Provided by: mdadm_3.3-2ubuntu7_i386 bug


       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,  FAULTY,  and

       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
       supported  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
       provides 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
       connected  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.

       Build  Build   an   array   that   doesn't   have  per-device  metadata
              (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  metadata (superblocks).
              Appropriate metadata is written to each  device,  and  then  the
              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 0,4,5,6,10 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:

       -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/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, --remove, or --replace, 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

       -c, --config=
              Specify the  config  file  or  directory.   Default  is  to  use
              /etc/mdadm/mdadm.conf  and  /etc/mdadm/mdadm.conf.d, or if those
              are missing then /etc/mdadm.conf and /etc/mdadm.conf.d.  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.

              If the name given is of a directory, then mdadm will collect all
              the files contained in the  directory  with  a  name  ending  in
              .conf,  sort  them  lexically, and process all of those files as
              config files.

       -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 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
                     component 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

              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
                     to "1.2" (the commonly preferred 1.x format).   "default"
                     is equivalent to "1.2".

              ddf    Use the "Industry Standard" DDF (Disk Data Format) format
                     defined by SNIA.  When creating a DDF array  a  CONTAINER
                     will be created, and normal arrays can be created in that

              imsm   Use the Intel(R) Matrix Storage Manager metadata  format.
                     This  creates  a  CONTAINER which is managed in a similar
                     manner to DDF, and is supported by an option-rom on  some


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

              When mdadm needs to print the name  for  a  device  it  normally
              finds  the  name  in  /dev  which  refers  to  the device and is
              shortest.  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
              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, 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.

       -z, --size=
              Amount  (in  Kibibytes)  of space to use from each drive in RAID
              levels 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.

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

              Sometimes a replacement drive can be a little smaller  than  the
              original  drives  though  this  should  be  minimised  by  IDEMA
              standards.  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
              persistent: 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
              appropriate before  the  number  of  devices  in  the  array  is

              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
              size  of  the  array to be whatever the real amount of available
              space is.

       -c, --chunk=
              Specify chunk size of kibibytes.  The default when  creating  an
              array  is 512KB.  To ensure compatibility with earlier versions,
              the default when Building and array with no persistent  metadata
              is  64KB.   This  is  only  meaningful  for RAID0, RAID4, RAID5,
              RAID6, and RAID10.

              RAID4, RAID5, RAID6, and RAID10 require the chunk size to  be  a
              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-asymmetric,        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
              identical 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-symmetric-6,      right-symmetric-6,     left-asymmetric-6,
              right-asymmetric-6, and parity-first-6.

              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.

              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', 'offset', and

              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).

              When  an  array  is  converted  between  RAID5  and   RAID6   an
              intermediate  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

              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').

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

              When  creating  an  array  on  devices which are 100G or larger,
              mdadm automatically adds an internal bitmap as it  will  usually
              be beneficial.  This can be suppressed with --bitmap=none .

              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 defaults to 64Meg, or larger if necessary
              to 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
              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

              When  the  devices  that will be part of a new array were filled
              with zeros before creation  the  operator  knows  the  array  is
              actually  clean.  If  that  is  the  case, such as after running
              badblocks, 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
              automatic 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

              Arrays with 1.x metadata can leave a gap between  the  start  of
              the  device  and  the start of array data.  This gap can be used
              for various metadata.   The  start  of  data  is  known  as  the
              data-offset.   Normally  an  appropriate data offset is computed
              automatically.  However it can be useful to  set  it  explicitly
              such  as  when re-creating an array which was originally created
              using a different version of mdadm which  computed  a  different

              Setting the offset explicitly over-rides the default.  The value
              given is in Kilobytes unless an 'M' or 'G' suffix is given.

              Since Linux 3.4, --data-offset can also be used with --grow  for
              some  RAID  levels  (initially  on  RAID10).   This  allows  the
              data-offset to be changed as part of the reshape process.   When
              the  data  offset  is changed, no backup file is required as the
              difference in offsets is used to provide the same functionality.

              When the new offset is earlier than the old offset,  the  number
              of devices in the array cannot shrink.  When it is after the old
              offset, the number of devices in the array cannot increase.

