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       mdadm - manage MD devices aka Linux Software RAID


       mdadm [mode] <raiddevice> [options] <component-devices>


       RAID  devices  are  virtual devices created from two or more real block
       devices.  This  allows  multiple  devices  (typically  disk  drives  or
       partitions  thereof)  to  be combined into a single device to hold (for
       example) a single filesystem.  Some RAID levels include redundancy  and
       so can survive some degree of device failure.

       Linux  Software  RAID  devices are implemented through the md (Multiple
       Devices) device driver.

       Currently, Linux supports LINEAR md devices,  RAID0  (striping),  RAID1
       (mirroring), RAID4, RAID5, RAID6, RAID10, MULTIPATH, and FAULTY.

       MULTIPATH  is  not a Software RAID mechanism, but does involve multiple
       devices: each device is a path to one common physical storage device.

       FAULTY is also not true RAID, and it  only  involves  one  device.   It
       provides  a layer over a true device that can be used to inject faults.


       mdadm has several major modes of operation:

              Assemble the components of a previously created  array  into  an
              active  array.  Components  can  be  explicitly  given or can be
              searched for.  mdadm checks that the components do form  a  bona
              fide  array,  and can, on request, fiddle superblock information
              so as to assemble a faulty array.

       Build  Build an array that doesn’t have  per-device  superblocks.   For
              these  sorts  of  arrays,  mdadm  cannot  differentiate  between
              initial creation and subsequent assembly of an array.   It  also
              cannot  perform any checks that appropriate components have been
              requested.  Because of this, the Build mode should only be  used
              together with a complete understanding of what you are doing.

       Create Create a new array with per-device superblocks.

       Follow or Monitor
              Monitor  one  or  more  md devices and act on any state changes.
              This is only meaningful for raid1, 4,  5,  6,  10  or  multipath
              arrays,  as  only these have interesting state.  raid0 or linear
              never have missing, spare, or failed drives, so there is nothing
              to monitor.

       Grow   Grow  (or shrink) an array, or otherwise reshape it in some way.
              Currently supported growth options including changing the active
              size  of  component  devices  and  changing the number of active
              devices in RAID levels 1/4/5/6, as well as adding or removing  a
              write-intent bitmap.

       Incremental Assembly
              Add a single device to an appropriate array.  If the addition of
              the device makes the array runnable, the array will be  started.
              This  provides  a convenient interface to a hot-plug system.  As
              each device is detected, mdadm has a chance  to  include  it  in
              some array as appropriate.

       Manage This is for doing things to specific components of an array such
              as adding new spares and removing faulty devices.

       Misc   This is an ’everything else’ mode that  supports  operations  on
              active  arrays,  operations on component devices such as erasing
              old superblocks, and information gathering operations.

              This mode does not act on a specific device or array, but rather
              it  requests  the  Linux  Kernel  to  activate any auto-detected


Options for selecting a mode are:

       -A, --assemble
              Assemble a pre-existing array.

       -B, --build
              Build a legacy array without superblocks.

       -C, --create
              Create a new array.

       -F, --follow, --monitor
              Select Monitor mode.

       -G, --grow
              Change the size or shape of an active array.

       -I, --incremental
              Add a single device into  an  appropriate  array,  and  possibly
              start the array.

              Request  that  the kernel starts any auto-detected arrays.  This
              can only work if md is compiled into the kernel — not if it is a
              module.   Arrays  can  be auto-detected by the kernel if all the
              components are in primary MS-DOS partitions with partition  type
              FD.    In-kernel   autodetect   is   not   recommended  for  new
              installations.  Using mdadm to  detect  and  assemble  arrays  —
              possibly  in  an  initrd  —  is  substantially more flexible and
              should be preferred.

       If a device is given before any options, or  if  the  first  option  is
       --add,  --fail,  or --remove, then the MANAGE mode is assume.  Anything
       other than these will cause the Misc mode to be assumed.

Options that are not mode-specific are:

       -h, --help
              Display general help message or, after one of the above options,
              a mode-specific help message.

              Display  more  detailed help about command line parsing and some
              commonly used options.

       -V, --version
              Print version information for mdadm.

       -v, --verbose
              Be more verbose about what is happening.  This can be used twice
              to be extra-verbose.  The extra verbosity currently only affects
              --detail --scan and --examine --scan.

       -q, --quiet
              Avoid printing purely informative messages.   With  this,  mdadm
              will  be  silent  unless  there is something really important to

       -b, --brief
              Be less verbose.  This is  used  with  --detail  and  --examine.
              Using  --brief  with  --verbose  gives  an intermediate level of

       -f, --force
              Be more forceful about  certain  operations.   See  the  various
              modes  for  the  exact  meaning  of  this  option  in  different

       -c, --config=
              Specify    the    config    file.     Default    is    to    use
              /etc/mdadm/mdadm.conf,    or    if   that   is   missing,   then
              /etc/mdadm.conf.  If the config file given  is  partitions  then
              nothing  will  be  read, but mdadm will act as though the config
              file  contained  exactly  DEVICE  partitions   and   will   read
              /proc/partitions to find a list of devices to scan.  If the word
              none is given for the config file, then mdadm will act as though
              the config file were empty.

       -s, --scan
              Scan  config  file  or /proc/mdstat for missing information.  In
              general, this option gives mdadm permission to get  any  missing
              information   (like  component  devices,  array  devices,  array
              identities, and alert destination) from the  configuration  file
              (see  previous  option);  one  exception is MISC mode when using
              --detail or --stop, in which case --scan says to get a  list  of
              array devices from /proc/mdstat.

       -e ,  --metadata=
              Declare the style of superblock (raid metadata) to be used.  The
              default is 0.90 for --create, and to guess for other operations.
              The  default can be overridden by setting the metadata value for
              the CREATE keyword in mdadm.conf.

              Options are:

              0, 0.90, default
                     Use the original 0.90  format  superblock.   This  format
                     limits   arrays   to  28  component  devices  and  limits
                     component devices of levels 1 and greater to 2 terabytes.

              1, 1.0, 1.1, 1.2
                     Use  the  new  version-1 format superblock.  This has few
                     restrictions.   The  different  sub-versions  store   the
                     superblock  at  different locations on the device, either
                     at the end (for 1.0), at the start (for 1.1) or  4K  from
                     the start (for 1.2).

