNED continues to be served against its pre-migration layout (L1) until the metadata server observes the assigned proxy server's OPEN(CLAIM_PROXY) for that file's pa_file_fh. LAYOUTGET on a file in ASSIGNED MUST return the pre-migration L1 layout (or MUST return NFS4ERR_DELAY only if the metadata server has already begun issuing L3 in anticipation of an imminent proxy-server pickup; the DELAY window MUST NOT persist across delivery attempts). This is "at-least-once delivery, no acceptance implied": the metadata server MUST NOT block client I/O for the duration of an unacknowledged assignment. To bound how long an assigned-but-unacknowledged migration lingers, the metadata server MUST enforce a bounded delivery count and a bounded delivery deadline:

The metadata server MUST include the same proxy_assignment4 in at most a policy-defined maximum A_MAX consecutive PROXY_PROGRESS replies without observing the acknowledging OPEN(CLAIM_PROXY). A RECOMMENDED default is A_MAX = 3.
If the metadata server has issued A_MAX copies of an assignment without acknowledgment, or a policy-defined delivery-deadline interval T_DELIVERY has elapsed since the first delivery, whichever occurs first, the metadata server MUST rescind the assignment via PROXY_OP_CANCEL_PRIOR in the next PROXY_PROGRESS reply. It MAY then reassign the same (pa_file_fh, kind) to a different eligible proxy server, restarting the delivery counter, or return the file to READY.
If a proxy server declines to accept new assignments because its local queue is full, it SHOULD simply not send PROXY_PROGRESS until it has capacity, relying on the metadata server's rescission-and-reassignment path above.

When a registered proxy server loses its session -- its lease expires, its underlying transport is torn down, or its registration is otherwise dropped -- the metadata server MUST treat each migration the proxy server had in flight as if PROXY_DONE(FAIL) had been issued: L1 stays in force, L2 and L3 are discarded. The metadata server MAY then reassign the work to another eligible proxy server; the (pa_file_fh, pa_target_deviceid) invariant continues to hold across reassignment. A proxy server that reconnects with the same prr_registration_id retains ownership of its in-flight migrations regardless of whether its EXCHANGE_ID returns the prior clientid or a fresh one; no reassignment is needed. The proxy server reclaims its per-file layouts via the metadata-server-recovery path in , presenting each migration's proxy_stateid to OPEN(CLAIM_PROXY). The prior prr_registration_id is available for re-presentation only while the metadata server holds it in the registration table; a metadata-server restart that discards the table forces the proxy server onto the fresh-registration retry path in step 2, at which point migration ownership resets. A host that does not implement the proxy server role simply does not call PROXY_REGISTRATION and is never selected for a PROXY_OP_MOVE or PROXY_OP_REPAIR assignment. A deployment with no registered proxy server falls back to per-chunk CB_CHUNK_REPAIR for single-shard repair, to admin-coordinated offline procedures for policy transitions and data server evacuation, and to blocking data server maintenance -- the data server cannot drain through a proxy server, so it must remain reachable to clients throughout its service life. Deployments SHOULD ensure at least one registered proxy server exists per failure domain to avoid a single point of failure on move operations.

Layout Shape During a Proxy Operation While a proxy operation is active, a client sees only one part of the mechanism: the layout the metadata server hands it. Everything else in this document -- the session, the ops, the credential-forwarding rules -- is between the metadata server and the proxy server. The layout shape is therefore what a client implementer needs to read to know how its code interacts with a proxied file. On a LAYOUTGET, the metadata server chooses one of three outcomes:

A direct-data server layout, when no proxy operation is in flight for the file and the client's coding-type support set includes the file's coding. This is the unchanged flexible file v2 layout path.
A single-data server layout naming the proxy server, when a PROXY_OP_MOVE or PROXY_OP_REPAIR migration is in flight for the file, or when the client's coding-type support set does not include the file's coding and a registered proxy server can translate. The layout's single ffv2_data_server4 entry has FFV2_DS_FLAGS_PROXY set on its ffs_ds_flags (see ), marking the entry as a proxy server rather than a direct data server. The client uses this layout as it would any flexible file v2 layout, sending CHUNK ops to the named data server; the proxy server internally dispatches reads and writes to the source and destination data servers.
NFS4ERR_CODING_NOT_SUPPORTED (see ), when the client's coding-type support set does not include the file's coding and no registered proxy server can translate.

A client that supports the flexible file v2 layout -- which is the precondition for any of this -- needs no proxy-specific I/O code: the proxy case arrives as a single-data server layout and drives the same CHUNK ops as any other flexible file v2 layout. The FFV2_DS_FLAGS_PROXY bit on the data server entry is available for clients that choose to distinguish proxy-fronted I/O (for telemetry, alternative retry policy, or avoidance under application preference); a client that ignores the bit interoperates unchanged.

Atomic commit on PROXY_DONE When the proxy server issues PROXY_DONE(pd_stateid, pd_status=NFS4_OK), the metadata server atomically (in one transaction):

promotes L2 to be the file's layout (D dropped, G promoted);
drops L1 and L3 from the file's layout records;
retires the in-flight migration record;
issues CB_LAYOUTRECALL for the file's outstanding client-facing layouts that name the proxy server; and
defers REMOVE_MIRROR(D) until those layouts are returned.

On its next LAYOUTGET each client receives the post-migration layout (L2): the real data servers, with no proxy server. When PROXY_DONE indicates failure (or PROXY_CANCEL is issued):

L1 is promoted unchanged -- the file falls back to its pre-migration mirror set.
L2 is dropped; the half-filled G instance is internally unlinked.
L3 is dropped and the migration record retired.
CB_LAYOUTRECALL is issued for the layouts that name the proxy server; on the next LAYOUTGET clients receive L1.

The swap window Because every client wrote through the proxy server, no client ever addressed D directly, and there are no client writes in flight to D when the swap occurs. The deferred REMOVE_MIRROR(D) covers only the proxy server's own trailing writes: by the time it issues PROXY_DONE the proxy server, as the sole writer, has quiesced its M2 fan-out, so the deferral window is short and contains no client-visible activity. When CB_LAYOUTRECALL arrives, a client holding a layout that names the proxy server returns it and issues a fresh LAYOUTGET in the usual way. In-flight client I/O to the proxy server across that boundary is handled by the in-flight-I/O rules for a proxy server change (see "In-Flight I/O When the proxy server Changes").