              When creating  an  array,  --data-offset  can  be  specified  as
              variable.   In the case each member device is expected to have a
              offset appended to the name, separated by a colon.   This  makes
              it  possible to recreate exactly an array which has varying data
              offsets (as can happen when different versions of mdadm are used
              to add different devices).

              This  option is complementary to the --freeze-reshape option for
              assembly. It is needed when --grow operation is interrupted  and
              it  is not restarted automatically due to --freeze-reshape usage
              during array assembly.  This option is used together with -G , (
              --grow  )  command  and  device  for  a  pending  reshape  to be
              continued.  All parameters  required  for  reshape  continuation
              will be read from array metadata.  If initial --grow command had
              required --backup-file= option to be  set,  continuation  option
              will require to have exactly the same backup file given as well.

              Any  other parameter passed together with --continue option will
              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
              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.

       -o, --readonly
              Start  the array read only rather than read-write as normal.  No
              writes will be allowed to the array, and no resync, recovery, or
              reshape will be started.

       -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-
              partitionable 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
              have a number added, e.g.  /dev/md/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 device number based on that  name.   If  the  device
              name is not in one of these formats, then a unused device number
              will be allocated.  The device 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.
              Names  that  are  not  in  'standard' format are only allowed in

              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

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.

              --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
              to  be out-of-date.  If mdadm cannot find enough working devices
              to start the array, but can find some devices that are  recorded
              as  having failed, then it will mark those devices as working so
              that the array can be started.  An array which requires  --force
              to be started may contain data corruption.  Use it carefully.

       -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  while  reshaping  an  array  (e.g.
              changing number of devices or chunk size) 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, 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,
              summaries, uuid, namle, homehost, resync, byteorder,  devicesize,
              no-bitmap, bbl, no-, metadata, 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 option 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.

              The metadata option only works on v0.90 metadata arrays and will
              convert  them  to  v1.0  metadata.   The array must not be dirty
              (i.e. it must not need a sync) and it must  not  have  a  write-
              intent bitmap.

              The  old  metadata  will  remain on the devices, but will appear
              older than the new metadata and so will usually be ignored.  The
              old  metadata  (or  indeed  the  new metadata) can be removed by
              giving the appropriate --metadata= option to --zero-superblock.

              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

              The bbl option will reserve space in each device for a bad block
              list.  This will be 4K in size and positioned near  the  end  of
              any free space between the superblock and the data.

              The  no-bbl option will cause any reservation of space for a bad
              block list to be  removed.   If  the  bad  block  list  contains
              entries,  this  will fail, as removing the list could cause data

              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
              operation 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.

For Manage mode:

       -t, --test
              Unless a more serious error occurred, mdadm  will  exit  with  a
              status  of  2  if  no changes were made to the array and 0 if at
              least one change was made.  This can be useful when an  indirect
              specifier  such  as  missing,  detached  or  faulty  is  used in
              requesting an  operation  on  the  array.   --test  will  report
              failure if these specifiers didn't find any match.

       -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
              immediately 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 previously 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  normally  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

              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  used  with  v1.x  metadata, --re-add can be accompanied by
              --update=devicesize, --update=bbl, or --update=no-bbl.  See  the
              description  of  these  option when used in Assemble mode for an
              explanation of their 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.

              If the device name given is faulty  then  mdadm  will  find  all
              devices  in  the  array  that are marked faulty, remove them and
              attempt to immediately re-add them.  This can be useful  if  you
              are certain that the reason for failure has been resolved.

       -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,  detached and names like set-A 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.   The  third  will
              remove a set as describe below under --fail.

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

              For RAID10 arrays where the number of copies evenly divides  the
              number  of devices, the devices can be conceptually divided into
              sets where each set contains a single complete copy of the  data
              on  the  array.   Sometimes a RAID10 array will be configured so
              that these sets are on separate controllers.  In this  case  all
              the devices in one set can be failed by giving a name like set-A
              or set-B to --fail.  The appropriate set names are  reported  by

              same as --fail.

              Mark  listed  devices  as  requiring  replacement.  As soon as a
              spare is available, it will be  rebuilt  and  will  replace  the
              marked  device.   This is similar to marking a device as faulty,
              but the device remains in service during the recovery process to
              increase   resilience   against  multiple  failures.   When  the
              replacement process finishes, the replaced device will be marked
              as faulty.