              This  will  override any HOMEHOST setting in the config file and
              provides the identity of the host which should be considered the
              home for any arrays.

              When  creating  an  array,  the homehost will be recorded in the
              superblock.  For version-1 superblocks, it will be  prefixed  to
              the  array name.  For version-0.90 superblocks, part of the SHA1
              hash of the hostname will be stored in the  later  half  of  the

              When  reporting  information  about an array, any array which is
              tagged for the given homehost will be reported as such.

              When using Auto-Assemble,  only  arrays  tagged  for  the  given
              homehost will be assembled.

For create, build, or grow:

       -n, --raid-devices=
              Specify  the  number of active devices in the array.  This, plus
              the number of spare devices (see below) must equal the number of
              component-devices  (including "missing" devices) that are listed
              on the command line for --create.   Setting  a  value  of  1  is
              probably  a  mistake  and  so requires that --force be specified
              first.  A value of 1 will then be allowed for linear, multipath,
              raid0 and raid1.  It is never allowed for raid4 or raid5.
              This  number  can  only be changed using --grow for RAID1, RAID5
              and RAID6 arrays, and only on kernels  which  provide  necessary

       -x, --spare-devices=
              Specify  the  number  of  spare  (eXtra)  devices in the initial
              array.  Spares can also be added and removed later.  The  number
              of  component  devices listed on the command line must equal the
              number of raid devices plus the number of spare devices.

       -z, --size=
              Amount (in Kibibytes) of space to use from each  drive  in  RAID
              level  1/4/5/6.   This must be a multiple of the chunk size, and
              must leave about 128Kb of space at the end of the drive for  the
              RAID  superblock.   If  this is not specified (as it normally is
              not) the smallest drive (or partition) sets the size, though  if
              there  is  a  variance  among  the  drives of greater than 1%, a
              warning is issued.

              This value can be set with --grow for RAID level 1/4/5/6. If the
              array  was created with a size smaller than the currently active
              drives, the extra space can be accessed using --grow.  The  size
              can  be given as max which means to choose the largest size that
              fits on all current drives.

       -c, --chunk=
              Specify chunk size of kibibytes.  The default is 64.

              Specify rounding factor for linear array (==chunk size)

       -l, --level=
              Set raid level.  When used with --create, options  are:  linear,
              raid0,  0,  stripe, raid1, 1, mirror, raid4, 4, raid5, 5, raid6,
              6, raid10, 10, multipath, mp, faulty.  Obviously some  of  these
              are synonymous.

              When  used  with  --build, only linear, stripe, raid0, 0, raid1,
              multipath, mp, and faulty are valid.

              Not yet supported with --grow.

       -p, --layout=
              This option configures the  fine  details  of  data  layout  for
              raid5,  and  raid10  arrays,  and controls the failure modes for

              The  layout  of  the  raid5  parity  block   can   be   one   of
              left-asymmetric,        left-symmetric,        right-asymmetric,
              right-symmetric, la, ra, ls, rs.  The default is left-symmetric.

              When setting the failure mode for level faulty, the options are:
              write-transient, wt, read-transient, rt,  write-persistent,  wp,
              read-persistent,  rp, write-all, read-fixable, rf, clear, flush,

              Each failure mode can be followed by a number, which is used  as
              a  period between fault generation.  Without a number, the fault
              is generated once on the first relevant request.  With a number,
              the  fault  will be generated after that many requests, and will
              continue to be generated every time the period elapses.

              Multiple failure modes can be current  simultaneously  by  using
              the --grow option to set subsequent failure modes.

              "clear"  or  "none"  will remove any pending or periodic failure
              modes, and "flush" will clear any persistent faults.

              To set the parity with --grow, the level of the array ("faulty")
              must be specified before the fault mode is specified.

              Finally,  the  layout  options for RAID10 are one of ’n’, ’o’ or
              ’f’ followed by a small  number.   The  default  is  ’n2’.   The
              supported options are:

              n signals ’near’ copies. Multiple copies of one data block are
              at similar offsets in different devices.

              o signals ’offset’  copies.   Rather  than  the  chunks  being
              duplicated within a stripe, whole stripes are duplicated but are
              rotated by one device  so  duplicate  blocks  are  on  different
              devices.   Thus  subsequent  copies  of  a block are in the next
              drive, and are one chunk further down.

              f signals ’far’ copies (multiple copies  have  very  different
              offsets).  See md(4) for more detail about ’near’ and ’far’.

              The  number  is  the  number  of copies of each datablock.  2 is
              normal, 3 can be useful.  This number can be at  most  equal  to
              the  number of devices in the array.  It does not need to divide
              evenly into that number (e.g. it is perfectly legal to  have  an
              ’n2’ layout for an array with an odd number of devices).

              same as --layout (thus explaining the p of -p).

       -b, --bitmap=
              Specify  a  file  to  store  a write-intent bitmap in.  The file
              should not exist unless --force is also given.   The  same  file
              should  be  provided  when  assembling  the  array.  If the word
              internal is given, then the bitmap is stored with  the  metadata
              on  the array, and so is replicated on all devices.  If the word
              none is given with --grow mode, then any bitmap that is  present
              is removed.

              To  help catch typing errors, the filename must contain at least
              one slash (’/’) if it is a real file (not ’internal’ or ’none’).

              Note:  external bitmaps are only known to work on ext2 and ext3.
              Storing bitmap files on other filesystems may result in  serious

              Set  the  chunksize  of the bitmap. Each bit corresponds to that
              many Kilobytes of storage.  When using a file based bitmap,  the
              default  is  to  use  the  smallest  size that is at-least 4 and
              requires no more than  2^21  chunks.   When  using  an  internal
              bitmap,  the  chunksize is automatically determined to make best
              use of available space.

       -W, --write-mostly
              subsequent devices  lists  in  a  --build,  --create,  or  --add
              command  will  be  flagged as ’write-mostly’.  This is valid for
              RAID1 only and means that the ’md’  driver  will  avoid  reading
              from  these  devices  if at all possible.  This can be useful if
              mirroring over a slow link.

              Specify that write-behind mode  should  be  enabled  (valid  for
              RAID1  only).  If  an  argument  is  specified,  it will set the
              maximum number of outstanding writes allowed. The default  value
              is  256.   A  write-intent  bitmap  is  required in order to use
              write-behind mode, and write-behind is only attempted on  drives
              marked as write-mostly.

