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 parti-
       tions  thereof)  to be combined into a single device to hold (for exam-
       ple) 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, FAULTY, and CON-

       MULTIPATH is not a Software RAID mechanism, but does  involve  multiple
       devices:  each  device is a path to one common physical storage device.
       New installations should not use md/multipath as it is  not  well  sup-
       ported  and  has  no  ongoing development.  Use the Device Mapper based
       multipath-tools instead.

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

       CONTAINER  is  different again.  A CONTAINER is a collection of devices
       that are managed as a set.  This is similar to the set of devices  con-
       nected to a hardware RAID controller.  The set of devices may contain a
       number of different RAID arrays each utilising some  (or  all)  of  the
       blocks  from  a  number  of  the  devices in the set.  For example, two
       devices in a 5-device set might form a RAID1 using the  whole  devices.
       The  remaining  three  might  have  a RAID5 over the first half of each
       device, and a RAID0 over the second half.

       With a CONTAINER, there is one set of metadata that  describes  all  of
       the arrays in the container.  So when mdadm creates a CONTAINER device,
       the device just represents the metadata.  Other  normal  arrays  (RAID1
       etc) can be created inside the container.

       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.

              array comprising those devices is activated.  A 'resync' process
              is started to make sure that the array is consistent (e.g.  both
              sides  of a mirror contain the same data) but the content of the
              device is left otherwise untouched.  The array can  be  used  as
              soon  as  it has been created.  There is no need to wait for the
              initial resync to finish.

       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  Linear and RAID levels 0/1/4/5/6, changing the RAID
              level between 0, 1, 5, and 6, and between 0 and 10, changing the
              chunk  size  and  layout  for RAID 0,4,5,6, 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.  Optionally, when the --fail flag  is
              passed  in  we  will  remove  the  device  from any active array
              instead of adding it.

              If a CONTAINER is passed to mdadm in this mode, then any  arrays
              within that container will be assembled and started.

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

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

       -I, --incremental
              Add/remove a single device to/from  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,  and  all  use  v0.90 metadata.  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 assumed.  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

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

       -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 containers and will
              read /proc/partitions to find a list of  devices  to  scan,  and
              /proc/mdstat  to  find  a list of containers to examine.  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 iden-
              tities, 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 RAID metadata (superblock) to be used.  The
              default is 1.2 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
                     Use the original 0.90  format  superblock.   This  format
                     limits  arrays  to 28 component devices and limits compo-
                     nent devices of levels 1 and greater to 2 terabytes.   It
                     is  also possible for there to be confusion about whether
                     the superblock applies to a whole device or just the last
                     partition, if that partition starts on a 64K boundary.

              1, 1.0, 1.1, 1.2 default
                     Use  the new version-1 format superblock.  This has fewer
                     restrictions.  It can easily be moved between hosts  with
                     different  endian-ness,  and  a recovery operation can be
                     checkpointed and restarted.  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).  "1" is equivalent


              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
              metadata.  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 UUID.

              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 home-
              host will be allowed to use 'local' names (i.e.  not  ending  in
              '_' followed by a digit string).  See below under Auto Assembly.

              When  mdadm  needs  to  print  the name for a device it normally
              finds the name in /dev which refers to the device and is  short-
              est.   When  a  path component is given with --prefer mdadm will
              prefer a longer name if it contains that component.  For example
              --prefer=by-uuid  will  prefer  a name in a subdirectory of /dev
              called by-uuid.

              This functionality is currently only provided  by  --detail  and

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 prob-
              ably  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, RAID5 or RAID6.
              This  number  can only be changed using --grow for RAID1, RAID4,
              RAID5 and RAID6 arrays, and only on kernels  which  provide  the
              necessary support.

       -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.

              Sometimes  a  replacement drive can be a little smaller than the
              original drives though this should be minimised by  IDEMA  stan-
              dards.   Such  a  replacement  drive will be rejected by md.  To
              guard against this it can be useful  to  set  the  initial  size
              slightly  smaller  than  the smaller device with the aim that it
              will still be larger than any replacement.