The CLAIM_PROXY open claim The proxy server opens the file with a new OPEN claim, CLAIM_PROXY. A bare OPEN(CLAIM_NULL) cannot serve here: it would be indistinguishable from an ordinary or racing OPEN on the registered-proxy server session, leaving the metadata server to infer proxy intent from session state -- which it cannot do reliably. CLAIM_PROXY makes the proxy OPEN explicit and carries, in one step, what the metadata server needs to bind the proxy server to the file. open_claim_type4 gains one enumerant and open_claim4 one union arm:

The CLAIM_PROXY open claim CLAIM_PROXY is filehandle-based and opens an existing file: the proxy server issues PUTFH of the assignment's pa_file_fh followed by OPEN(CLAIM_PROXY), with no directory filehandle and no component name, in the manner of CLAIM_FH. CLAIM_PROXY MUST NOT be combined with OPEN4_CREATE. The operand carries two values:

ocp_proxy_stateid:: the proxy_stateid4 the metadata server minted for this assignment and returned in the PROXY_PROGRESS work assignment (). It is the correlator that identifies this OPEN as the proxy OPEN for a specific assignment; the metadata server does not infer proxy intent from the session.
ocp_proxy_fh:: the filehandle under which the proxy server will serve the file to clients: the data-server filehandle that appears in the layout the metadata server hands an encoding-incapable client, and the filehandle a non-pNFS client obtains by LOOKUP against the proxy server. Only the proxy server can mint this filehandle; it is opaque to the metadata server, which records it and copies it verbatim into the layouts it issues. Carrying it in the OPEN binds it atomically with the proxy OPEN.

The metadata server MUST verify that ocp_proxy_stateid is valid, that it names an outstanding assignment, that the assignment was made to the calling clientid, and that the current filehandle is that assignment's file. An invalid or stale stateid draws NFS4ERR_BAD_STATEID. A proxy server MUST NOT issue OPEN(CLAIM_PROXY) unless it holds a successful PROXY_REGISTRATION (); successful registration is what establishes that the metadata server implements this extension. OPEN(CLAIM_PROXY) returns the ordinary OPEN result: an open stateid, and an open_delegation4 which the metadata server MUST set to OPEN_DELEGATE_NONE -- a delegation to the proxy server would conflict with the migration the proxy server is itself driving. The proxy server opens with OPEN4_SHARE_ACCESS_BOTH and OPEN4_SHARE_DENY_NONE. A retransmitted or re-issued OPEN(CLAIM_PROXY) is handled exactly as for any other claim: the session replay cache absorbs retransmits, and a genuine repeated OPEN by the same open-owner is an ordinary share-state operation. The proxy server then obtains the L3 composite layout with an ordinary LAYOUTGET; the metadata server serves L3 because the calling clientid holds an in-flight migration record for the file. The L3 layout stateid is a normal NFSv4 layout stateid. It governs the CHUNK_READ, CHUNK_WRITE, CHUNK_FINALIZE, and CHUNK_COMMIT operations the proxy server issues against the source and target data servers, in the standard way. It is distinct from proxy_stateid4 (), which is a control-plane handle for the migration as a whole and is never presented to LAYOUTGET; the metadata server keys its in-flight migration record on the proxy_stateid. Separating the two -- one for I/O on a layout, one for the migration -- keeps the migration record's lifetime independent of any LAYOUTGET / LAYOUTRETURN cycle the proxy server performs during the data-movement phase.

Drain interaction The DRAINING state on D is observable to external clients only through the absence of new layouts naming D: while D is DRAINING, the metadata server does not place D in any new mirror set. Before the migration becomes active for an existing file whose layout names D, the metadata server issues CB_LAYOUTRECALL on every outstanding layout for the file whose mirror set includes D. Once those layouts have been returned -- or administratively revoked when a client's CB back-channel fails to ack within the recall window -- the migration is in flight. From that point until the assigned proxy server completes its OPEN(CLAIM_PROXY) () and registers the filehandle under which it will serve the file, the metadata server cannot yet build a client-facing layout, and answers every LAYOUTGET for the file with NFS4ERR_DELAY; clients retry. This window MUST be bounded: if the assigned proxy server does not complete its OPEN(CLAIM_PROXY) within a deadline tied to its registration lease, the metadata server reassigns or abandons the assignment and stops returning NFS4ERR_DELAY. Once the proxy server has opened, subsequent LAYOUTGETs for the file return a layout naming the proxy server. This omit-and-replace ordering guarantees that no client write hits D after the migration has started. The alternative -- keep-and-shadow, in which the layout view continues to include D and the proxy server shadows client writes from D to G as they happen -- requires the proxy server to expose itself as a flexible file v2 data server (an INTERPOSED instance taking the place of D in the visible layout, with the proxy server funneling writes to both D and G). This shape is defined in the per-instance delta model below (informative) but is not exercised by the wire ops in this revision.

Per-instance migration deltas (informative) The L1/L2/L3 framing above describes one valid implementation approach -- whole-layout swap -- that captures the simplest case (single mirror replacement under a Client Side Mirroring encoding). A metadata server implementation that supports more general migrations (e.g., a single shard add to an erasure-coded file, or a partial mirror-set rotation under a Reed-Solomon encoding) MAY record migration state as per-instance deltas on the file's existing layout records, rather than as a complete L2/L3 pair. In this informative model, each migration record carries an array of per-instance deltas, each delta describing a transformation on one position within one segment of layout_segments. Four instance states are useful:

STABLE:: unchanged; client writes go here directly.
DRAINING:: a slot being decommissioned; under omit-and-replace, the LAYOUTGET view-build path omits this slot and replaces it with the matching INCOMING.
INCOMING:: a new slot the proxy server is filling; under omit-and-replace, the LAYOUTGET view-build path emits this slot in place of the matching DRAINING.
INTERPOSED:: a slot whose visible endpoint is the proxy server, with the proxy server internally fanning writes out to one or more target data servers. Used by keep-and-shadow (forward-compat; not produced by the wire ops in this revision).

The current published layout (layout_segments) is built through the deltas: when LAYOUTGET runs while a migration is active, the layout-build path consults the migration record and emits the during-migration view by applying the deltas to the base segments. layout_segments itself is never mutated until PROXY_DONE(NFS4_OK) collapses the deltas into the base records permanently. This per-instance model and the L1/L2/L3 swap model agree on the wire-visible behavior in the simplest case (single mirror replacement, omit-and-replace). The wire ops in this draft do not require either implementation; a metadata server chooses whichever matches its layout-record machinery. The wire ops in this draft do not constrain the choice; the per-instance delta model is one known implementation strategy that has been used to track the four record-builder invariants and a lease-aware reaper for the migration record / proxy_stateid tables across the lifecycle described above.

Client Behavior During a proxy operation the layout the metadata server hands a client is a single data server flexible file v2 layout naming the proxy server. The client treats it as any other flexible file v2 layout, sending CHUNK ops to the named data server under its existing layout stateid. Nothing in the client's path distinguishes "the data server is a proxy server" from "the data server is a real data server"; that distinction lives entirely on the metadata server side. The proxy server accepts those CHUNK ops under the client's existing layout stateid because the metadata server has registered the stateid via TRUST_STATEID on the proxy server, per the tight-coupling semantics in . The client handles proxy-server-side errors (NFS4ERR_DELAY, connection loss, NFS4ERR_BAD_STATEID) exactly as it would any other data server error: report LAYOUTERROR to the metadata server and expect either a new layout or a proxy server reassignment in return.

