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raidtab(5)									       raidtab(5)

       raidtab - configuration file for md (RAID) devices

       /etc/raidtab is the default configuration file for the raid tools (raidstart and company).
       It defines how RAID devices are configured on a system.

       /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 config-
       uration 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  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 kilobytes. Has to be a power of 2 and has  a  compila-
	      tion-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

       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!

       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.

       raidstart(8), raid0run(8), mkraid(8), raidstop(8)

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