              This value can be set with --grow for RAID level 1/4/5/6  though
              CONTAINER  based arrays such as those with IMSM metadata may not
              be able to support this.  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.

              Before  reducing the size of the array (with --grow --size=) you
              should make sure that space isn't needed.  If the device holds a
              filesystem,  you would need to resize the filesystem to use less

              After reducing the array size you should  check  that  the  data
              stored  in the device is still available.  If the device holds a
              filesystem, then an  'fsck'  of  the  filesystem  is  a  minimum
              requirement.  If there are problems the array can be made bigger
              again with no loss with another --grow --size= command.

              This value cannot be used when creating a CONTAINER such as with
              DDF  and  IMSM metadata, though it perfectly valid when creating
              an array inside a container.

       -Z, --array-size=
              This is only meaningful with --grow and its effect is  not  per-
              sistent:  when  the  array  is stopped and restarted the default
              array size will be restored.

              Setting the array-size causes the array  to  appear  smaller  to
              programs  that  access  the  data.   This is particularly needed
              before reshaping an array so that it will be  smaller.   As  the
              reshape  is  not  reversible, but setting the size with --array-
              size is, it is required that the array size is reduced as appro-
              priate before the number of devices in the array is reduced.

              Before  reducing the size of the array you should make sure that
              space isn't needed.  If the device holds a filesystem, you would
              need to resize the filesystem to use less space.

              After  reducing  the  array  size you should check that the data
              stored in the device is still available.  If the device holds  a
              filesystem,  then  an  'fsck'  of  the  filesystem  is a minimum
              requirement.  If there are problems the array can be made bigger
              again with no loss with another --grow --array-size= command.

              A  suffix  of  'M'  or 'G' can be given to indicate Megabytes or
              Gigabytes respectively.  A value of max  restores  the  apparent
              power of 2.  In any case it must be a multiple of 4KB.

              A  suffix  of  'M'  or 'G' can be given to indicate Megabytes or
              Gigabytes respectively.

              Specify rounding factor for a Linear array.  The  size  of  each
              component will be rounded down to a multiple of this size.  This
              is a synonym for --chunk but highlights  the  different  meaning
              for Linear as compared to other RAID levels.  The default is 64K
              if a kernel earlier than 2.6.16 is in use, and is  0K  (i.e.  no
              rounding) in later kernels.

       -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, container.  Obviously some
              of these are synonymous.

              When a CONTAINER metadata type is requested, only the  container
              level is permitted, and it does not need to be explicitly given.

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

              Can be used with --grow to change the RAID level in some  cases.
              See LEVEL CHANGES below.

       -p, --layout=
              This  option  configures  the  fine  details  of data layout for
              RAID5, RAID6, and RAID10 arrays, and controls the failure  modes
              for faulty.

              The layout of the RAID5 parity block can be one of left-asymmet-
              ric, left-symmetric, right-asymmetric, right-symmetric, la,  ra,
              ls, rs.  The default is left-symmetric.

              It is also possible to cause RAID5 to use a RAID4-like layout by
              choosing parity-first, or parity-last.

              Finally   for   RAID5   there   are   DDF-compatible    layouts,
              ddf-zero-restart, ddf-N-restart, and ddf-N-continue.

              These  same  layouts  are available for RAID6.  There are also 4
              layouts that will provide an intermediate stage  for  converting
              between  RAID5 and RAID6.  These provide a layout which is iden-
              tical to  the  corresponding  RAID5  layout  on  the  first  N-1
              devices,  and  has  the  'Q' syndrome (the second 'parity' block
              used by RAID6) on the last device.  These layouts are: left-sym-
              metric-6,  right-symmetric-6,  left-asymmetric-6, right-asymmet-
              ric-6, and parity-first-6.
              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.

