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       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 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

       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!


       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


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

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