              Tell  mdadm that the array pre-existed and is known to be clean.
              It can be useful when trying to recover from a major failure  as
              you  can  be  sure  that  no  data  will  be affected unless you
              actually write to the array.  It can also be used when  creating
              a  RAID1  or  RAID10  if  you  want to avoid the initial resync,
              however  this  practice  —  while  normally  safe   —   is   not
              recommended.    Use  this  only  if you really know what you are

              This is needed when --grow is used to  increase  the  number  of
              raid-devices   in  a  RAID5  if  there   are  no  spare  devices
              available.  See the section below on RAID_DEVICE  CHANGES.   The
              file  should  be  stored  on  a separate device, not on the raid
              array being reshaped.

       -N, --name=
              Set a name for the array.  This is currently only effective when
              creating  an  array  with a version-1 superblock.  The name is a
              simple textual  string  that  can  be  used  to  identify  array
              components when assembling.

       -R, --run
              Insist  that mdadm run the array, even if some of the components
              appear to be active in another array  or  filesystem.   Normally
              mdadm will ask for confirmation before including such components
              in an array.  This option causes that question to be suppressed.

       -f, --force
              Insist  that  mdadm  accept  the  geometry  and layout specified
              without question.  Normally mdadm will not allow creation of  an
              array with only one device, and will try to create a raid5 array
              with one missing drive (as this makes the  initial  resync  work
              faster).  With --force, mdadm will not try to be so clever.

       -a, --auto{=no,yes,md,mdp,part,p}{NN}
              Instruct  mdadm  to  create  the device file if needed, possibly
              allocating  an  unused  minor  number.   "md"  causes   a   non-
              partitionable  array  to be used.  "mdp", "part" or "p" causes a
              partitionable array (2.6 and later) to be used.  "yes"  requires
              the  named  md  device to have a ’standard’ format, and the type
              and minor number will be determined from this.  See DEVICE NAMES

              The  argument can also come immediately after "-a".  e.g. "-ap".

              If --auto is not given on the command  line  or  in  the  config
              file, then the default will be --auto=yes.

              If  --scan  is  also given, then any auto= entries in the config
              file will override the --auto instruction given on  the  command

              For  partitionable arrays, mdadm will create the device file for
              the whole array and for the first  4  partitions.   A  different
              number  of partitions can be specified at the end of this option
              (e.g.  --auto=p7).  If the device name ends with  a  digit,  the
              partition  names  add  a ’p’, and a number, e.g. "/dev/home1p3".
              If there is no trailing digit, then  the  partition  names  just
              have a number added, e.g. "/dev/scratch3".

              If  the md device name is in a ’standard’ format as described in
              DEVICE NAMES, then it will be created, if  necessary,  with  the
              appropriate  number  based  on that name.  If the device name is
              not in one of these formats, then a unused minor number will  be
              allocated.   The minor number will be considered unused if there
              is no active array for that number, and there  is  no  entry  in
              /dev for that number and with a non-standard name.

              Normally  when --auto causes mdadm to create devices in /dev/md/
              it will also create symlinks from /dev/ with names starting with
              md  or md_.  Use --symlink=no to suppress this, or --symlink=yes
              to enforce this even if it is suppressing mdadm.conf.

For assemble:

       -u, --uuid=
              uuid of array to assemble. Devices which don’t  have  this  uuid
              are excluded

       -m, --super-minor=
              Minor  number  of  device  that  array was created for.  Devices
              which don’t have this minor number are excluded.  If you  create
              an  array  as  /dev/md1,  then  all superblocks will contain the
              minor number  1,  even  if  the  array  is  later  assembled  as

              Giving the literal word "dev" for --super-minor will cause mdadm
              to use  the  minor  number  of  the  md  device  that  is  being
              assembled.   e.g.  when  assembling  /dev/md0, --super-minor=dev
              will look for super blocks with a minor number of 0.

       -N, --name=
              Specify the name of the array to assemble.   This  must  be  the
              name that was specified when creating the array.  It must either
              match the name stored in the  superblock  exactly,  or  it  must
              match  with  the  current  homehost prefixed to the start of the
              given name.

       -f, --force
              Assemble the array even if some superblocks appear out-of-date

       -R, --run
              Attempt to start the array even if fewer drives were given  than
              were  present  last  time the array was active.  Normally if not
              all the expected drives are found and --scan is not  used,  then
              the  array  will  be  assembled  but not started.  With --run an
              attempt will be made to start it anyway.

              This is the reverse of --run in that it inhibits the startup  of
              array  unless  all  expected  drives  are present.  This is only
              needed with --scan, and can be used if the physical  connections
              to devices are not as reliable as you would like.

       -a, --auto{=no,yes,md,mdp,part}
              See this option under Create and Build options.

       -b, --bitmap=
              Specify  the  bitmap  file  that  was  given  when the array was
              created.  If an array has an internal bitmap, there is  no  need
              to specify this when assembling the array.

              If  --backup-file was used to grow the number of raid-devices in
              a RAID5, and the system crashed  during  the  critical  section,
              then  the  same --backup-file must be presented to --assemble to
              allow possibly corrupted data to be restored.

       -U, --update=
              Update the superblock on each device while assembling the array.
              The  argument  given  to  this  flag  can  be  one  of sparc2.2,
              summaries, uuid, name, homehost, resync, byteorder,  devicesize,
              or super-minor.

              The  sparc2.2 option will adjust the superblock of an array what
              was created on a Sparc  machine  running  a  patched  2.2  Linux
              kernel.  This kernel got the alignment of part of the superblock
              wrong.  You can use the --examine --sparc2.2 option to mdadm  to
              see what effect this would have.

              The  super-minor option will update the preferred minor field on
              each superblock to match the minor number  of  the  array  being
              assembled.   This can be useful if --examine reports a different
              "Preferred Minor" to --detail.  In some cases this  update  will
              be  performed  automatically by the kernel driver. In particular
              the update happens automatically at the first write to an  array
              with  redundancy  (RAID  level 1 or greater) on a 2.6 (or later)

              The uuid option will change the uuid of the array.  If a UUID is
              given  with  the  --uuid  option that UUID will be used as a new
              UUID and will NOT be used to help identify the  devices  in  the
              array.  If no --uuid is given, a random UUID is chosen.

