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)