              Finally,  the  layout  options for RAID10 are one of 'n', 'o' or
              'f' followed by a small number.  The default is 'n2'.  The  sup-
              ported 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', 'offset', and

              The number is the number of copies of each datablock.  2 is nor-
              mal,  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).

              When an array is converted between RAID5 and RAID6 an intermedi-
              ate RAID6 layout is used in which the second parity block (Q) is
              always on the last device.  To convert  a  RAID5  to  RAID6  and
              leave it in this new layout (which does not require re-striping)
              use --layout=preserve.  This will try to avoid any restriping.

              The converse of this is --layout=normalise which will  change  a
              non-standard RAID6 layout into a more standard arrangement.

              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').
              fit the bitmap into the available space.

              A  suffix  of  'M'  or 'G' can be given to indicate Megabytes or
              Gigabytes respectively.

       -W, --write-mostly
              subsequent devices listed in a --build, --create, or --add  com-
              mand 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 mirror-
              ing 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 maxi-
              mum 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 actu-
              ally 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 doing.

              When  the  devices  that will be part of a new array were filled
              with zeros before creation the operator knows the array is actu-
              ally  clean.  If  that  is  the case, such as after running bad-
              blocks, this argument can be used to tell mdadm  the  facts  the
              operator knows.

              When  an  array  is resized to a larger size with --grow --size=
              the new space is normally resynced in that  same  way  that  the
              whole  array  is  resynced at creation.  From Linux version 3.0,
              --assume-clean can be used with that command to avoid the  auto-
              matic resync.

              This  is  needed  when  --grow is used to increase the number of
              raid-devices in a RAID5 or RAID6 if there are no  spare  devices
              available,  or  to shrink, change RAID level or layout.  See the
              GROW MODE section below on RAID-DEVICES CHANGES.  The file  must
              be  stored  on  a  separate  device, not on the RAID array being

              be ignored.

       -N, --name=
              Set a name for the array.  This is currently only effective when
              creating an array with a version-1 superblock, or an array in  a
              DDF  container.  The name is a simple textual string that can be
              used to identify array components when assembling.  If  name  is
              needed  but  not specified, it is taken from the basename of the
              device that is being created.  e.g. when  creating  /dev/md/home
              the name will default to home.

       -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 with-
              out  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{=yes,md,mdp,part,p}{NN}
              Instruct mdadm how to create the device file if needed, possibly
              allocating an unused minor number.  "md" causes a non-partition-
              able array to be used (though since Linux  2.6.28,  these  array
              devices are in fact partitionable).  "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.   With
              mdadm  3.0,  device creation is normally left up to udev so this
              option is unlikely to be needed.  See DEVICE NAMES below.

              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/md/home1p3.
              If there is no trailing digit, then  the  partition  names  just
              This is meaningful with --create or --build.

       -a, --add
              This option can be used in Grow mode in two cases.

              If the target array is a Linear array, then --add can be used to
              add one or more devices to the array.  They are simply catenated
              on  to  the end of the array.  Once added, the devices cannot be

              If the --raid-disks option is being used to increase the  number
              of devices in an array, then --add can be used to add some extra
              devices to be included in the array.  In most cases this is  not
              needed  as  the  extra devices can be added as spares first, and
              then the number of  raid-disks  can  be  changed.   However  for
              RAID0,  it  is  not  possible to add spares.  So to increase the
              number of devices in a RAID0, it is necessary  to  set  the  new
              number  of devices, and to add the new devices, in the same com-

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  assem-
              bled.   e.g.  when  assembling  /dev/md0, --super-minor=dev will
              look for super blocks with a minor number of 0.

              --super-minor is only relevant for v0.90  metadata,  and  should
              not normally be used.  Using --uuid is much safer.

       -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 the metadata on some devices appears
              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 cre-
              ated.  If an array has an internal bitmap, there is no  need  to
              specify this when assembling the array.

              If  --backup-file was used while reshaping an array (e.g. chang-
              ing number of devices or chunk size) and the system crashed dur-
              ing  the  critical  section, then the same --backup-file must be
              presented to --assemble to allow possibly corrupted data  to  be
              restored, and the reshape to be completed.