              The  name  option will change the name of the array as stored in
              the  superblock.   This  is   only   supported   for   version-1

              The  homehost option will change the homehost as recorded in the
              superblock.  For version-0 superblocks,  this  is  the  same  as
              updating  the  UUID.   For  version-1 superblocks, this involves
              updating the name.

              The resync option will  cause  the  array  to  be  marked  dirty
              meaning that any redundancy in the array (e.g. parity for raid5,
              copies for raid1) may be incorrect.  This will  cause  the  raid
              system  to  perform  a  "resync"  pass  to  make  sure  that all
              redundant information is correct.

              The byteorder option allows arrays to be moved between  machines
              with  different  byte-order.   When assembling such an array for
              the first time after  a  move,  giving  --update=byteorder  will
              cause  mdadm  to  expect  superblocks  to  have  their byteorder
              reversed, and will correct  that  order  before  assembling  the
              array.    This  is  only  valid  with  original  (Version  0.90)

              The  summaries  option  will  correct  the  summaries   in   the
              superblock.  That  is  the  counts  of  total,  working, active,
              failed, and spare devices.

              The devicesize will rarely be of use.  It applies to version 1.1
              and 1.2 metadata only (where the metadata is at the start of the
              device) and is only useful when the component device has changed
              size  (typically become larger).  The version 1 metadata records
              the amount of the device that can be used to store data, so if a
              device  in  a  version  1.1  or  1.2  array  becomes larger, the
              metadata will still be visible, but the extra  space  will  not.
              In  this  case  it  might  be  useful to assemble the array with
              --update=devicesize.  This will cause  mdadm  to  determine  the
              maximum  usable  amount  of  space on each device and update the
              relevant field in the metadata.

              This flag is only meaningful with auto-assembly (see  discussion
              below).   In that situation, if no suitable arrays are found for
              this homehost, mdadm will rescan for any arrays at all and  will
              assemble them and update the homehost to match the current host.

For Manage mode:

       -a, --add
              hot-add listed devices.

              re-add a device that was recently removed from an array.

       -r, --remove
              remove listed devices.  They must  not  be  active.   i.e.  they
              should  be  failed  or  spare devices.  As well as the name of a
              device file (e.g.  /dev/sda1) the words failed and detached  can
              be  given to --remove.  The first causes all failed device to be
              removed.  The second  causes  any  device  which  is  no  longer
              connected  to  the  system  (i.e  an ’open’ returns ENXIO) to be
              removed.  This will only succeed for devices that are spares  or
              have already been marked as failed.

       -f, --fail
              mark  listed devices as faulty.  As well as the name of a device
              file, the word detached can  be  given.   This  will  cause  any
              device  that  has  been detached from the system to be marked as
              failed.  It can then be removed.

              same as --fail.

       Each of these options require that the first device listed is the array
       to  be acted upon, and the remainder are component devices to be added,
       removed, or marked as faulty.   Several  different  operations  can  be
       specified for different devices, e.g.
            mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1
       Each operation applies to all devices listed until the next  operation.

       If  an  array  is  using a write-intent bitmap, then devices which have
       been removed can be re-added in a way that avoids a full reconstruction
       but  instead just updates the blocks that have changed since the device
       was removed.  For arrays with persistent metadata (superblocks) this is
       done  automatically.  For arrays created with --build mdadm needs to be
       told that this device we removed recently with --re-add.

       Devices can only be removed from an array if they  are  not  in  active
       use,  i.e.  that must be spares or failed devices.  To remove an active
       device, it must first be marked as faulty.

For Misc mode:

       -Q, --query
              Examine a device to see (1) if it is an md device and (2) if  it
              is  a  component  of  an  md  array.   Information about what is
              discovered is presented.

       -D, --detail
              Print detail of one or more md devices.

       -Y, --export
              When used with --detail or --examine, output will  be  formatted
              as key=value pairs for easy import into the environment.

       -E, --examine
              Print content of md superblock on device(s).

              If  an array was created on a 2.2 Linux kernel patched with RAID
              support, the superblock will have been created  incorrectly,  or
              at  least  incompatibly  with  2.4 and later kernels.  Using the
              --sparc2.2 flag with --examine will fix  the  superblock  before
              displaying  it.  If this appears to do the right thing, then the
              array   can   be   successfully   assembled   using   --assemble

       -X, --examine-bitmap
              Report  information about a bitmap file.  The argument is either
              an external bitmap file or an array  component  in  case  of  an
              internal bitmap.

       -R, --run
              start a partially built array.

       -S, --stop
              deactivate array, releasing all resources.

       -o, --readonly
              mark array as readonly.

       -w, --readwrite
              mark array as readwrite.

              If  the  device  contains  a  valid  md superblock, the block is
              overwritten with  zeros.   With  --force  the  block  where  the
              superblock  would be is overwritten even if it doesn’t appear to
              be valid.

       -t, --test
              When used with --detail, the exit status  of  mdadm  is  set  to
              reflect the status of the device.

       -W, --wait
              For  each  md  device  given,  wait for any resync, recovery, or
              reshape activity to finish before returning.  mdadm will  return
              with  success  if  it  actually  waited for every device listed,
              otherwise it will return failure.

For Incremental Assembly mode:

       --rebuild-map, -r
              Rebuild the map file (/var/run/mdadm/map)  that  mdadm  uses  to
              help track which arrays are currently being assembled.

       --run, -R
              Run  any  array assembled as soon as a minimal number of devices
              are available, rather than waiting until  all  expected  devices
              are present.

       --scan, -s
              Only  meaningful  with -R this will scan the map file for arrays
              that are being incrementally assembled and will try to start any
              that  are  not  already started.  If any such array is listed in
              mdadm.conf as requiring an external bitmap, that bitmap will  be
              attached first.

For Monitor mode:

       -m, --mail
              Give a mail address to send alerts to.

       -p, --program, --alert
              Give a program to be run whenever an event is detected.

       -y, --syslog
              Cause  all events to be reported through ’syslog’.  The messages
              have facility of ’daemon’ and varying priorities.

       -d, --delay
              Give a delay in seconds.  mdadm polls the  md  arrays  and  then
              waits this many seconds before polling again.  The default is 60

       -f, --daemonise
              Tell mdadm to run as  a  background  daemon  if  it  decides  to
              monitor  anything.  This causes it to fork and run in the child,
              and to disconnect form the terminal.   The  process  id  of  the
              child  is  written  to stdout.  This is useful with --scan which
              will only continue monitoring if a mail address or alert program
              is found in the config file.

