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MDADM(8)                             System Manager's Manual                             MDADM(8)

       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  sin‐
       gle 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

       Currently,  Linux  supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4,

       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/mul‐
       tipath 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 meta‐
       data.  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.  Compo‐
              nents can be explicitly given or can be searched for.  mdadm checks that the compo‐
              nents 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  compo‐
              nents  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 oth‐
              erwise 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 meaning‐
              ful 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  sup‐
              ported  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

       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  inter‐
              face  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 con‐
              tainer 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, opera‐
              tions on component devices such as erasing old superblocks, and information gather‐
              ing operations.

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

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

              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 ini‐
              trd — is substantially more flexible and should be preferred.

       If a device is given before any options, or if the first option is and of --add, --re-add,
       --add-spare,  --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

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

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

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

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

       -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 con‐
              tained 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 set‐
              ting 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 boundary.

              1, 1.0, 1.1, 1.2 default
                     Use  the  new version-1 format superblock.  This has fewer restrictions.  It
                     can easily be moved between hosts with different endian-ness, and a recovery
                     operation  can  be  checkpointed  and restarted.  The different sub-versions
                     store the superblock at different locations on the device, either at the end
                     (for  1.0),  at  the start (for 1.1) or 4K from the start (for 1.2).  "1" is
                     equivalent 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 container.

              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 platforms:


              This  will  override any HOMEHOST setting in the config file and provides the iden‐
              tity 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  ver‐
              sion-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 Assembly.

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

              This functionality is currently only provided by --detail and --monitor.

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 vari‐
              ance 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 respec‐

              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 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 --size= command.

              This value cannot be used when creating a CONTAINER such as with DDF and IMSM meta‐
              data, 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 reduced.

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

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

              A suffix of 'M' or 'G' can be given to  indicate  Megabytes  or  Gigabytes  respec‐
              tively.   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 an 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  respec‐

              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  high‐
              lights  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 round‐
              ing) 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

       -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  par‐
              ity-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 pro‐
              vide 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-asymmet‐
              ric-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, none.

              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 rele‐
              vant 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 off‐
              sets 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 'far'.

              The number is the number of copies of each datablock.  2 is normal, 3 can  be  use‐
              ful.   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-strip‐
              ing) use --layout=preserve.  This will try to avoid any restriping.

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

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

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

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

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

              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 respec‐

       -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

              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 doing.

              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 nor‐
              mally resynced in that same way that the whole array is resynced at creation.  From
              Linux  version 3.0, --assume-clean can be used with that command to avoid the auto‐
              matic resync.

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

              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 auto‐
              matically.  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 com‐
              puted a different offset.

              Setting the offset explicitly over-rides the default.  The value given is in  Kilo‐
              bytes 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 dif‐
              ferent 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 sup‐

       -f, --force
              Insist  that mdadm accept the geometry and layout specified without question.  Nor‐
              mally 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 line.

              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.

              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 num‐
              ber  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 "/dev/md/".

              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 removed.

              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 command.

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 /dev/md2.

              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  speci‐
              fied  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, name, homehost, resync,
              byteorder, devicesize, no-bitmap, bbl, no-bbl, 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 --exam‐
              ine 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) kernel.

              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 superblocks.

              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  redun‐
              dancy  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) superblocks.

              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 meta‐

              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 meta‐
              data)  can  be  removed  by  giving   the   appropriate   --metadata=   option   to

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

              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 corruption.

              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  speci‐
              fiers 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 redun‐
              dancy.  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 all.

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

              When  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

              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.

              Add  a device as a spare.  This is similar to --add except that it does not attempt
              --re-add first.  The device will be added as a spare even if it looks like it could
              be an recent member of the array.

       -r, --remove
              remove  listed  devices.   They  must not be active.  i.e. they should be failed or
              spare devices.

              As well as the name of a device file (e.g.  /dev/sda1) the words  failed,  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

              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 config‐
              ured 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 --detail.

              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 fin‐
              ishes, 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 read‐
              ing from these devices if possible.

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

       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.  Sev‐
       eral 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  con‐
              trollers  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, --examine, or --incremental output will
              be formatted as key=value pairs for easy import into the environment.

              With --incremental The value MD_STARTED indicates  whether  an  array  was  started
              (yes)  or  not,  which  may include a reason (unsafe, nothing, no).  Also the value
              MD_FOREIGN indicates if the array is expected on this host (no),  or  seems  to  be
              from elsewhere (yes).