When the Layout Is Recalled If the metadata server recalls the layout mid-operation (the proxy server failed and is being replaced, or the operation completed and normal data server layouts are being reissued), the client issues LAYOUTRETURN as usual and reacquires via LAYOUTGET. The new layout may name a different proxy server, a different mirror set, or -- if the proxy operation has completed -- the real data servers directly.

In-Flight I/O When the Proxy Server Changes In-flight I/O to the old proxy server when the metadata server recalls the layout MAY complete at the old proxy server; results remain valid under the old proxy server's authority. New I/O issued after LAYOUTRETURN MUST go through the data server(s) the new layout names: a replacement proxy server, or the real data servers if the proxy operation has completed.

State Machine A file's participation in a proxy operation passes through five states.

READY:: No operation is in flight.
ASSIGNED:: The metadata server has queued an assignment for a proxy server, but that proxy server has not acknowledged it via OPEN + LAYOUTGET.
PROXY_ACTIVE:: The proxy server is driving a move or repair, and external clients hold the transitional L3 composite layout.
COMMITTING:: The proxy server has issued PROXY_DONE(OK), and the metadata server is recalling L3 from the clients holding it.
DONE:: Clients are on the post-move L2 layout and the source data servers are retired.

The state is metadata-server-local: clients never observe these state names directly, but a client's behavior is shaped by which layout the metadata server is currently handing out. A given file spends most of its lifetime in READY; a proxy operation is a relatively short excursion through the other four states, after which the file returns to READY with a new layout in place -- or, on cancellation or failure, with the old layout preserved. The diagram below shows the states and the principal transitions, including the failure exits from ASSIGNED and PROXY_ACTIVE back to READY. The Transitions table that follows enumerates each transition with its trigger and effect.

Transitions

From	To	Trigger	Actions
READY	ASSIGNED	metadata server decides to move or repair	metadata server queues a `proxy_assignment4` (`PROXY_OP_MOVE` or `PROXY_OP_REPAIR`) for delivery in the next PROXY_PROGRESS reply to the selected proxy server; creates the in-flight migration record
ASSIGNED	PROXY_ACTIVE	proxy server picks up the assignment	proxy server issues `OPEN(CLAIM_PROXY)` + LAYOUTGET against `pa_file_fh`; metadata server begins serving clients a layout naming the proxy server
PROXY_ACTIVE	COMMITTING	proxy server issues PROXY_DONE with `pd_status=NFS4_OK`	metadata server begins CB_LAYOUTRECALL fan-out to clients holding L3 (issued during PROXY_ACTIVE); L1 was already recalled and drained before PROXY_ACTIVE was entered
COMMITTING	DONE	All clients have LAYOUTRETURNed	metadata server issues post-move layouts (L2); source DSes retired
DONE	READY	metadata server finalizes retirement bookkeeping	file returns to steady state under the new layout; no wire event is required
ASSIGNED	READY	metadata-server-initiated cancellation: metadata server includes a `PROXY_OP_CANCEL_PRIOR` assignment in the next PROXY_PROGRESS reply	metadata server drops the in-flight record; proxy server drops the assignment from its in-flight queue
PROXY_ACTIVE	COMMITTING	proxy server issues PROXY_DONE with a failing `pd_status`, or PROXY_CANCEL, or the proxy server's lease expires with no replacement available	metadata server begins CB_LAYOUTRECALL fan-out to clients still on the L3 (proxy) layout so they revert to L1; L2 and G are discarded
COMMITTING	READY	All clients that held L3 have LAYOUTRETURNed	file returns to steady state under L1 (the pre-operation layout); no L2 promotion occurs

Proxy Server Failure and Recovery

Proxy Server Crash During PROXY_ACTIVE When a proxy server crashes mid-operation, client I/O routed through it receives NFS4ERR_DELAY (if the proxy server is reachable but unhealthy) or connection errors (if unreachable), and the affected clients report LAYOUTERROR to the metadata server. The metadata server MAY select a replacement proxy server from the registered pool and queue a fresh proxy_assignment4 (PROXY_OP_MOVE or PROXY_OP_REPAIR) for that proxy server in its next PROXY_PROGRESS reply, with the source layout updated to reflect current reality -- destination data servers that the failed proxy server populated are now part of the source set -- and the destination layout unchanged; the replacement proxy server resumes from wherever the failed proxy server left off. Before the replacement proxy server's layout becomes live, the metadata server MUST fence the failed proxy server: it revokes the failed proxy server's L3 layout stateid (REVOKE_STATEID) and, where the data server protocol supports it, fences the failed proxy server at the source and target data servers. Fencing closes the window in which a delayed write from a failed-but-not-dead proxy server could land after the replacement proxy server has taken over -- a two-proxy-server instance of the write race the single-writer model otherwise prevents. The metadata server then issues CB_LAYOUTRECALL on the old layout and the replacement proxy server's layout becomes live for new LAYOUTGETs. If the metadata server cannot find a replacement within a policy timeout, it MUST cancel the operation: revert to the pre-move source layout, do not issue a destination layout, and mark the destination data servers for cleanup or retry.

Cascading Proxy Server Failure A second proxy server failure on the same operation SHOULD escalate to deployment management rather than trigger another automatic replacement. Recurring failures across multiple proxy servers indicate an environmental issue no proxy server can work around -- an unreachable source data server, a misconfiguration, or a starved replacement pool -- that operator attention will resolve sooner than another retry.

Source Data Server Crash During PROXY_ACTIVE A source data server crash reduces the proxy server's read parallelism but does not block forward progress as long as the erasure code can still reconstruct the file from the surviving source data servers. If the source set degrades past reconstructibility, the operation transitions to whole-file repair semantics automatically: ranges that can still be reconstructed succeed; ranges that cannot terminate the operation with NFS4ERR_PAYLOAD_LOST.

Destination Data Server Crash During PROXY_ACTIVE A destination data server crash is handled as a normal data server failure on the destination side. The proxy server, acting as a client to the destination data servers, reports LAYOUTERROR to the metadata server, which MAY substitute a spare or mark the destination FFV2_DS_FLAGS_REPAIR. The proxy server continues pushing to the remaining destinations, and clients are unaffected.

Metadata Server Crash Recovery Clients and the proxy server detect metadata server session loss and enter RECLAIM per / S10.2.1. The proxy server's recovery extends the standard NFSv4.1 client-recovery sequence with one proxy-specific step -- re-registration -- and one explicit safety rule: if the proxy server cannot reclaim a layout for a migration it had in flight, the proxy server drops that migration rather than continuing with stale state.