       -i, --pid-file
              When  mdadm  is  running  in  daemon  mode, write the pid of the
              daemon process to the specified file, instead of printing it  on
              standard output.

       -1, --oneshot
              Check  arrays only once.  This will generate NewArray events and
              more  significantly  DegradedArray  and  SparesMissing   events.
                      mdadm --monitor --scan -1
              from  a  cron  script  will  ensure  regular notification of any
              degraded arrays.

       -t, --test
              Generate a TestMessage alert for every array found  at  startup.
              This  alert  gets  mailed and passed to the alert program.  This
              can be used for  testing  that  alert  message  do  get  through


       Usage: mdadm --assemble md-device options-and-component-devices...

       Usage: mdadm --assemble --scan md-devices-and-options...

       Usage: mdadm --assemble --scan options...

       This  usage  assembles  one  or  more  raid  arrays  from  pre-existing
       components.  For each array, mdadm needs to know  the  md  device,  the
       identity  of the array, and a number of component-devices. These can be
       found in a number of ways.

       In the first usage example (without the --scan) the first device  given
       is  the md device.  In the second usage example, all devices listed are
       treated as md devices and assembly is attempted.  In the  third  (where
       no  devices  are  listed)  all  md  devices  that  are  listed  in  the
       configuration file are assembled.

       If precisely one device is listed, but --scan is not given, then  mdadm
       acts  as  though --scan was given and identity information is extracted
       from the configuration file.

       The  identity  can  be  given  with  the  --uuid   option,   with   the
       --super-minor  option,  will  be taken from the md-device record in the
       config file, or will be  taken  from  the  super  block  of  the  first
       component-device listed on the command line.

       Devices  can  be  given on the --assemble command line or in the config
       file. Only devices which have an md superblock which contains the right
       identity will be considered for any array.

       The  config  file  is  only  used  if explicitly named with --config or
       requested with (a  possibly  implicit)  --scan.   In  the  later  case,
       /etc/mdadm/mdadm.conf is used.

       If  --scan is not given, then the config file will only be used to find
       the identity of md arrays.

       Normally the array will be started after it is assembled.   However  if
       --scan  is  not  given  and  insufficient drives were listed to start a
       complete (non-degraded) array, then the array is not started (to  guard
       against  usage  errors).   To  insist that the array be started in this
       case (as may work for RAID1, 4, 5, 6, or 10), give the --run flag.

       If the md device does not exist, then it will be created providing  the
       intent  is  clear.  i.e.  the  name  must be in a standard form, or the
       --auto option must be given to  clarify  how  and  whether  the  device
       should be created.  This can be useful for handling partitioned devices
       (which don’t have a stable device  number  —  it  can  change  after  a
       reboot)  and  when  using  "udev" to manage your /dev tree (udev cannot
       handle  md  devices  because  of  the  unusual  device   initialisation

       If  the  option  to  "auto"  is "mdp" or "part" or (on the command line
       only) "p", then mdadm will create  a  partitionable  array,  using  the
       first free one that is not in use and does not already have an entry in
       /dev (apart from numeric /dev/md* entries).

       If the option to "auto" is "yes" or  "md"  or  (on  the  command  line)
       nothing,  then  mdadm  will  create a traditional, non-partitionable md

       It is expected that the "auto" functionality will  be  used  to  create
       device   entries  with  meaningful  names  such  as  "/dev/md/home"  or
       "/dev/md/root", rather than names based on the numerical array  number.

       When  using  option  "auto" to create a partitionable array, the device
       files for the first 4 partitions  are  also  created.  If  a  different
       number  is required it can be simply appended to the auto option.  e.g.
       "auto=part8".  Partition names are created by appending a digit  string
       to  the  device  name,  with an intervening "p" if the device name ends
       with a digit.

       The --auto option is also available in  Build  and  Create  modes.   As
       those  modes  do  not use a config file, the "auto=" config option does
       not apply to these modes.

   Auto Assembly
       When --assemble is used with --scan and no devices  are  listed,  mdadm
       will  first  attempt  to  assemble  all the arrays listed in the config

       If a homehost has been specified (either in the config file or  on  the
       command line), mdadm will look further for possible arrays and will try
       to assemble anything that it finds which is tagged as belonging to  the
       given  homehost.   This is the only situation where mdadm will assemble
       arrays without being given specific device name or identity information
       for the array.

       If  mdadm  finds a consistent set of devices that look like they should
       comprise an array, and if the superblock is tagged as belonging to  the
       given  home host, it will automatically choose a device name and try to
       assemble the array.  If the array uses version-0.90 metadata, then  the
       minor  number as recorded in the superblock is used to create a name in
       /dev/md/ so  for  example  /dev/md/3.   If  the  array  uses  version-1
       metadata, then the name from the superblock is used to similarly create
       a name in /dev/md (the  name  will  have  any  ’host’  prefix  stripped

       If  mdadm  cannot  find  any  array  for  the given host at all, and if
       --auto-update-homehost is given, then mdadm will search again  for  any
       array  (not just an array created for this host) and will assemble each
       assuming --update=homehost.  This will  change  the  host  tag  in  the
       superblock  so that on the next run, these arrays will be found without
       the  second  pass.   The  intention  of  this  feature  is  to  support
       transitioning a set of md arrays to using homehost tagging.

       The reason for requiring arrays to be tagged with the homehost for auto
       assembly is to guard  against  problems  that  can  arise  when  moving
       devices from one host to another.


       Usage:  mdadm  --build  md-device  --chunk=X --level=Y --raid-devices=Z

       This usage is similar to --create.  The difference is that  it  creates
       an array without a superblock. With these arrays there is no difference
       between initially creating the array and  subsequently  assembling  the
       array,  except  that hopefully there is useful data there in the second

       The level may raid0, linear, multipath, or  faulty,  or  one  of  their
       synonyms. All devices must be listed and the array will be started once


       Usage: mdadm --create md-device --chunk=X --level=Y
                   --raid-devices=Z devices

       This usage will initialise a new md array, associate some devices  with
       it, and activate the array.

       If  the  --auto  option  is  given  (as described in more detail in the
       section on Assemble mode), then the md device will be  created  with  a
       suitable device number if necessary.

