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raidtab(5) File Formats Manual raidtab(5)
NAME
raidtab - configuration file for md (RAID) devices
DESCRIPTION
/etc/raidtab is the default configuration file for the raid tools (raidstart and company). It defines how RAID devices are configured on a
system.
FORMAT
/etc/raidtab has multiple sections, one for each md device which is being configured. Each section begins with the raiddev keyword. The
order of items in the file is important. Later raiddev entries can use earlier ones (which allows RAID-10, for example), and the parsing
code isn't overly bright, so be sure to follow the ordering in this man page for best results.
Here's a sample md configuration file:
#
# sample raiddev configuration file
# 'old' RAID0 array created with mdtools.
#
raiddev /dev/md0
raid-level 0
nr-raid-disks 2
persistent-superblock 0
chunk-size 8
device /dev/hda1
raid-disk 0
device /dev/hdb1
raid-disk 1
raiddev /dev/md1
raid-level 5
nr-raid-disks 3
nr-spare-disks 1
persistent-superblock 1
parity-algorithm left-symmetric
device /dev/sda1
raid-disk 0
device /dev/sdb1
raid-disk 1
device /dev/sdc1
raid-disk 2
device /dev/sdd1
spare-disk 0
Here is more information on the directives which are in raid configuration files; the options are listen in this file in the same order
they should appear in the actual configuration file.
raiddev device
This introduces the configuration section for the stated device.
nr-raid-disks count
Number of raid devices in the array; there should be count raid-disk entries later in the file. (current maximum limit for RAID
devices -including spares- is 12 disks. This limit is already extended to 256 disks in experimental patches.)
nr-spare-disks count
Number of spare devices in the array; there should be count spare-disk entries later in the file. Spare disks may only be used with
RAID4 and RAID5, and allow the kernel to automatically build new RAID disks as needed. It is also possible to add/remove spares run-
time via raidhotadd/raidhotremove, care has to be taken that the /etc/raidtab configuration exactly follows the actual configuration
of the array. (raidhotadd/raidhotremove does not change the configuration file)
persistent-superblock 0/1
newly created RAID arrays should use a persistent superblock. A persistent superblock is a small disk area allocated at the end of
each RAID device, this helps the kernel to safely detect RAID devices even if disks have been moved between SCSI controllers. It
can be used for RAID0/LINEAR arrays too, to protect against accidental disk mixups. (the kernel will either correctly reorder disks,
or will refuse to start up an array if something has happened to any member disk. Of course for the 'fail-safe' RAID variants
(RAID1/RAID5) spares are activated if any disk fails.)
Every member disk/partition/device has a superblock, which carries all information necessary to start up the whole array. (for
autodetection to work all the 'member' RAID partitions should be marked type 0xfd via fdisk) The superblock is not visible in the
final RAID array and cannot be destroyed accidentally through usage of the md device files, all RAID data content is available for
filesystem use.
parity-algorithm which
The parity-algorithm to use with RAID5. It must be one of left-asymmetric, right-asymmetric, left-symmetric, or right-symmetric.
left-symmetric is the one that offers maximum performance on typical disks with rotating platters.
chunk-size size
Sets the stripe size to size kilobytes. Has to be a power of 2 and has a compilation-time maximum of 4M. (MAX_CHUNK_SIZE in the ker-
nel driver) typical values are anything from 4k to 128k, the best value should be determined by experimenting on a given array, alot
depends on the SCSI and disk configuration.
device devpath
Adds the device devpath to the list of devices which comprise the raid system. Note that this command must be followed by one of
raid-disk, spare-disk, or parity-disk. Also note that it's possible to recursively define RAID arrays, ie. to set up a RAID5 array
of RAID5 arrays. (thus achieving two-disk failure protection, at the price of more disk space spent on RAID5 checksum blocks)
raid-disk index
The most recently defined device is inserted at position index in the raid array.
spare-disk index
The most recently defined device is inserted at position index in the spare disk array.
parity-disk index
The most recently defined device is moved to the end of the raid array, which forces it to be used for parity.
failed-disk index
The most recently defined device is inserted at position index in the raid array as a failed device. This allows you to create raid
1/4/5 devices in degraded mode - useful for installation. Don't use the smallest device in an array for this, put this after the
raid-disk definitions!
NOTES
The raidtools are derived from the md-tools and raidtools packages, which were originally written by Marc Zyngier, Miguel de Icaza, Gadi
Oxman, Bradley Ward Allen, and Ingo Molnar.
SEE ALSO
raidstart(8), raid0run(8), mkraid(8), raidstop(8)
raidtab(5)