       --with This can follow a list of --replace devices.  The devices listed
              after --with will be preferentially used to replace the  devices
              listed  after  --replace.   These  device  must already be spare
              devices in the array.

              Subsequent devices that are added  or  re-added  will  have  the
              'write-mostly' flag set.  This is only valid for RAID1 and means
              that the 'md' driver will avoid reading from  these  devices  if

              Subsequent  devices  that  are  added  or re-added will have the
              'write-mostly' flag cleared.

       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
              discovered 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. If used without
              argument, mdadm will scan  all  controllers  looking  for  their
              capabilities.  Otherwise, mdadm will only look at the controller
              specified by the argument in form of an absolute filepath  or  a
              link, e.g.  /sys/devices/pci0000:00/0000:00:1f.2.

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

       -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
              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 --update=sparc2.2.

       -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.  Note that running  this  on  an  array  device
              (e.g.  /dev/md0) does not report the bitmap for that array.

              List  the  bad-blocks  recorded  for the device, if a bad-blocks
              list has been configured.  Currently only 1.x metadata  supports
              bad-blocks lists.


              Save  metadata from lists devices, or restore metadata to listed

       -R, --run
              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
              overwritten 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
              subarray 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-subarray 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,
              otherwise it will return failure.

              For  each  md  device  given,  or each device in /proc/mdstat if
              --scan is given, arrange for the array to  be  marked  clean  as
              soon  as  possible.  mdadm will return with success if the array
              uses external metadata and we successfully waited.   For  native
              arrays  this  returns  immediately  as the kernel handles dirty-
              clean transitions at shutdown.  No action is taken if  safe-mode
              handling is disabled.

For Incremental Assembly mode:

       --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
              normally 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
              kernel alerts mdadm immediately when there is any change.

       -r, --increment
              Give  a  percentage  increment.   mdadm  will generate RebuildNN
              events with the given percentage increment.

       -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 from 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

              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
       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 no arrays are described by the
       configuration file, then any arrays that can be found on unused devices
       will be 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, the --name option, or
       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 or /etc/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
       different  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
       number (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,
       and are started "read-auto" so that nothing is written  to  any  device
       until the array is written to. i.e.  automatic resync etc is delayed.

       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

       This  behaviour  can  be  modified  by  the AUTO line in the mdadm.conf
       configuration 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
       difference  between  initially  creating  the  array  and  subsequently
       assembling  the array, except that hopefully there is useful data there
       in the second 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
       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 array is
       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
       problems 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
       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.

       If the array type supports a write-intent bitmap, and if the devices in
       the array exceed 100G is size, an  internal  write-intent  bitmap  will
       automatically be added unless some other option is explicitly requested
       with the --bitmap option.  In any case  space  for  a  bitmap  will  be
       reserved so that one can be added layer with --grow --bitmap=internal.

       If  the  metadata type supports it (currently only 1.x metadata), space
       will be allocated to store a bad block  list.   This  allows  a  modest
       number  of  bad  blocks to be recorded, allowing the drive to remain in
       service while only partially functional.

       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
       former 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.


       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 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-subarray  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 supported.

              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
              components of the one array are grouped together and reported in
              a single entry suitable for inclusion in mdadm.conf.

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

              If the device contains RAID metadata, a file will be created  in
              the  directory and the metadata will be written to it.  The file
              will be the same size  as  the  device  and  have  the  metadata
              written  in  the  file  at the same locate that it exists in the
              device.  However the file will be "sparse" so  that  only  those
              blocks  containing  metadata  will be allocated. The total space
              used will be small.

              The file name used in the directory will be the base name of the
              device.    Further  if any links appear in /dev/disk/by-id which
              point to the device, then hard links to the file will be created
              in directory based on these by-id names.

              Multiple  devices  can  be listed and their metadata will all be
              stored in the one directory.