Proxy Server Recovery Sequence The proxy server, after detecting metadata server session loss, performs the following steps in order:

EXCHANGE_ID + CREATE_SESSION with the proxy server's prior client_owner4. Standard NFSv4.1 client recovery; the proxy server's clientid4 is restored when the metadata server recognizes the prior client_owner4.
PROXY_REGISTRATION to re-establish the proxy-server role on the new session. The proxy server attempts renewal first, presenting its prior prr_registration_id, and inspects the result.

Renewal accepted (NFS4ERR_STALE_CLIENTID NOT returned):

the metadata server retained the registration across its own restart; the proxy server keeps the same prr_registration_id and the migration records tied to it remain in force. Proceed to step 3.

NFS4ERR_STALE_CLIENTID:

the metadata server lost its registration table across the restart (see the unknown-or-stale case in ). The proxy server MUST retry PROXY_REGISTRATION with pra_registration_id = PROXY_REGISTRATION_ID_NEW (0) to obtain a fresh registration. The fresh prr_registration_id returned by the metadata server is unrelated to the prior one and carries no prior migration ownership; the metadata server's assignment queue for this proxy server in step 3 will necessarily be empty, and every retained sidecar entry falls through to step 5 to be dropped per .

NFS4ERR_PERM:

another party is currently registered under this prr_registration_id with a different authenticated identity; the proxy server MUST NOT retry with the prior ID. This is a deployment or credentialing fault, not a routine recovery outcome, and is out of scope for the autopilot recovery path.

Until PROXY_REGISTRATION completes, the metadata server treats the client as an ordinary NFSv4 client and MUST NOT deliver proxy assignments to it.
PROXY_PROGRESS to pull the assignment queue. If the metadata server has retained any in-flight migrations owned by this proxy server, it delivers them in the reply -- each as a fresh proxy_assignment4 carrying a fresh proxy_stateid (the prior proxy_stateid is from a prior boot and is permanently stale; see ). For each re-delivered assignment, the proxy server attempts to match it to a retained sidecar entry by (pa_file_fh, pa_target_deviceid). Matched assignments proceed to step 4; unmatched assignments are handled as fresh assignments.
Per-file layout reclaim, for each matched assignment: OPEN_RECLAIM(CLAIM_PREVIOUS, pa_file_fh) per , followed by LAYOUTGET(reclaim=true) per with the proxy server's retained layout stateid as the reclaim key. On success the metadata server returns a fresh layout stateid for the resumed migration and the proxy server continues from where it left off. On NFS4ERR_NO_GRACE, NFS4ERR_RECLAIM_BAD, or any other reclaim failure, the proxy server MUST drop the migration: it issues PROXY_CANCEL with the fresh proxy_stateid delivered in step 3 to signal the metadata server that the migration cannot be resumed, discards the retained sidecar entry, and halts any pending I/O for the file.
Sidecar entries the metadata server did not re-deliver. For any migration the proxy server retained in its sidecar but the metadata server did not include in step 3, the proxy server MUST drop that migration: discard the sidecar entry and halt any pending I/O for the file. No signal to the metadata server is needed -- the metadata server already has no record of this migration.
RECLAIM_COMPLETE when all per-file reclaims and drops are done.

Dropped migrations rejoin the metadata server's normal assignment path: a data server still DRAINING when recovery settles will, in due course, attract a fresh proxy_assignment4 -- to this proxy server or another -- under the (pa_file_fh, pa_target_deviceid) invariant of . The sidecar match in step 3 is best-effort across a metadata server reboot. deviceid4 is server-scoped and MAY change across a restart per ; a target deviceid the metadata server reassigns on restart will not match the proxy server's retained sidecar entry, the proxy server will not recognize the re-delivery as the same migration, and the affected sidecar entries are dropped per step 5. The autopilot re-drives the work as a fresh assignment. Implementations that preserve target deviceids across restart will resume mid-flight; those that do not will re-drive from scratch -- both are conformant.

Retention Requirement A proxy server MUST be able to supply each in-flight migration's layout stateid as the reclaim key after a proxy-server-process restart. The natural implementation retains it in proxy-server-local storage (e.g., a small sidecar file or DB table keyed by pa_file_fh) when the metadata server grants the L3 layout; how it is retained is an implementation matter. A proxy server that cannot supply the prior stateid cannot reclaim its layouts after a restart, and the affected migrations are dropped per step 5 above.

Proxy Server Identity Continuity A proxy server implementation SHOULD retain its client_owner4 across proxy-server-process restart so that post-restart EXCHANGE_ID recovers the same clientid and the in-flight migration records remain valid. If the proxy server's client_owner4 rotates (e.g., because proxy server process state was lost), the new EXCHANGE_ID gets a fresh clientid4. The in-flight migration records are keyed on proxy_stateid and authorized to the proxy server by its prr_registration_id (see PROXY_DONE authorization step 4, ), so the records survive the clientid rollover provided the metadata server still holds the prior prr_registration_id in its registration table: the reconnecting proxy server, on presenting the same prr_registration_id, retains authority over its in-flight migrations and reclaims each one's per-file layout via OPEN(CLAIM_PROXY) with the record's proxy_stateid. When the metadata server did not retain the registration -- either because a metadata-server restart discarded the registration table (see step 2) or because the migration records themselves were not retained across the metadata server's recovery -- the drop rules in apply and the autopilot re-issues fresh assignments at its discretion.

Lost Migration Records The drop rules in cover the lost-state case end-to-end: a migration the metadata server did not retain is dropped by the proxy server -- either because the metadata server does not re-deliver it in PROXY_PROGRESS, or because the per-file reclaim returns NFS4ERR_NO_GRACE / NFS4ERR_RECLAIM_BAD -- and the autopilot is free to re-drive the work as a fresh assignment. No proxy-specific signalling beyond standard NFSv4 reclaim errors and PROXY_CANCEL is needed.

Security Considerations This document adds security-relevant behavior in two places: the session the proxy server establishes with the metadata server, and the data path clients take through the proxy server during a proxy operation. The session is narrower than the data path -- only the metadata server talks to the proxy server over it, and the metadata server has long been a trusted coordinator in the pNFS model -- but it carries operations that affect every client whose layouts reach a proxy server. The data path is broader, because it exposes the proxy server to every client whose layout names it; a compromised proxy server on that path has the same observational and modification reach as a compromised data server, and in the translating-proxy server case a larger reach because of the elevated identity the proxy server typically runs with. Each threat the design addresses or explicitly leaves out of scope is named below. Credential forwarding, the most consequential and the most easily implemented incorrectly, is expanded in .