       As  devices  are  added,  they  are checked to see if they contain raid
       superblocks or filesystems.  They  are  also  checked  to  see  if  the
       variance in device size exceeds 1%.

       If  any  discrepancy is found, the array will not automatically be run,
       though the presence of a --run can override this caution.

       To create a "degraded" array in which some devices are missing,  simply
       give  the  word  "missing"  in place of a device name.  This will cause
       mdadm to leave the corresponding slot in the array empty.  For a  RAID4
       or  RAID5 array at most one slot can be "missing"; for a RAID6 array at
       most two slots.  For a RAID1 array, only one real device  needs  to  be
       given.  All of the others can be "missing".

       When creating a RAID5 array, mdadm will automatically create a degraded
       array with an extra spare drive.  This is because  building  the  spare
       into a degraded array is in general faster than resyncing the parity on
       a non-degraded, but not clean, array.  This feature can  be  overridden
       with the --force option.

       When  creating  an array with version-1 metadata a name for the host is
       required.  If this is not given with  the  --name  option,  mdadm  will
       chose  a  name  based  on  the last component of the name of the device
       being created.  So if /dev/md3 is being created, then the name  3  will
       be  chosen.   If /dev/md/home is being created, then the name home will
       be used.

       A new array will normally get a randomly assigned 128bit UUID which  is
       very  likely to be unique.  If you have a specific need, you can choose
       a UUID for the array by giving the  --uuid=  option.   Be  warned  that
       creating two arrays with the same UUID is a recipe for disaster.  Also,
       using --uuid= when creating a v0.90 array will  silently  override  any
       --homehost= setting.

       The General Management options that are valid with --create are:

       --run  insist  on running the array even if some devices look like they
              might be in use.

              start the array readonly — not supported yet.


       Usage: mdadm device options... devices...

       This usage will allow individual devices in  an  array  to  be  failed,
       removed  or  added.  It is possible to perform multiple operations with
       on command. For example:
         mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
       will firstly mark /dev/hda1 as faulty in /dev/md0 and will then  remove
       it  from the array and finally add it back in as a spare.  However only
       one md array can be affected by a single command.


       Usage: mdadm options ...  devices ...

       MISC mode includes a number of  distinct  operations  that  operate  on
       distinct devices.  The operations are:

              The  device  is examined to see if it is (1) an active md array,
              or (2) a component of an md array.  The  information  discovered
              is reported.

              The  device should be an active md device.  mdadm will display a
              detailed description of the array.  --brief or --scan will cause
              the output to be less detailed and the format to be suitable for
              inclusion in /etc/mdadm/mdadm.conf.  The exit  status  of  mdadm
              will normally be 0 unless mdadm failed to get useful information
              about the device(s); however, if the  --test  option  is  given,
              then the exit status will be:

              0      The array is functioning normally.

              1      The array has at least one failed device.

              2      The  array  has  multiple  failed devices such that it is

              4      There was an error while trying to get information  about
                     the device.

              The  device  should  be  a component of an md array.  mdadm will
              read the md superblock of the device and display  the  contents.
              If  --brief  or  --scan is given, then multiple devices that are
              components of the one array are grouped together and reported in
              a  single entry suitable for inclusion in /etc/mdadm/mdadm.conf.

              Having --scan without listing any devices will cause all devices
              listed in the config file to be examined.

       --stop The   devices   should   be  active  md  arrays  which  will  be
              deactivated, as long as they are not currently in use.

       --run  This will fully activate a partially assembled md array.

              This will mark an active array as read-only, providing  that  it
              is not currently being used.

              This will change a readonly array back to being read/write.

       --scan For  all  operations  except  --examine,  --scan  will cause the
              operation to be applied to all arrays  listed  in  /proc/mdstat.
              For  --examine,  --scan  causes all devices listed in the config
              file to be examined.


       Usage: mdadm --monitor options... devices...

       This usage causes mdadm to periodically poll a number of md arrays  and
       to report on any events noticed.  mdadm will never exit once it decides
       that there are arrays to be checked, so it should normally  be  run  in
       the background.

       As  well  as  reporting  events,  mdadm may move a spare drive from one
       array to another if they  are  in  the  same  spare-group  and  if  the
       destination array has a failed drive but no spares.

       If  any devices are listed on the command line, mdadm will only monitor
       those devices. Otherwise all arrays listed in  the  configuration  file
       will  be  monitored.   Further,  if  --scan is given, then any other md
       devices that appear in /proc/mdstat will also be monitored.

       The result of monitoring the arrays is the generation of events.  These
       events  are  passed  to  a  separate  program (if specified) and may be
       mailed to a given E-mail address.

       When passing events to a program, the program  is  run  once  for  each
       event,  and  is  given  2 or 3 command-line arguments: the first is the
       name of the event (see below), the second is the name of the md  device
       which  is  affected,  and  the third is the name of a related device if
       relevant (such as a component device that has failed).

       If --scan is given, then  a  program  or  an  E-mail  address  must  be
       specified  on  the  command line or in the config file.  If neither are
       available, then mdadm will not monitor anything.  Without --scan, mdadm
       will continue monitoring as long as something was found to monitor.  If
       no program or email is given, then each event is reported to stdout.

       The different events are:

                  An md array which previously was configured  appears  to  no
                  longer be configured. (syslog priority: Critical)

                  If  mdadm  was  told  to  monitor an array which is RAID0 or
                  Linear, then it will report DeviceDisappeared with the extra
                  information  Wrong-Level.   This is because RAID0 and Linear
                  do not  support  the  device-failed,  hot-spare  and  resync
                  operations which are monitored.