              If the file needed for the above option is not available for any
              reason an empty file can be given together with this  option  to
              indicate that the backup file is invalid.  In this case the data
              that was being rearranged at the time  of  the  crash  could  be
              irrecoverably  lost,  but  the  rest  of  the array may still be
              recoverable.  This option should only be used as a  last  resort
              if there is no way to recover the backup file.

       -U, --update=
              Update the superblock on each device while assembling the array.
              The argument given to this flag can be  one  of  sparc2.2,  sum-
              maries,  uuid,  name,  homehost,  resync, byteorder, devicesize,
              no-bitmap, 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 ker-
              nel.  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
              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 mean-
              ing 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  redun-
              dant 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 option will rarely be of use.  It applies to ver-
              sion 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.

              The  no-bitmap  option can be used when an array has an internal
              bitmap which is corrupt in some way so that assembling the array
              normally  fails.   It  will  cause  any  internal  bitmap  to be

              Option is intended to be used in start-up scripts during  initrd
              boot phase.  When array under reshape is assembled during initrd
              phase, this option stops reshape after reshape critical  section
              is  being restored. This happens before file system pivot opera-
              tion and avoids loss of file system context.  Losing file system
              context would cause reshape to be broken.

              Reshape  can  be continued later using the --continue option for
              the grow command.

       -a, --add
              hot-add  listed  devices.   If a device appears to have recently
              been part of the array (possibly it failed or was  removed)  the
              device  is  re-added  as  described  in the next point.  If that
              fails or the device was never part of the array, the  device  is
              added as a hot-spare.  If the array is degraded, it will immedi-
              ately start to rebuild data onto that spare.

              Note that this and the following options are only meaningful  on
              array with redundancy.  They don't apply to RAID0 or Linear.

              re-add a device that was previous removed from an array.  If the
              metadata on the device reports that it is a member of the array,
              and  the slot that it used is still vacant, then the device will
              be added back to the array in the same position.  This will nor-
              mally  cause  the data for that device to be recovered.  However
              based on the event count on the device, the  recovery  may  only
              require  sections  that  are flagged a write-intent bitmap to be
              recovered or may not require any recovery at all.

              When used on an array that has no metadata (i.e.  it  was  built
              with  --build)  it will be assumed that bitmap-based recovery is
              enough to make the device fully consistent with the array.

              When --re-add can be accompanied  by  --update=devicesize.   See
              the description of this option when used in Assemble mode for an
              explanation of its use.

              If the device name given is missing then mdadm will try to  find
              any  device  that  looks like it should be part of the array but
              isn't and will try to re-add all such devices.

       -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  con-
              nected  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.


       Each of these options requires that the  first  device  listed  is  the
       array  to  be acted upon, and the remainder are component devices to be
       added, removed, marked as faulty, etc.   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 dis-
              covered is presented.

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

              Print details of the platform's RAID  capabilities  (firmware  /
              hardware topology) for a given metadata format.

       -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 contents of the metadata stored on  the  named  device(s).
              Note  the  contrast  between  --examine and --detail.  --examine
              applies to devices which  are  components  of  an  array,  while
              --detail applies to a whole array which is currently active.

              If an array was created on a SPARC machine with a 2.2 Linux ker-
              nel 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
              start a partially assembled array.  If --assemble did  not  find
              enough  devices  to  fully  start the array, it might leaving it
              partially assembled.  If you wish, you can  then  use  --run  to
              start the array in degraded mode.

       -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 over-
              written with zeros.  With --force the block where the superblock
              would be is overwritten even if it doesn't appear to be valid.

              If the device is a container and the argument to --kill-subarray
              specifies an inactive subarray in the container, then the subar-
              ray  is  deleted.   Deleting all subarrays will leave an 'empty-
              container'   or   spare   superblock   on   the   drives.    See
              --zero-superblock  for  completely  removing a superblock.  Note
              that some formats depend on the subarray index for generating  a
              UUID,  this  command will fail if it would change the UUID of an
              active subarray.