Proxy Server Authority:: A proxy server in PROXY_ACTIVE sees all client I/O for the proxied file. A compromised proxy server can observe or modify file data. Deployments MUST treat proxy-server-capable hosts as at least as trusted as the data servers they proxy for. PROXY_REGISTRATION SHOULD be gated by deployment-level authorization; arbitrary hosts that present the op without prior provisioning SHOULD be rejected.
Transport security across the operation:: The proxy server's connections to source and destination data servers are independent of the client's connection to the proxy server. A proxy server MAY read from an AUTH_SYS source and write to a TLS destination (or any other combination). The proxy server is responsible for enforcing the effective security policy (e.g., do not downgrade encrypted data to a plaintext data server).
Principal binding during a proxy operation:: For proxy-server-to-data-server traffic (the proxy server reading source data servers and writing destination data servers to carry out a PROXY_OP_MOVE or PROXY_OP_REPAIR assignment), the proxy server presents a principal to those data servers that they will accept; this is the proxy server's own service identity unless constrained delegation or equivalent is arranged. Forwarding the client's identity to the peer data servers for proxy-server-driven data movement is NOT required and is typically NOT practical (the client is not in the conversation at that point). See for the case of client-initiated file I/O through a translating proxy server, where the credential-forwarding rule is different and stricter.
Proxy Server Impersonation:: A malicious metadata server could steer clients to a hostile proxy server by issuing a layout whose single ffv2_data_server4 entry names an attacker-controlled host (with FFV2_DS_FLAGS_PROXY set). Registration is proxy-initiated, so the attack is not against PROXY_REGISTRATION -- it is against the client's willingness to follow any data-server address the metadata server hands it. The existing metadata server trust model already grants the metadata server this capability for any data server, via CB_LAYOUTRECALL and the ability to issue any layout it chooses; PROXY_REGISTRATION does not weaken it, and the proxy case is not more exposed than the ordinary data-server case. Clients that require stronger proxy server identity verification SHOULD apply deployment-level authorization to the proxy server's transport-security credentials.
Registration lease expiry:: If a proxy server's lease expires mid-operation, the metadata server MUST abandon the operation: discard the in-flight migration record, revert the affected layouts to the pre-operation state, and arrange cleanup of any half-populated destination data servers. The metadata server MUST NOT continue to route client I/O to a proxy server whose registration has lapsed.

Credential Forwarding and the Privilege Boundary A translating proxy server (see ) has structurally elevated privilege by design. To perform its management tasks -- moves, repairs, evacuations, cross-tenant re-exports -- the deployment grants the proxy server's service identity broad access: typically not-root-squashed, often read/write to every file in the namespace, and session authority to every data server. That privilege is intentional. An encoding-ignorant client that reaches the proxy server, however, arrives with its own RPC credentials that the proxy server does not itself need in order to function. An NFSv3 client's uid/gid, an AUTH_SYS-squashed identity, an RPCSEC_GSS principal -- none of these are the proxy server's own. If the proxy server ignores the client's credentials and issues metadata server or data server operations under its own service identity when translating client I/O, every client that reaches the proxy server silently inherits the proxy server's privilege. This is a protocol-level privilege-escalation vector, and this document calls it out rather than hiding it. The normative requirements below apply whenever a proxy server is translating client-initiated file I/O (as distinct from proxy-server-driven move / repair work, which runs under the proxy server's own authority on directives from the metadata server). They form a cohesive set: credential pass-through is the core requirement; no-squash-inversion closes the most common way pass-through can be implemented incorrectly; authorization-remains-with-metadata server names the responsibility on the metadata server side of the same contract; service-identity-is-for-the-control-plane draws the line between the op paths where the proxy server uses its own credentials and the op paths where it does not; and the failure-mode rule specifies the correct refusal behavior rather than letting a silent fall-through become the escape hatch.

Credential pass-through:

The proxy server MUST present the client's credentials (RPC auth flavor and principal) on every metadata server or data server operation it issues as a consequence of a client-initiated request. Specifically, a client READ that the proxy server expands into LAYOUTGET + CHUNK_READ MUST carry the client's credentials on both the LAYOUTGET against the metadata server and the CHUNK_READ against the data servers. The proxy server MUST NOT substitute its own service identity for client-initiated operations.

No squash inversion:

If the client arrives with a root-squashed identity (for example, uid 0 mapped to nobody by the NFSv3 export configuration on the client-facing side of the proxy server), the proxy server MUST preserve the squashed identity when forwarding. The proxy server MUST NOT translate a client's squashed credentials back into unsquashed root, even though the proxy server's own identity is typically unsquashed.

Authorization remains with the metadata server:

When a client-initiated operation reaches the metadata server over a proxy server <-> metadata server session, the metadata server MUST use the RPC credentials carried on that compound for authorization and MUST NOT substitute the proxy server's session-level identity. Equivalently: the metadata server performs access-control checks against the forwarded client credentials, not against the proxy server's service identity, for any client-initiated file operation. The proxy server is a translator, not an authority. This is what prevents proxy server deployment from becoming a blanket ACL override.

Proxy Server Service Identity Is for the Control Plane Only:

The proxy server MUST use its own service identity for the following, and MUST NOT use it for any other operation on behalf of a forwarded client (per the pass-through rule above):

The metadata server <-> proxy server session (the session the proxy server opens to the metadata server, on which PROXY_REGISTRATION, PROXY_PROGRESS, PROXY_DONE, and PROXY_CANCEL all flow on the fore-channel; the session's back-channel is not used by this draft).
Peer-data server session setup for proxy-server-driven data movement (reading source data servers, writing destination data servers under a PROXY_OP_MOVE assignment the metadata server has delivered via PROXY_PROGRESS).
proxy server housekeeping.

The proxy server's service identity MUST NOT be used for client-initiated file data operations.

Failure mode on missing credentials:

If the proxy server cannot forward a client's credentials for some reason (e.g., the client presented AUTH_NONE, or the client-facing side used a security flavor the proxy server cannot propagate), the proxy server MUST reject the client operation with the equivalent of NFS4ERR_ACCESS (or NFS3ERR_ACCES for NFSv3 clients). The proxy server MUST NOT fall back to serving the operation under its own identity.

Deployment-level requirements:

PROXY_REGISTRATION MUST be deployment-authorized. An unknown host presenting PROXY_REGISTRATION MUST be rejected. This is the only wire-level defense against a hostile entity registering as a proxy server and then receiving client-forwarded credentials.
The metadata server <-> proxy server session MUST use RPCSEC_GSS or RPC-over-TLS with mutual authentication. AUTH_SYS as the session-authentication flavor on the metadata server <-> proxy server session is forbidden. This is distinct from forwarded client credentials: an NFSv3 /AUTH_SYS client's credentials MAY ride inside a mutually-authenticated (RPCSEC_GSS or RPC-over-TLS) session as the per-operation credential of a proxy-forwarded compound (see the credential-forwarding rules above); what is forbidden is AUTH_SYS on the session itself. Equivalently, AUTH_SYS is never sufficient for PROXY_REGISTRATION (which authenticates the session), but AUTH_SYS is the ordinary per-operation flavor for an NFSv3 client whose I/O the proxy server forwards under the client's identity.
Deployments SHOULD audit both the proxy server's credential-forwarding behavior (the proxy server logs what it forwards) and the metadata server's authorization checks (the metadata server logs what principal authorized each operation). Divergence between the two indicates a credential-forwarding bug or compromise.