                  An   md  array  started  reconstruction.  (syslog  priority:

                  Where NN is 20, 40, 60, or 80, this indicates  that  rebuild
                  has  passed  that  many  percentage  of  the  total. (syslog
                  priority: Warning)

                  An md array that was  rebuilding,  isn’t  any  more,  either
                  because   it  finished  normally  or  was  aborted.  (syslog
                  priority: Warning)

           Fail   An active component device of an array has  been  marked  as
                  faulty. (syslog priority: Critical)

                  A  spare component device which was being rebuilt to replace
                  a faulty device has failed. (syslog priority: Critical)

                  A spare component device which was being rebuilt to  replace
                  a  faulty  device has been successfully rebuilt and has been
                  made active.  (syslog priority: Info)

                  A new md array has been detected in the  /proc/mdstat  file.
                  (syslog priority: Info)

                  A  newly noticed array appears to be degraded.  This message
                  is not generated when mdadm notices a  drive  failure  which
                  causes  degradation,  but  only  when  mdadm notices that an
                  array is degraded when it first  sees  the  array.   (syslog
                  priority: Critical)

                  A spare drive has been moved from one array in a spare-group
                  to another to allow a failed drive to be replaced.   (syslog
                  priority: Info)

                  If  mdadm  has been told, via the config file, that an array
                  should have a certain number of  spare  devices,  and  mdadm
                  detects  that  it  has  fewer than this number when it first
                  sees the array, it  will  report  a  SparesMissing  message.
                  (syslog priority: Warning)

                  An  array  was  found  at  startup,  and the --test flag was
                  given.  (syslog priority: Info)

       Only Fail,  FailSpare,  DegradedArray,  SparesMissing  and  TestMessage
       cause  Email  to be sent.  All events cause the program to be run.  The
       program is run with two or three arguments: the event name,  the  array
       device and possibly a second device.

       Each event has an associated array device (e.g.  /dev/md1) and possibly
       a second device.  For  Fail,  FailSpare,  and  SpareActive  the  second
       device  is  the  relevant  component  device.  For MoveSpare the second
       device is the array that the spare was moved from.

       For mdadm to move spares from  one  array  to  another,  the  different
       arrays   need   to   be  labeled  with  the  same  spare-group  in  the
       configuration file.  The spare-group name can be any string; it is only
       necessary that different spare groups use different names.

       When  mdadm  detects  that  an  array in a spare group has fewer active
       devices than necessary  for  the  complete  array,  and  has  no  spare
       devices,  it  will  look for another array in the same spare group that
       has a full complement of working drive  and  a  spare.   It  will  then
       attempt  to  remove  the  spare from the second drive and add it to the
       first.  If the removal succeeds but the adding fails, then it is  added
       back to the original array.


       The  GROW  mode  is  used  for  changing the size or shape of an active
       array.  For this to work, the kernel must support the necessary change.
       Various  types  of  growth  are  being  added  during  2.6 development,
       including restructuring a raid5 array to have more active devices.

       Currently the only support available is to

       ·   change the "size" attribute for RAID1, RAID5 and RAID6.

       ·   increase the "raid-disks" attribute of RAID1, RAID5, and RAID6.

       ·   add a  write-intent  bitmap  to  any  array  which  supports  these
           bitmaps, or remove a write-intent bitmap from such an array.

       Normally  when  an array is built the "size" it taken from the smallest
       of the drives.  If all the small drives in an  arrays  are,  one  at  a
       time,  removed  and replaced with larger drives, then you could have an
       array of  large  drives  with  only  a  small  amount  used.   In  this
       situation,  changing  the  "size" with "GROW" mode will allow the extra
       space to start being used.  If the size is increased  in  this  way,  a
       "resync" process will start to make sure the new parts of the array are

       Note that when an array changes size, any filesystem that may be stored
       in  the  array  will  not  automatically  grow  to  use the space.  The
       filesystem will need to be explicitly told to use the extra space.

       A RAID1 array can work with  any  number  of  devices  from  1  upwards
       (though  1  is  not very useful).  There may be times which you want to
       increase or decrease the number of active devices.  Note that  this  is
       different to hot-add or hot-remove which changes the number of inactive

       When reducing the number of devices in a RAID1 array, the  slots  which
       are  to be removed from the array must already be vacant.  That is, the
       devices which were in those slots must be failed and removed.

       When the number of devices  is  increased,  any  hot  spares  that  are
       present will be activated immediately.

       Increasing the number of active devices in a RAID5 is much more effort.
       Every block in the array will need to be read and written back to a new
       location.   From  2.6.17,  the  Linux Kernel is able to do this safely,
       including restart and interrupted "reshape".

       When relocating the first few stripes on a raid5, it is not possible to
       keep  the  data  on  disk  completely  consistent  and crash-proof.  To
       provide the required safety, mdadm disables writes to the  array  while
       this  "critical  section"  is  reshaped, and takes a backup of the data
       that is in that section.  This backup is normally stored in  any  spare
       devices that the array has, however it can also be stored in a separate
       file specified with the --backup-file option.  If this option is  used,
       and  the  system  does  crash during the critical period, the same file
       must be passed to --assemble to restore the backup and  reassemble  the

       A  write-intent  bitmap  can  be  added  to, or removed from, an active
       array.  Either internal bitmaps, or bitmaps stored in a separate  file,
       can  be added.  Note that if you add a bitmap stored in a file which is
       in a filesystem that is on the raid array being  affected,  the  system
       will deadlock.  The bitmap must be on a separate filesystem.


       Usage: mdadm --incremental [--run] [--quiet] component-device

       Usage: mdadm --incremental --rebuild

       Usage: mdadm --incremental --run --scan

       This mode is designed to be used in conjunction with a device discovery
       system.  As devices are found in a system, they can be passed to  mdadm
       --incremental to be conditionally added to an appropriate array.

       mdadm  performs a number of tests to determine if the device is part of
       an array, and which array it should be  part  of.   If  an  appropriate
       array  is  found, or can be created, mdadm adds the device to the array
       and conditionally starts the array.

       Note that mdadm will only add devices to an array which were previously
       working  (active  or spare) parts of that array.  It does not currently
       support automatic inclusion of a new drive as a spare in some array.

       mdadm --incremental requires a bug-fix in all kernels  through  2.6.19.
       Hopefully,  this  will be fixed in 2.6.20; alternately, apply the patch
       which is included with the mdadm source distribution.  If mdadm detects
       that   this   bug  is  present,  it  will  abort  any  attempt  to  use

       The tests that mdadm makes are as follow:

       +      Is the device permitted by mdadm.conf?  That is, is it listed in
              a  DEVICES  line  in  that  file.  If DEVICES is absent then the
              default it to allow any device.  Similar if DEVICES contains the
              special  word  partitions then any device is allowed.  Otherwise
              the device name given to mdadm must match one of  the  names  or
              patterns in a DEVICES line.