              If the device is a container and the argument to --update-subar-
              ray  specifies  a  subarray  in  the  container, then attempt to
              update the given superblock field in the subarray. See below  in
              MISC MODE for details.

       -t, --test
              When  used  with  --detail,  the  exit status of mdadm is set to
              reflect the status of the device.  See below in  MISC  MODE  for

       -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, oth-
              erwise it will return failure.

       --rebuild-map, -r
              Rebuild the map file (/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.

       --fail, -f
              This  allows  the  hot-plug  system  to remove devices that have
              fully disappeared from the kernel.  It will first fail and  then
              remove the device from any array it belongs to.  The device name
              given should be a kernel device name such as "sda", not  a  name
              in /dev.

              Only  used  with  --fail.   The 'path' given will be recorded so
              that if a new device appears at the  same  location  it  can  be
              automatically  added  to the same array.  This allows the failed
              device to be automatically replaced  by  a  new  device  without
              metadata  if it appears at specified path.   This option is nor-
              mally only set by a udev script.

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
              seconds.   Since  2.6.16, there is no need to reduce this as the
              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  dae-
              mon  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.  Run-
                      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 suc-

              This  inhibits  the  functionality  for  moving  spares  between
              arrays.   Only  one  monitoring  process started with --scan but
              without this flag is allowed, otherwise the two could  interfere
              with each other.

       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 compo-
       nents.  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 configura-
       tion file are assembled.  If no arrays are described by the  configura-
       tion  file, then any arrays that can be found on unused devices will be
       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 not all expected drives were listed, 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 udev is active, mdadm does not create any entries in /dev but leaves
       that  to udev.  It does record information in /run/mdadm/map which will
       allow udev to choose the correct name.

       If mdadm detects that udev  is  not  configured,  it  will  create  the
       devices in /dev itself.

       In Linux kernels prior to version 2.6.28 there were two distinctly dif-
       ferent types of md devices that could be created:  one  that  could  be
       partitioned  using  standard partitioning tools and one that could not.
       Since 2.6.28 that distinction is no longer relevant  as  both  type  of
       devices  can  be partitioned.  mdadm will normally create the type that
       originally could not be partitioned as it has a well defined major num-
       ber (9).

       Prior to 2.6.28, it is important that mdadm chooses the correct type of
       array device to use.  This can be controlled with  the  --auto  option.
       In  particular,  a value of "mdp" or "part" or "p" tells mdadm to use a
       partitionable device rather than the default.

       In the no-udev case, the value given to --auto can  be  suffixed  by  a
       number.   This  tells  mdadm to create that number of partition devices
       rather than the default of 4.

       The value given to --auto can also be given in the  configuration  file
       as a word starting auto= on the ARRAY line for the relevant array.

   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  no  arrays  are  listed  in  the  config  (other  than those marked
       <ignore>) it will look  through  the  available  devices  for  possible
       arrays  and  will try to assemble anything that it finds.  Arrays which
       are tagged as belonging to the given homehost  will  be  assembled  and
       started  normally.   Arrays  which do not obviously belong to this host
       are given names that are expected not to conflict with anything  local,
       This  behaviour can be modified by the AUTO line in the mdadm.conf con-
       figuration file.  This line can indicate that  specific  metadata  type
       should,  or  should  not,  be  automatically assembled.  If an array is
       found which is not listed in mdadm.conf and has a metadata format  that
       is  denied  by  the AUTO line, then it will not be assembled.  The AUTO
       line can also request that all arrays  identified  as  being  for  this
       homehost  should  be  assembled regardless of their metadata type.  See
       mdadm.conf(5) for further details.