What the protocol cannot defend against:

A compromised proxy server has direct access to whatever credentials pass through it. Credential confidentiality collapses the moment the proxy server is under adversary control. Mitigation is operational: restrict which hosts can register as a proxy server, audit PROXY_REGISTRATION events, rotate deployment-level keys.
A deployment that configures a proxy server to run as root while the client is root-squashed has already violated rule 2 above; no wire mechanism detects a proxy server deliberately mis-implementing credential forwarding. Deployments SHOULD verify their proxy server implementation's credential-forwarding behavior through conformance testing before production use.

Future work (noted as an Open Question below): RPCSEC_GSSv3 structured privilege assertion per is the natural strong-authentication mechanism for proxy-server-forwarded credentials. This revision does not require GSSv3 because the broader NFSv4 deployment base does not yet support it; deployments that can use GSSv3 SHOULD prefer it over AUTH_SYS passthrough for the credential-forwarding channel.

Namespace Traversal Privilege A proxy server that translates client I/O has to know how the metadata server's namespace is shaped: which paths are exported, what filehandle each path resolves to, how the exports mount within one another. The proxy server acquires this information by traversing the metadata server's namespace -- LOOKUP, LOOKUPP, PUTFH, PUTROOTFH, GETFH on the proxy server <-> metadata server session. This traversal cannot always run under forwarded client credentials: at the point the proxy server needs to discover a new export (a client has not yet asked for it, or the proxy server has just restarted and has no filehandle cache) there is no client whose credentials the proxy server could forward. Deployments have two choices for how the proxy server acquires namespace shape:

Grant a narrow traversal privilege:: The metadata server MAY treat a registered proxy server's service identity as authorized for LOOKUP, LOOKUPP, PUTFH, PUTROOTFH, GETFH, and SEQUENCE on the proxy server <-> metadata server session without applying the metadata server's export-rule filtering that would normally gate those names. This is strictly a structural privilege: it permits the proxy server to see that paths exist and to obtain their filehandles, but grants no data access. All operations that carry or require data authorization (OPEN, READ, WRITE, LAYOUTGET, GETATTR of privileged attributes, etc.) MUST still run under the rules of : forwarded client credentials for client-initiated operations, and proxy server service identity only for control-plane operations. A deployment that grants this privilege discloses the metadata server's namespace shape to the proxy server's service identity -- specifically, names that the proxy server's source address would not be able to see through the metadata server's normal export filtering. Deployments SHOULD audit traversal compounds on registered-proxy server sessions so the disclosure is reviewable; the metadata server SHOULD log each LOOKUP / GETFH that benefits from the bypass.
Do not grant the privilege:: The proxy server is required to translate every client-originated LOOKUP into a separate LOOKUP against the metadata server under the forwarding client's credentials, caching only what the client's credentials authorized the metadata server to return. This eliminates the namespace-shape disclosure but costs an extra metadata server round-trip per client LOOKUP-miss and leaves the proxy server unable to pre-discover exports.

This document does not normatively prefer one approach over the other. Implementations SHOULD document which they use; deployment guidance for the common combined data server+proxy server case is that granting the privilege is the expected choice, acceptable given the proxy server is already a trusted control-plane peer of the metadata server. The traversal privilege is distinct from and narrower than root_squash bypass. A forwarded-uid-0 client operation (OPEN, READ, etc.), even when the privilege is granted, is still subject to normal root_squash handling on the proxy server's source-address rule at the metadata server; the privilege applies only to the six ops enumerated above.

Proxy-server-side Policy Enforcement (informative) A proxy server implementation MAY perform per-client export-rule enforcement locally, rejecting operations the metadata server would also reject before forwarding them. This is a performance optimization: it keeps bad requests off the proxy server <-> metadata server wire and lets the proxy server return NFS4ERR_WRONGSEC / NFS4ERR_ACCESS without paying a round-trip. Local enforcement is not a security boundary. Rule 3 of names the metadata server as the authority for every client-initiated file operation. A proxy server that performs local enforcement is checking its own cached copy of the metadata server's per-client rules; if the copy is stale, wrong, or absent, the proxy server MUST forward the operation and let the metadata server decide. A proxy server implementation that declines to perform local enforcement is conformant with this specification. Deployments that want local enforcement need a mechanism for the proxy server to acquire the metadata server's per-export client-rule list. This document does not standardize such a mechanism; implementation-specific options include a control-plane probe-protocol extension, out-of-band admin distribution, or a future revision of this specification. Any such mechanism MUST limit distribution to proxy servers that the deployment has authorized (the rules are sensitive deployment policy) and MUST support a refresh path so proxy servers see rule changes within a bounded time.

Implementations This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.

reffs reffs is an open-source NFSv4.2 server, metadata server, and erasure-coding client. The reffs source ships a metadata server, a Proxy Server, and a multi-encoding client harness used as the working implementation for this draft. reffs is licensed AGPL-3.0-or-later. reffs implements, at the time of writing:

The proxy listener model (one process serving its native NFS port and a per-[[proxy_mds]] proxy server port from independent superblock namespaces, see ).
PROXY_REGISTRATION over RPCSEC_GSS-class auth, presently exercised via mutually-authenticated RPC-over-TLS () with a client-cert SHA-256 fingerprint allowlist. AUTH_SYS over plain TCP is rejected with NFS4ERR_PERM per .
PROXY_PROGRESS lease renewal and the empty-assignment idle path used by every steady-state proxy server poll.
Forwarding of LOOKUP, OPEN, READ, WRITE, GETATTR, CLOSE, LAYOUTGET, GETDEVICEINFO, LAYOUTRETURN, LAYOUTERROR through the proxy server to the upstream metadata server using the end-client's credentials.

Forward-channel ops not yet exercised end-to-end in the public implementation include PROXY_DONE / PROXY_CANCEL, which are issued only after a PROXY_PROGRESS reply that delivers a proxy_assignment4. The metadata-server-driven assignment model (move, repair) is wire-implemented but the only assignment kind exercised by the published demo is the implicit no-assignment heartbeat that every PROXY_PROGRESS produces.

