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raidtab.5NAME
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 impor
tant. 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. (cur
rent 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 runtime via raidhotadd/raid
hotremove, care has to be taken that the
/etc/raidtab configuration exactly follows the
actual configuration of the array. (raidho
tadd/raidhotremove does not change the configura
tion 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 pro
tect 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-sym
metric, 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 bytes. 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 configura
tion.
device devpath
Adds the device devpath to the list of devices
which comprise the raid system. Note that this com
mand 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)
Man(1) output converted with man2html |