              This is the reverse of --dump.  mdadm will locate a file in  the
              directory  that  has a name appropriate for the given device and
              will restore metadata from it.  Names that match /dev/disk/by-id
              names  are preferred, however if two of those refer to different
              files, mdadm will not choose between them  but  will  abort  the

              If  a  file name is given instead of a directory then mdadm will
              restore from that file to a single device, always  provided  the
              size  of  the  file  matches  that  of  the device, and the file
              contains valid metadata.

       --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.

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


       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
       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 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
                  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
                  or domain 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 or the spares  must
       be   allowed   to  migrate  through  matching  POLICY  domains  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.

       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

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

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

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

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

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

       Using GROW on containers is currently supported only for  Intel's  IMSM
       container  format.   The  number  of  devices  in  a  container  can be
       increased - which affects all arrays in the container - or an array  in
       a  container  can  be  converted  between levels where those levels are
       supported by the container, and the conversion is on  of  those  listed
       above.   Resizing arrays in an IMSM container with --grow --size is not
       yet supported.

       Grow functionality (e.g. expand a number of raid devices)  for  Intel's
       IMSM  container format has an experimental status. It is guarded by the
       MDADM_EXPERIMENTAL environment variable which must be set to '1' for  a
       GROW command to succeed.  This is for the following reasons:

       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
       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 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
       bitmap.  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

       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
       the number of devices  in  a  RAID5  safely,  including  restarting  an
       interrupted  "reshape".   From  2.6.31,  the  Linux  Kernel  is able to
       increase or decrease the number of devices in a RAID5 or RAID6.

       From 2.6.35, the Linux Kernel is able to convert a RAID0 in to a  RAID4
       or RAID5.  mdadm uses this functionality and the ability to add devices
       to a RAID4 to allow devices to be added to a RAID0.  When requested  to
       do  this,  mdadm  will  convert the RAID0 to a RAID4, add the necessary
       disks and make the reshape happen, and then convert the RAID4  back  to

       When  decreasing the number of devices, the size of the array will also
       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
       growing 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
       throughout the reshape  operation,  as  described  below  under  LAYOUT

       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.

       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 --fail component-device

       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
              metadata 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

       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
       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.

       If the array is not found in the config file and its metadata does  not
       identify  it  as  belonging to the "homehost", then mdadm will choose a
       name for the array which is certain not  to  conflict  with  any  array
       which  does  belong to this host.  It does this be adding an underscore
       and a small number to the name preferred by the metadata.

       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.


       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
              debugging 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.

              A  key value of IMSM metadata is that it allows interoperability
              with  boot  ROMs  on  Intel  platforms,  and  with  other  major
              operating  systems.  Consequently, mdadm will only allow an IMSM
              array to be created or modified if detects that it is running on
              an   Intel  platform  which  supports  IMSM,  and  supports  the
              particular configuration of IMSM that is being  requested  (some
              functionality requires newer OROM support).

              These  checks can be suppressed by setting IMSM_NO_PLATFORM=1 in
              the environment.  This can be useful for testing or for disaster
              recovery.   You  should  be  aware  that interoperability may be
              compromised by setting this value.

              Any string given in this variable is added to the start  of  the
              AUTO  line in the config file, or treated as the whole AUTO line
              if none is given.  It can be used to  disable  certain  metadata
              types when mdadm is called from a boot script.  For example
                  export MDADM_CONF_AUTO='-ddf -imsm'
              will  make  sure  that mdadm does not automatically assemble any
              DDF or IMSM arrays that  are  found.   This  can  be  useful  on
              systems configured to manage such arrays with dmraid.


         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.

         echo 'DEVICE /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.

         echo 'DEVICE /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 > /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

         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
       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.

   /etc/mdadm/mdadm.conf (or /etc/mdadm.conf)
       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.

   /etc/mdadm/mdadm.conf.d (or /etc/mdadm.conf.d)
       A directory containing configuration files which are  read  in  lexical

       When  --incremental  mode  is  used,  this  file  gets a list of arrays
       currently 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
       possible.  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 adding "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
       partitions such as "/dev/mdNNpXX" are possible.

       From  kernel version 2.6.29 standard names can be non-numeric following
       the form:


       where XXX is any string.  These names  are  supported  by  mdadm  since
       version 3.3 provided they are enabled in mdadm.conf.


       mdadm was previously known as mdctl.


       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


       Related man pages:

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