Demonstration A reproducible demonstration of cross-proxy proxying, exercising the layout-passthrough data path through proxy server A and proxy server B against a shared metadata server + 6 data servers, lives in the reffs source under deploy/sanity/. The demo does not exercise migration, repair, or any proxy_assignment4; its purpose is to show that a client's encoding-encoded write through one proxy server is recoverable byte-for-byte through a peer proxy server that shares the same metadata server. The matrix:

Path	Layout	Encoding	Result
/ffv1-csm	FF v1	plain mirror	PASS
/ffv1-stripes	FF v1	stripe k=6, m=0	PASS
/ffv2-csm	FF v2	plain mirror, CHUNK	PASS
/ffv2-rs	FF v2	RS(4,2), CHUNK	PASS
/ffv2-mj	FF v2	Mojette systematic (4,2)	PASS

For each row the client opens <path>/codec_<label>.bin through the proxy server A proxy listener, performs an encoding-encoded write of a 96 KiB random payload, then opens the same filehandle through the proxy server B proxy listener and reads it back. The client's cmp(1) of the original payload and the proxy server B-served payload returns no differences in all rows of rows. The demo is published with the reffs source; the matrix above is the empirical record from the most recent published run on the editors' infrastructure.

IANA Considerations This document requires no new IANA registries. It does consume the following NFSv4.2 code points, assigned via the extension process of . They are contiguous with, and shift with, the range assigned by :

Operations 96-99: PROXY_REGISTRATION, PROXY_PROGRESS, PROXY_DONE, PROXY_CANCEL (Section 6).
proxy_stateid4, a new server-issued stateid type (Section 6.1).

This document requests IANA to register one new bit in the "EXCHGID4FLAG*" flag space for the ExchangeID operation from . The requested value is 0x00200000, outside the existing MASK_PNFS block (0x00070000) and clear of the bit requests; IANA MAY assign a different value at its discretion, in which case the numeric value in and its uses throughout this document are updated to match the assignment. EXCHGID4 Flag Assignment

Flag Name	Value	RFC	Reference
EXCHGID4_FLAG_USE_PROXY_SERVER	0x00200000	RFCTBD11	, this doc

Interaction with the Base Specification The mechanism this document specifies is built on top of four constructs that defines: the chunk_guard4 compare-and-swap (CAS) primitive, the CHUNK_LOCK mechanism, the CB_CHUNK_REPAIR per-chunk repair callback, and the TRUST_STATEID / REVOKE_STATEID control plane. None of these are modified or extended here; this section states how each is used (or explicitly excluded) when a proxy server is active on a file. Two of the four (chunk_guard4, CHUNK_LOCK) describe what the proxy server does on the data server side of the mechanism; the other two (CB_CHUNK_REPAIR, TRUST_STATEID) describe how metadata-server-side bookkeeping composes with a live proxy operation.

chunk_guard4 The proxy server enforces chunk_guard4 CAS on the destination mirror set on behalf of clients. The proxy server MAY use the same guard values client writes carry through it, or generate fresh guard values on the destination side, provided uniqueness on the destination is preserved.

CHUNK_LOCK If a client holds a chunk lock on a file when a proxy operation activates, the lock follows the file: the proxy server takes ownership of the lock on the destination mirror set, and the metadata-server-escrow semantics (the Reserved cg_client_id Value subsection of ) apply if the original holder becomes unreachable during the operation.

CB_CHUNK_REPAIR Per-chunk CB_CHUNK_REPAIR and an in-flight proxy PROXY_OP_MOVE or PROXY_OP_REPAIR migration on the same file are mutually exclusive at any given time. The metadata server MUST NOT issue CB_CHUNK_REPAIR for a file currently in PROXY_ACTIVE; the proxy server handles any mid-move repair internally. If the metadata server decides a proxied file also needs per-chunk repair after the proxy operation completes, it issues CB_CHUNK_REPAIR against the post-move layout.

TRUST_STATEID / REVOKE_STATEID When the metadata server selects a proxy server for a proxy operation, it issues TRUST_STATEID on the proxy server for every client layout stateid that will route through the proxy server during PROXY_ACTIVE. On proxy server retirement the metadata server issues REVOKE_STATEID on the retired proxy server. This is the same mechanism defines for any data server in a tightly coupled deployment.

Open Questions The design is substantially complete but still has open points that need Working Group input or internal agreement before the first submission. They fall into three rough categories: wire-level details that need to be nailed down (richer capability advertising), architectural choices that affect the mechanism's shape (multiple concurrent proxies per file, transitive proxy, capability-scoped EXCHGID flag), and policy questions whose answers bind deployment choices more than wire behavior (metadata server operation-state persistence, RPCSEC_GSSv3 requirement level, DEVICEID_REGISTRATION generalization). Each item below briefly states the question and the candidate resolutions; none of them block this document's core mechanism but each may reshape a detail of it.

Multiple concurrent proxies per file:

The design assumes one proxy per file per operation. Should two proxies be allowed to pipeline a large file (proxy A drives the first 1 TB, proxy B drives the next)? The motivating case is a multi-terabyte move where a single proxy's bandwidth is the bottleneck; parallelizing across proxies would shorten the operation proportionally. The cost is state-machine complexity (two operation ids to track, partial-completion bookkeeping, range ownership between proxies) and layout complexity (the client sees two proxy server entries in ffv2s_data_servers and needs routing rules between them). One-proxy-per-file keeps the mechanism simple; if the bandwidth case turns out to dominate in practice, a follow-on extension can add parallelism later without invalidating the single-proxy path.

Transitive proxy:

If a file in PROXY_ACTIVE needs a second move (e.g., a data server maintenance window opens while a repair is already running), what happens? Queueing the second move postpones the maintenance, which may not be acceptable if the maintenance window is hard. Aborting the first move wastes the repair work already done and puts the file back into a degraded state. Allowing a proxy to act as the source for another proxy (a "chained" proxy setup) preserves the repair progress but doubles the state-machine work and introduces failure-mode compounds that the current design does not cover. The right answer probably depends on operator priorities and may need to be a configurable metadata server policy rather than a protocol rule.

Migration-state retention across restart:

The recovery model leaves retention of both the registration table and in-flight migration state across a metadata server restart to the implementation; a metadata server that retains neither is conformant, and a reconnecting proxy server whose prior prr_registration_id is unknown takes the fresh-ID retry path in step 2. Should the document nonetheless add a SHOULD recommending retention of both, so that a reboot does not discard the progress of a large move? Production deployments would likely want it; it is a quality-of-implementation recommendation only, with no effect on interoperability.

Registration as a capability-scoped authority:

Resolved in this revision: PROXY_REGISTRATION requires the session to present EXCHGID4_FLAG_USE_PROXY_SERVER (), distinguishing a proxy-server session from both regular pNFS-client and regular non-pNFS-client sessions. The registration itself remains the per-op capability declaration (encoding set, lease); the EXCHGID4 flag scopes the session that carries it.