       Note: Auto assembly cannot be used for assembling and  activating  some
       arrays  which are undergoing reshape.  In particular as the backup-file
       cannot be given, any reshape which requires a backup-file  to  continue
       cannot  be started by auto assembly.  An array which is growing to more
       devices and has passed the critical  section  can  be  assembled  using

       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 differ-
       ence between initially creating the array and  subsequently  assembling
       the array, except that hopefully there is useful data there in the sec-
       ond case.

       The level may raid0, linear, raid1, raid10, multipath,  or  faulty,  or
       one  of  their synonyms.  All devices must be listed and the array will
       be started  once  complete.   It  will  often  be  appropriate  to  use
       --assume-clean with levels raid1 or raid10.

       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.

       The named device will normally not exist when mdadm  --create  is  run,
       but will be created by udev once the array becomes active.

       As  devices  are  added,  they  are checked to see if they contain RAID
       superblocks or filesystems.  They are also checked to see if the  vari-
       ance 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
       required.  If this is not given with  the  --name  option,  mdadm  will
       choose  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.

       When creating a partition based array,  using  mdadm  with  version-1.x
       metadata, the partition type should be set to 0xDA (non fs-data).  This
       type selection allows for greater precision since using any other [RAID
       auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create prob-
       lems in the event of array recovery through a live cdrom.

       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 cre-
       ating  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.

       When creating an array within a CONTAINER mdadm can be given either the
       list of devices to use, or simply the name of the container.  The  for-
       mer case gives control over which devices in the container will be used
       for the array.  The latter case allows mdadm  to  automatically  choose
       which devices to use based on how much spare space is available.

       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.

       When a device is added to an active array, mdadm checks to  see  if  it
       has  metadata on it which suggests that it was recently a member of the
       array.  If it does, it tries to "re-add" the  device.   If  there  have
       been  no  changes  since  the device was removed, or if the array has a
       write-intent bitmap which has recorded  whatever  changes  there  were,
       then  the device will immediately become a full member of the array and
       those differences recorded in the bitmap will be resolved.

              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.

              Print  detail  of  the  platform's RAID capabilities (firmware /
              hardware topology).  If the metadata is  specified  with  -e  or
              --metadata= then the return status will be:

              0      metadata  successfully enumerated its platform components
                     on this system

              1      metadata is platform independent

              2      metadata failed to find its platform components  on  this

              If the device is a container and the argument to --update-subar-
              ray specifies a subarray  in  the  container,  then  attempt  to
              update  the  given superblock field in the subarray.  Similar to
              updating an array in "assemble" mode, the  field  to  update  is
              selected by -U or --update= option.  Currently only name is sup-

              The name option updates the subarray name in  the  metadata,  it
              may  not  affect the device node name or the device node symlink
              until the subarray is  re-assembled.   If  updating  name  would
              change the UUID of an active subarray this operation is blocked,
              and the command will end in an error.

              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

              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 oper-
              ation to be applied to all arrays listed in  /proc/mdstat.   For
              --examine,  --scan  causes all devices listed in the config file
              to be examined.

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

       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 or domain  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  speci-
       fied  on the command line or in the config file.  If neither are avail-
       able, then mdadm will not monitor anything.  Without --scan, mdadm will
                  information  Wrong-Level.   This is because RAID0 and Linear
                  do not support the device-failed, hot-spare and resync oper-
                  ations which are monitored.

                  An  md array started reconstruction. (syslog priority: Warn-

                  Where NN is a two-digit number (ie. 05, 48). This  indicates
                  that  rebuild has passed that many percent of the total. The
                  events are generated with fixed increment since 0. Increment
                  size  may be specified with a commandline option (default is
                  20). (syslog priority: Warning)

                  An md array that was  rebuilding,  isn't  any  more,  either
                  because  it finished normally or was aborted. (syslog prior-
                  ity: 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

                  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 or the spares must
       be allowed to migrate through matching POLICY domains in the configura-
       tion  file.   The spare-group name can be any string; it is only neces-
       sary 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.