       +      Does  the  device  have  a  valid  md superblock.  If a specific
              metadata version is request with --metadata or -e then only that
              style  of  metadata is accepted, otherwise mdadm finds any known
              version of metadata.  If no md metadata is found, the device  is

       +      Does  the  metadata  match  an expected array?  The metadata can
              match  in  two  ways.   Either  there  is  an  array  listed  in
              mdadm.conf  which identifies the array (either by UUID, by name,
              by device list, or by minor-number), or the  array  was  created
              with  a  homehost specified and that homehost matches the one in
              mdadm.conf or on the command line.  If  mdadm  is  not  able  to
              positively  identify the array as belonging to the current host,
              the device will be rejected.

       +      mdadm keeps a list of arrays that it has partially assembled  in
              /var/run/mdadm/map   (or  /var/run/  if  the  directory
              doesn’t exist).  If no array exists which matches  the  metadata
              on  the  new  device,  mdadm  must choose a device name and unit
              number.  It does this based on any name given in  mdadm.conf  or
              any  name  information  stored  in  the  metadata.  If this name
              suggests a unit number, that number will be  used,  otherwise  a
              free  unit number will be chosen.  Normally mdadm will prefer to
              create a partitionable array, however  if  the  CREATE  line  in
              mdadm.conf suggests that a non-partitionable array is preferred,
              that will be honoured.

       +      Once an appropriate array is found or created and the device  is
              added,  mdadm  must  decide if the array is ready to be started.
              It will normally compare the  number  of  available  (non-spare)
              devices to the number of devices that the metadata suggests need
              to be active.  If there are at least that many, the  array  will
              be  started.   This  means  that  if any devices are missing the
              array will not be restarted.

              As an alternative, --run may be passed to mdadm  in  which  case
              the  array  will  be  run  as  soon  as there are enough devices
              present for the data to be accessible.  For a raid1, that  means
              one  device  will start the array.  For a clean raid5, the array
              will be started as soon as all but one drive is present.

              Note that neither of these approaches is really  ideal.   If  it
              can be known that all device discovery has completed, then
                 mdadm -IRs
              can  be  run  which  will try to start all arrays that are being
              incrementally assembled.  They are started in  "read-auto"  mode
              in which they are read-only until the first write request.  This
              means that no metadata updates are made and no attempt at resync
              or  recovery happens.  Further devices that are found before the
              first write can still be added safely.


         mdadm --query /dev/name-of-device
       This will find out if a given device is a raid array,  or  is  part  of
       one, and will provide brief information about the device.

         mdadm --assemble --scan
       This  will  assemble and start all arrays listed in the standard config
       file.  This command will typically go in a system startup file.

         mdadm --stop --scan
       This will shut down all arrays that can be  shut  down  (i.e.  are  not
       currently in use).  This will typically go in a system shutdown script.

         mdadm --follow --scan --delay=120
       If (and only if) there is an Email address  or  program  given  in  the
       standard  config  file, then monitor the status of all arrays listed in
       that file by polling them ever 2 minutes.

         mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1
       Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and /dev/hdc1.

         echoDEVICE /dev/hd*[0-9] /dev/sd*[0-9]> mdadm.conf
         mdadm --detail --scan >> mdadm.conf
       This  will  create  a  prototype  config  file that describes currently
       active arrays that are known to be made from partitions of IDE or  SCSI
       drives.   This  file  should  be  reviewed  before being used as it may
       contain unwanted detail.

         echoDEVICE /dev/hd[a-z] /dev/sd*[a-z]> mdadm.conf
         mdadm --examine --scan --config=mdadm.conf >> mdadm.conf
       This will find arrays which could be assembled from  existing  IDE  and
       SCSI  whole  drives  (not partitions), and store the information in the
       format of a config file.  This file is very likely to contain  unwanted
       detail,  particularly  the devices= entries.  It should be reviewed and
       edited before being used as an actual config file.

         mdadm --examine --brief --scan --config=partitions
         mdadm -Ebsc partitions
       Create a list of devices by reading /proc/partitions,  scan  these  for
       RAID  superblocks, and printout a brief listing of all that were found.

         mdadm -Ac partitions -m 0 /dev/md0
       Scan all partitions and devices listed in /proc/partitions and assemble
       /dev/md0  out  of  all such devices with a RAID superblock with a minor
       number of 0.

         mdadm --monitor --scan --daemonise > /var/run/mdadm
       If config file contains a mail address or alert program, run  mdadm  in
       the  background  in monitor mode monitoring all md devices.  Also write
       pid of mdadm daemon to /var/run/mdadm.

         mdadm -Iq /dev/somedevice
       Try  to  incorporate  newly  discovered  device  into  some  array   as

         mdadm --incremental --rebuild --run --scan
       Rebuild  the array map from any current arrays, and then start any that
       can be started.

         mdadm /dev/md4 --fail detached --remove detached
       Any devices which are components of /dev/md4 will be marked  as  faulty
       and then remove from the array.

         mdadm --create --help
       Provide help about the Create mode.

         mdadm --config --help
       Provide help about the format of the config file.

         mdadm --help
       Provide general help.


       If  you’re using the /proc filesystem, /proc/mdstat lists all active md
       devices with information about them.  mdadm uses this  to  find  arrays
       when  --scan is given in Misc mode, and to monitor array reconstruction
       on Monitor mode.

       The config file lists which devices may  be  scanned  to  see  if  they
       contain  MD  super block, and gives identifying information (e.g. UUID)
       about known MD arrays.  See mdadm.conf(5) for more details.

       When --incremental mode is used,  this  file  gets  a  list  of  arrays
       currently  being  created.   If  /var/run/mdadm  does  not  exist  as a
       directory, then /var/run/ is used instead.


       While entries in the /dev directory can have any format you like, mdadm
       has  an  understanding of ’standard’ formats which it uses to guide its
       behaviour when creating device files via the --auto option.

       The standard names for non-partitioned arrays  (the  only  sort  of  md
       array available in 2.4 and earlier) are either of


       where  NN is a number.  The standard names for partitionable arrays (as
       available from 2.6 onwards) are either of


       Partition numbers should be indicated by added  "pMM"  to  these,  thus


       mdadm was previously known as mdctl.

       mdadm  is  completely separate from the raidtools package, and does not
       use the /etc/raidtab configuration file at all.


       For further information on mdadm usage, MD and the  various  levels  of
       RAID, see:


       (based upon Jakob Østergaard’s Software-RAID.HOWTO)

       The latest version of mdadm should always be available from


       mdadm.conf(5), md(4).

       raidtab(5), raid0run(8), raidstop(8), mkraid(8).