Richer capability advertising:

pra_encodings covers the transformation classes that matter for move / repair. Features that are implementation-internal (encryption, compression, alignment normalization) do not need to be advertised because they do not affect the wire contract. Features that DO affect the wire (e.g., support for some future sparse-read or TRIM op) would warrant a richer capability descriptor. Worth revisiting when those ops are defined.

RPCSEC_GSSv3 for translating-proxy credential forwarding:

Credential forwarding under AUTH_SYS is weak (uid spoofable, no integrity protection). RPCSEC_GSSv3 structured privilege assertion () is the natural strong-authentication mechanism, but its deployment base in the NFSv4 community is narrow. Should the draft REQUIRE GSSv3 for translating proxies, RECOMMEND it, or leave it as implementation-optional? The answer likely depends on how aggressively the working group wants to push GSSv3 adoption as a side effect of standardizing this mechanism.

DEVICEID_REGISTRATION generalization:

PROXY_REGISTRATION in this document is a proxy-specific capability-advertisement op: a data server opens a session to the metadata server and declares that it is proxy-capable, along with encoding-set membership and a lease. The same mechanism has broader applicability as a generic data server -> metadata server capability advertisement -- a DEVICEID_REGISTRATION op whose payload can carry:

Fault-zone coordinates (building, floor, room, rack, power domain, network domain, cooling domain). An admin who needs to power down a rack can drive the metadata server to recall all layouts referencing data servers in that zone and evacuate files via PROXY_OP_MOVE assignments before the outage.
Storage media type (SSD / HDD / tape / cloud tier), for layout-policy decisions.
Geographic location, for data-locality policy.
Transport security profile (TLS-capable, required mutual-TLS cert fingerprint).
Performance tier labels, for admin-assigned QoS.
Encryption-at-rest and compression-at-rest flags.
Scheduled maintenance windows, so the metadata server can preemptively drain a data server before a planned outage.

Under this framing, PROXY_REGISTRATION is one arm of a generic DEVICEID_REGISTRATION op: the proxy-capability arm. If the working group prefers the generalization, the op in this document re-homes as a specialization of DEVICEID_REGISTRATION, keeping its wire shape for the proxy arm and adding typed entries for the other capability classes. The broader op may land in , in a dedicated draft, or as an extension of this document; settlement of that scoping question is the open item. The op direction (data server -> metadata server) is the same for both specialized PROXY_REGISTRATION and generalized DEVICEID_REGISTRATION; that direction does not today exist as a session in the tight-coupling control plane of (which runs metadata server -> data server). A resolution of this item also settles whether the proxy-server draft introduces a new data-server-initiated session or whether the generalized version does.

Deferred The items below are explicit protocol extensions identified during design that this revision does not specify. They overlap with Out of Scope in ; where Out of Scope frames a deferral in the context of what the mechanism does do, this list reads as a standalone punch list of candidate follow-on work items, useful to a future revision's planner. A future editorial pass MAY merge this list into Out of Scope before submission.

Partial-range PROXY_OP_MOVE assignments.
Multi-proxy pipelines for very large files.
Automated proxy selection with load balancing.
Proxy-failure predicate (when should the metadata server pre-emptively replace a slow proxy?).
Integration with server-side copy () as an alternative for single-file moves within one namespace.
Delta-journaling during a move for online moves without dual-writes.

References Normative References Remote Procedure Call (RPC) Security Version 3 This document specifies version 3 of the Remote Procedure Call (RPC) security protocol (RPCSEC_GSS). This protocol provides support for multi-principal authentication of client hosts and user principals to a server (constructed by generic composition), security label assertions for multi-level security and type enforcement, structured privilege assertions, and channel bindings. This document updates RFC 5403. Network File System (NFS) Version 4 Minor Version 2 External Data Representation Standard (XDR) Description This document provides the External Data Representation (XDR) description for NFS version 4 minor version 2. Rules for NFSv4 Extensions and Minor Versions This document describes the rules relating to the extension of the NFSv4 family of protocols. It covers the creation of minor versions, the addition of optional features to existing minor versions, and the correction of flaws in features already published as Proposed Standards. The rules relating to the construction of minor versions and the interaction of minor version implementations that appear in this document supersede the minor versioning rules in RFC 5661 and other RFCs defining minor versions. Network File System (NFS) Version 4 Minor Version 1 Protocol This document describes the Network File System (NFS) version 4 minor version 1, including features retained from the base protocol (NFS version 4 minor version 0, which is specified in RFC 7530) and protocol extensions made subsequently. The later minor version has no dependencies on NFS version 4 minor version 0, and is considered a separate protocol. This document obsoletes RFC 5661. It substantially revises the treatment of features relating to multi-server namespace, superseding the description of those features appearing in RFC 5661. Towards Remote Procedure Call Encryption by Default This document describes a mechanism that, through the use of opportunistic Transport Layer Security (TLS), enables encryption of Remote Procedure Call (RPC) transactions while they are in transit. The proposed mechanism interoperates with Open Network Computing (ONC) RPC implementations that do not support it. This document updates RFC 5531. Parallel NFS (pNFS) Flexible File Layout Version 2 Hammerspace Parallel NFS (pNFS) allows a separation between the metadata (onto a metadata server) and data (onto a storage device) for a file. The Flexible File Version 2 Layout Type is defined in this document as an extension to pNFS that allows the use of storage devices that require only a limited degree of interaction with the metadata server and use already-existing protocols. Data protection is also added to provide integrity. Both Client-side mirroring and the erasure coding algorithms are used for data protection. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References NFS Version 3 Protocol Specification This paper describes the NFS version 3 protocol. This paper is provided so that people can write compatible implementations. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Network File System (NFS) Version 4 Minor Version 2 Protocol This document describes NFS version 4 minor version 2; it describes the protocol extensions made from NFS version 4 minor version 1. Major extensions introduced in NFS version 4 minor version 2 include the following: Server-Side Copy, Application Input/Output (I/O) Advise, Space Reservations, Sparse Files, Application Data Blocks, and Labeled NFS. Parallel NFS (pNFS) Flexible File Layout Parallel NFS (pNFS) allows a separation between the metadata (onto a metadata server) and data (onto a storage device) for a file. The flexible file layout type is defined in this document as an extension to pNFS that allows the use of storage devices that require only a limited degree of interaction with the metadata server and use already-existing protocols. Client-side mirroring is also added to provide replication of files. Improving Awareness of Running Code: The Implementation Status Section This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section. This will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. This process is not mandatory. Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications. This document obsoletes RFC 6982, advancing it to a Best Current Practice.

Acknowledgments David Flynn and Trond Myklebust shaped the proxy-server architecture, in particular the split between proxy registration and metadata-server-issued directives. Sorin Faibish reviewed the document. Chris Inacio, Brian Pawlowski, Chuck Lever, and Gorry Fairhurst guided this process.