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 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 countraid-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 countspare-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 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 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 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.
