Adding an Uncacheable Attribute to NFSv4.2

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Table of Contents

1. Introduction

In the Network File System version 4.2 (NFSv4.2) (see [RFC7863]) , a client queries for either a file's or directory's attributes via either GETATTR (see Section 18.7 of [RFC8881]) or READDIR (see Section 18.23 of [RFC8881]) to the server. These directory entries (dirents) can be cached locally by the client.¶

As these cached dirents are shared by all users on the clients and the client can not determine access to these dirents, all users are presented with the same set of attributes. We introduce the new uncacheable attribute, which informs the client not to cache a dirent for that file or directory object. As such each time the client queries for those attributes, the response can be tailored to that user. That decision can be based on either Access Control List (ACL) on a file or directory object (see Section 6 of [RFC8881]) or proprietary policies.¶

Besides the caching of metadata for both files and directories, the client can also cache data for files. The uncacheable attribute also informs the client to avoid its page cache. It is very similar to passing the O_DIRECT to the open call. This can be useful for files that are not shared and also for ones which do not meet the expectations of the caching patterns of the page cache.¶

Using the process detailed in [RFC8178], the revisions in this document become an extension of NFSv4.2 [RFC7862]. They are built on top of the external data representation (XDR) [RFC4506] generated from [RFC7863].¶

2. Uncacheable Dirents

If a file object or directory has the uncacheable attribute set, then the client MUST NOT cache their dirent attributes. I.e., just because it presented the attributes to user A on the first request, then on a subsequent request, it MUST query for those attributes again. Also, the client MUST NOT share attributes between different users.¶

3. Uncacheable Files

If a file object is marked as uncacheable, that also means all modifications to the file MUST immediately be sent from the client to the server. I.e., the data is also not cacheable.¶

4. XDR for Offline Attribute

<CODE BEGINS>
///
/// typedef bool            fattr4_uncacheable;
///
/// const FATTR4_UNCACHEABLE            = 87;
///


<CODE ENDS>

5. Extraction of XDR

This document contains the external data representation (XDR) [RFC4506] description of the uncacheable attribute. The XDR description is embedded in this document in a way that makes it simple for the reader to extract into a ready-to-compile form. The reader can feed this document into the following shell script to produce the machine readable XDR description of the new flags:¶

<CODE BEGINS>
#!/bin/sh
grep '^ *///' $* | sed 's?^ */// ??' | sed 's?^ *///$??'


<CODE ENDS>

That is, if the above script is stored in a file called 'extract.sh', and this document is in a file called 'spec.txt', then the reader can do:¶

<CODE BEGINS>
sh extract.sh < spec.txt > uncacheable_prot.x


<CODE ENDS>

The effect of the script is to remove leading white space from each line, plus a sentinel sequence of '///'. XDR descriptions with the sentinel sequence are embedded throughout the document.¶

Note that the XDR code contained in this document depends on types from the NFSv4.2 nfs4_prot.x file (generated from [RFC7863]). This includes both nfs types that end with a 4, such as offset4, length4, etc., as well as more generic types such as uint32_t and uint64_t.¶

While the XDR can be appended to that from [RFC7863], the various code snippets belong in their respective areas of that XDR.¶

6. Security Considerations

Clients MUST NOT make access decisions to uncacheable dirents, these decisions MUST be made by the server. With the uncacheable attribute, dirents can be annotated such that attributes can be presented to the user based on the server's decision.¶

7. IANA Considerations

There are no IANA considerations.¶

Author's Address