From Linux Man Pages
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 configura-
tion 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 27 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 runtime 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 car-
ries 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-symmet-
ric. 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
kernel 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.
RELATED
raidstart(8), raid0run(8), mkraid(8), raidstop(8)
CATEGORY