MD(4)

NAME
md - Multiple Device driver aka Linux Software Raid

SYNOPSIS
/dev/mdn
/dev/md/n

DESCRIPTION
The md driver provides virtual devices that are created from one or more independant underlying devices. This array of devices often contains redundancy, and hence the acronym RAID which stands for a Redundant Array of Independant Devices.

md support RAID levels 1 (mirroring) 4 (striped array with parity device) and 5 (striped array with distributed parity information. If a single underlying device fails while using one of these level, they array will continue to function.

md also supports a number of pseudo RAID (non-redundant) configuations including RAID0 (striped array), LINEAR (catenated array) and MULTIPATH (a set of different interfaces to the same device).

  RAID SUPER BLOCK
With the exception of Legacy Arrays described below, each device that is incorportated into an MD array has a super block written towards the end of the device. This superblock records information about the structure and state of the array so that the array an be reliably re-assembled after a shutdown. The superblock is 4K long and is written into a 64K aligned block that start at least 64K and less than 128K from the end of the device (i.e. to get the address of the superblock round the size of the device down to a multiple of 64K and then subtract 64K). The available size of each device is the ammount of space before the super block, so between 64K and 128K is lost when a device in incorporated into an MD array. The superblock contains, among other things:

LEVEL
    The

UUID
    a 128 bit Universally Unique Identifier that identifies the array that this device is part of.

  LINEAR

  RAID0

  RAID1

  RAID4

  RAID5

  REBUILD/RESYNC

FILES

/proc/mdstat
    Contains information about the status of currently running array.

/proc/sys/dev/raid/speed_limit_min
    A readable and writable file that reflects the current goal rebuild speed for times when non-rebuild activity is current on an array. The speed is in Kibibytes per second, and is a per-device rate, not a per-array rate (which means that an array with more disc will shuffle more data for a given speed). The default is 100.

/proc/sys/dev/raid/speed_limit_max
    A readable and writable file that reflects the current goal rebuild speed for times when no non-rebuild activity is current on an array. The default is 100,000.

SEE ALSO
mdadm(8), mkraid(8).