       If the spare group for a degraded array is not defined, mdadm will look
       at the rules of spare migration specified by POLICY lines in mdadm.conf
       and then follow similar steps as above if a matching spare is found.

       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.

       Currently the supported changes include

       o   change the "size" attribute for RAID1, RAID4, RAID5 and RAID6.

       o   increase or decrease the "raid-devices" attribute of RAID0,  RAID1,
           RAID4, RAID5, and RAID6.

       o   change the chunk-size and layout of RAID0, RAID4, RAID5 and RAID6.

       o   convert  between  RAID1 and RAID5, between RAID5 and RAID6, between
           RAID0, RAID4, and RAID5, and  between  RAID0  and  RAID10  (in  the
           near-2 mode).

       o   add  a  write-intent  bitmap to any array which supports these bit-
           maps, or remove a write-intent bitmap from such an array.

       1.     Intel's  native  IMSM  check-pointing  is  not fully tested yet.
              This can causes IMSM incompatibility during the grow process: an
              array  which is growing cannot roam between Microsoft Windows(R)
              and Linux systems.

       2.     Interrupting a grow operation is not recommended, because it has
              not been fully tested for Intel's IMSM container format yet.

       Note: Intel's native checkpointing doesn't use --backup-file option and
       it is transparent for assembly feature.

       Normally when an array is built the "size" is 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 situa-
       tion, 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 synchro-

       Note that when an array changes size, any filesystem that may be stored
       in the array will not automatically grow or shrink to use or vacate the
       space.  The filesystem will need to be explicitly told to use the extra
       space after growing, or to reduce  its  size  prior  to  shrinking  the

       Also the size of an array cannot be changed while it has an active bit-
       map.  If an array has a bitmap, it must be removed before the size  can
       be changed. Once the change is complete a new bitmap can be created.

       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.

       Changing the number of active devices in a RAID5 or RAID6 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  increase
       decrease.   If  there was data in the array, it could get destroyed and
       this is not reversible, so you should firstly shrink the filesystem  on
       the array to fit within the new size.  To help prevent accidents, mdadm
       requires that the size of the  array  be  decreased  first  with  mdadm
       --grow  --array-size.   This  is a reversible change which simply makes
       the end of the array inaccessible.  The integrity of any data can  then
       be checked before the non-reversible reduction in the number of devices
       is request.

       When relocating the first few stripes on a RAID5 or RAID6,  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.  For grows, this backup may be 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,  and  is
       required  to  be  specified  for shrinks, RAID level changes and layout
       changes.  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 array.  When shrinking rather than  grow-
       ing  the array, the reshape is done from the end towards the beginning,
       so the "critical section" is at the end of the reshape.

       Changing the RAID level of any array happens instantaneously.   However
       in  the  RAID5 to RAID6 case this requires a non-standard layout of the
       RAID6 data, and in the RAID6 to RAID5 case that non-standard layout  is
       required  before  the  change  can be accomplished.  So while the level
       change is instant, the accompanying layout change can take quite a long
       time.  A --backup-file is required.  If the array is not simultaneously
       being grown or shrunk, so that the array size will remain  the  same  -
       for  example,  reshaping  a  3-drive  RAID5  into a 4-drive RAID6 - the
       backup file will be used not just for a "cricital section" but through-
       out the reshape operation, as described below under LAYOUT CHANGES.

       Changing  the  chunk-size of layout without also changing the number of
       devices as the same time will involve re-writing all  blocks  in-place.
       To  ensure  against  data  loss in the case of a crash, a --backup-file
       must be provided for these changes.  Small sections of the  array  will
       be  copied  to  the  backup file while they are being rearranged.  This
       means that all the data is copied twice, once to the backup and once to
       the  new  layout  on  the  array,  so this type of reshape will go very

       If the reshape is interrupted for any reason, this backup file must  be
       made  available  to  mdadm  --assemble so the array can be reassembled.
       Consequently the file cannot be stored on the device being reshaped.

       Usage: mdadm --incremental --rebuild-map

       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.