       -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 incom‐
              patibly  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 config‐
              ured.  Currently only 1.x metadata supports bad-blocks lists.


              Save metadata from lists devices, or restore metadata to listed devices.

       -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 inac‐
              tive  subarray in the container, then the subarray is deleted.  Deleting all subar‐
              rays 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 sub‐
              array 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 details.

       -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 transi‐
              tions at shutdown.  No action is taken if safe-mode handling is disabled.

              Set the "sync_action" for all md devices given  to  one  of  idle,  frozen,  check,
              repair.   Setting  to  idle  will  abort  any  currently running action though some
              actions will automatically restart.  Setting  to  frozen  will  abort  any  current
              action and ensure no other action starts automatically.

              Details of check and repair can be found it md(4) under SCRUBBING AND MISMATCHES.

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 meta‐
              data if it appears at specified path.   This option is normally only set by a  udev

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  sec‐
              onds  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

       -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.  Running
                      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 successfully.

              This inhibits the functionality for moving spares between arrays.  Only  one  moni‐
              toring  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 com‐
       ponent-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

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

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

       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

       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 file.

       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  meta‐
       data,  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 first).

       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 for‐
       mat  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  auto-

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

       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  syn‐
       onyms.   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 filesys‐
       tems.  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  parti‐
       tion  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, allow‐
       ing 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 descrip‐
              tion 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 unusable.

              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 system

              If the device is a container and the argument to --update-subarray specifies a sub‐
              array  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 multi‐
              ple 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 con‐
              fig 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

              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 operation.

              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 verbosity.

       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 moni‐

       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 any‐
       thing.  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  (e.g. recovery, resync, reshape, check,
                  repair). (syslog priority: Warning)

                  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  pri‐
                  ority: 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:

                  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  cer‐
                  tain  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 prior‐
                  ity: 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 match‐
       ing 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

       ·   change the chunk-size and layout of RAID0, RAID4, RAID5, RAID6 and RAID10.

       ·   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  for‐
       mat  has an experimental status. It is guarded by the MDADM_EXPERIMENTAL environment vari‐
       able which must be set to '1' for a GROW command to succeed.  This is  for  the  following

       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 synchronised.

       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 array.

       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  com‐
       plete a new bitmap can be created.

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

       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

       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 RAID0.

       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  inac‐
       cessible.   The integrity of any data can then be checked before the non-reversible reduc‐
       tion 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

       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  sec‐
       tion" but throughout the reshape operation, as described below under LAYOUT CHANGES.

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

       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 bit‐
       map 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  [optional-aliases-for-

       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 condition‐
       ally 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  meta‐
       data 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.   Simi‐
              larly  if  DEVICES contains the special word partitions then any device is allowed.
              Otherwise the device name given to mdadm, or one of the aliases given, or an  alias
              found in the filesystem, must match one of the names or patterns in a DEVICES line.

              This  is the only context where the aliases are used.  They are usually provided by
              a udev rules mentioning ${DEVLINKS}.

       +      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, oth‐
              erwise mdadm finds any known version of metadata.  If no md metadata is found,  the
              device may be still added to an array as a spare if POLICY allows.

       mdadm  keeps  a  list  of arrays that it has partially assembled in /run/mdadm/map.  If no
       array exists which matches the metadata on the new device, mdadm must choose a device name
       and  unit number.  It does this based on any name given in mdadm.conf or any name informa‐
       tion 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-parti‐
       tionable 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  recov‐
       ery  happens.   Further  devices  that are found before the first write can still be added

       This section describes environment variables that affect how mdadm operates.

              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.

              If  mdadm  detects that systemd is in use it will normally request systemd to start
              various background tasks (particularly mdmon) rather than forking and running  them
              in the background.  This can be suppressed by setting MDADM_NO_SYSTEMCTL=1.

              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.

              If an array is stopped while it is performing a reshape and that reshape was making
              use of a backup file, then when the array is re-assembled mdadm will sometimes com‐
              plain  that  the backup file is too old.  If this happens and you are certain it is
              the   right   backup   file,   you   can   over-ride   this   check   by    setting
              MDADM_GROW_ALLOW_OLD=1 in the environment.

              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 exam‐
                  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/mon.pid
       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

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

         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 appropriate.

         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 infor‐
       mation 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 order.

       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 home‐
       host 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 "/dev/md/d1p2".

       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  possi‐

       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).

v3.3.2                                                                                   MDADM(8)

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