       Conversely, it can also be used with the --fail flag  to  do  just  the
       opposite  and  find  whatever  array a particular device is part of and
       remove the device from that array.

       If the device passed is a CONTAINER device created by a  previous  call
       to  mdadm,  then rather than trying to add that device to an array, all
       the arrays described by the metadata of the container will be started.

       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 normally only add devices to an array which were
       previously working (active or spare) parts of that array.  The  support
       for  automatic  inclusion  of  a  new  drive  as  a spare in some array
       requires a configuration through POLICY in config file.

       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 meta-
              data version is requested 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 may
              be still added to an array as a spare if POLICY allows.

       mdadm  keeps  a  list  of  arrays  that  it  has partially assembled in
       /run/mdadm/map.  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 num-
       ber will be used, otherwise a free unit number will  be  chosen.   Nor-
       mally 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.
       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 incremen-
       tally assembled.  They are started in "read-auto" mode  in  which  they
       are  read-only until the first write request.  This means that no meta-
       data 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.

       This section describes environment  variables  that  affect  how  mdadm

              Setting  this  value  to 1 will prevent mdadm from automatically
              launching mdmon.  This variable is intended primarily for debug-
              ging mdadm/mdmon.

              Normally,  mdadm  does  not create any device nodes in /dev, but
              leaves that task to udev.  If udev appears not to be configured,
              or  if  this  environment variable is set to '1', the mdadm will
              create and devices that are needed.

         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 cur-
       rently 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 --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 > /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 /run/mdadm/

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

         mdadm --incremental --rebuild-map --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 --grow /dev/md4 --level=6 --backup-file=/root/backup-md4
       The array /dev/md4 which is currently a RAID5 array will  be  converted
       to  RAID6.   There should normally already be a spare drive attached to
       the array as a RAID6 needs one more drive than a matching RAID5.

         mdadm --create /dev/md/ddf --metadata=ddf --raid-disks 6 /dev/sd[a-f]
       Create a DDF array over 6 devices.

         mdadm --create /dev/md/home -n3 -l5 -z 30000000 /dev/md/ddf
       Create a RAID5 array over any 3 devices in the given DDF set.  Use only
       30 gigabytes of each device.

         mdadm -A /dev/md/ddf1 /dev/sd[a-f]
       Assemble a pre-exist ddf array.

         mdadm -I /dev/md/ddf1
       Assemble  all  arrays  contained  in  the ddf array, assigning names as

         mdadm --create --help

       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 con-
       tain 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 cur-
       rently being created.

       mdadm understand two sorts of names for array devices.

       The first is the so-called 'standard' format name,  which  matches  the
       names used by the kernel and which appear in /proc/mdstat.

       The  second  sort  can  be  freely chosen, but must reside in /dev/md/.
       When giving a device name to mdadm to  create  or  assemble  an  array,
       either full path name such as /dev/md0 or /dev/md/home can be given, or
       just the suffix of the second sort of name, such as home can be given.

       When mdadm chooses device names  during  auto-assembly  or  incremental
       assembly,  it  will sometimes add a small sequence number to the end of
       the name to avoid conflicted between multiple arrays that have the same
       name.  If mdadm can reasonably determine that the array really is meant
       for this host, either by a hostname in the metadata, or by the presence
       of the array in mdadm.conf, then it will leave off the suffix if possi-
       ble.  Also if the homehost is specified as <ignore> mdadm will only use
       a  suffix  if  a  different array of the same name already exists or is
       listed in the config file.

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


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


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

       From kernel version, 2.6.28 the "non-partitioned array" can actually be
       partitioned.  So the "md_dNN" names are no longer  needed,  and  parti-
       tions such as "/dev/mdNNpXX" are possible.


       (based upon Jakob Ostergaard's Software-RAID.HOWTO)

       The latest version of mdadm should always be available from


       Related man pages:

       mdmon(8), mdadm.conf(5), md(4).

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

v3.2.5                                                                MDADM(8)
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