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 parti-
       tions  thereof)  to be combined into a single device to hold (for exam-
       ple) a single filesystem.  Some RAID levels include redundancy  and  so
       can survive some degree of device failure.

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

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

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

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

       CONTAINER  is  different again.  A CONTAINER is a collection of devices
       that are managed as a set.  This is similar to the set of devices  con-
       nected to a hardware RAID controller.  The set of devices may contain a
       number of different RAID arrays each utilising some  (or  all)  of  the
       blocks  from  a number of the devices in the set.  For example, two de-
       vices 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 de-
       vice, and a RAID0 over the second half.

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

       mdadm has several major modes of operation:

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

       Build  Build  an  array  that  doesn't  have  per-device  metadata (su-
              perblocks).  For these sorts of arrays, mdadm cannot differenti-
              ate  between  initial creation and subsequent assembly of an ar-
              ray.  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).  Ap-
              propriate metadata is written to each device, and then the array
              comprising  those  devices  is activated.  A 'resync' process is
              started to make sure that the array  is  consistent  (e.g.  both
              sides  of a mirror contain the same data) but the content of the
              device is left otherwise untouched.  The array can  be  used  as
              soon  as  it has been created.  There is no need to wait for the
              initial resync to finish.

       Follow or Monitor
              Monitor one or more md devices and act  on  any  state  changes.
              This  is only meaningful for RAID1, 4, 5, 6, 10 or multipath ar-
              rays, as only these have interesting  state.   RAID0  or  Linear
              never have missing, spare, or failed drives, so there is nothing
              to monitor.

       Grow   Grow (or shrink) an array, or otherwise reshape it in some  way.
              Currently supported growth options including changing the active
              size of component devices and changing the number of active  de-
              vices  in  Linear  and  RAID levels 0/1/4/5/6, changing the RAID
              level between 0, 1, 5, and 6, and between 0 and 10, changing the
              chunk  size  and layout for RAID 0,4,5,6,10 as well as adding or
              removing a write-intent bitmap and changing the array's  consis-
              tency policy.

       Incremental Assembly
              Add a single device to an appropriate array.  If the addition of
              the device makes the array runnable, the array will be  started.
              This  provides  a convenient interface to a hot-plug system.  As
              each device is detected, mdadm has a chance  to  include  it  in
              some  array as appropriate.  Optionally, when the --fail flag is
              passed in we will remove the device from any  active  array  in-
              stead of adding it.

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

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

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

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

Options for selecting a mode are:
       -A, --assemble
              Assemble a pre-existing array.

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

       -C, --create
              Create a new array.

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

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

       -I, --incremental
              Add/remove  a  single  device  to/from an appropriate array, and
              possibly start the array.

              Request that the kernel starts any auto-detected  arrays.   This
              can  only work if md is compiled into the kernel -- not if it is
              a module.  Arrays can be auto-detected by the kernel if all  the
              components  are in primary MS-DOS partitions with partition type
              FD, and all use v0.90 metadata.   In-kernel  autodetect  is  not
              recommended  for  new  installations.  Using mdadm to detect and
              assemble arrays -- possibly in an  initrd  --  is  substantially
              more flexible and should be preferred.

       If  a device is given before any options, or if the first option is one
       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 options.

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

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

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

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

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

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

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

       -e, --metadata=
              Declare the style of RAID metadata (superblock) to be used.  The
              default  is 1.2 for --create, and to guess for other operations.
              The default can be overridden by setting the metadata value  for
              the CREATE keyword in mdadm.conf.

              Options are:

              0, 0.90
                     Use  the  original  0.90  format superblock.  This format
                     limits arrays to 28 component devices and  limits  compo-
                     nent  devices of levels 1 and greater to 2 terabytes.  It
                     is also possible for there to be confusion about  whether
                     the superblock applies to a whole device or just the last
                     partition, if that partition starts on a 64K boundary.

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

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

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


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

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

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

              When using Auto-Assemble, only arrays tagged for the given home-
              host  will  be  allowed to use 'local' names (i.e. not ending in
              '_' followed by a digit string).  See below under Auto Assembly.

              The special name "any" can be used as a wild card.  If an  array
              is  created  with  --homehost=any  then  the  name "any" will be
              stored in the array and it can be assembled in the same  way  on
              any  host.   If an array is assembled with this option, then the
              homehost recorded on the array will be ignored.

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

              This  functionality  is  currently only provided by --detail and

              specifies the cluster name for the md device. The md device  can
              be  assembled  only on the cluster which matches the name speci-
              fied. If this option is not provided, mdadm tries to detect  the
              cluster name automatically.

For create, build, or grow:
       -n, --raid-devices=
              Specify  the  number of active devices in the array.  This, plus
              the number of spare devices (see below) must equal the number of
              component-devices  (including "missing" devices) that are listed
              on the command line for --create.  Setting a value of 1 is prob-
              ably  a mistake and so requires that --force be specified first.
              A value of 1 will then be allowed for linear,  multipath,  RAID0
              and RAID1.  It is never allowed for RAID4, RAID5 or RAID6.
              This  number  can only be changed using --grow for RAID1, RAID4,
              RAID5 and RAID6 arrays, and only on kernels  which  provide  the
              necessary support.

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

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

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

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

              This value can be set with --grow for RAID level 1/4/5/6  though
              CONTAINER  based arrays such as those with IMSM metadata may not
              be able to support this.  If the array was created with  a  size
              smaller than the currently active drives, the extra space can be
              accessed using --grow.  The size can be given as max which means
              to choose the largest size that fits on all current drives.

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

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

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

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

              Setting the array-size causes the array  to  appear  smaller  to
              programs  that access the data.  This is particularly needed be-
              fore reshaping an array so that it will be smaller.  As the  re-
              shape  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 re-
              quirement.  If there are problems the array can be  made  bigger
              again with no loss with another --grow --array-size= command.

              A  suffix of 'K', 'M' or 'G' can be given to indicate Kilobytes,
              Megabytes or Gigabytes respectively.  A value  of  max  restores
              the apparent size of the array to be whatever the real amount of
              available space is.

              Clustered arrays do not support this parameter yet.

       -c, --chunk=
              Specify chunk size of kilobytes.  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 'K', 'M' or 'G' can be given to indicate Kilobytes,
              Megabytes or Gigabytes respectively.

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

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

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

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

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

       -p, --layout=
              This  option  configures  the  fine  details  of data layout for
              RAID5, RAID6, and RAID10 arrays, and controls the failure  modes
              for faulty.  It can also be used for working around a kernel bug
              with RAID0, but generally doesn't need to be used explicitly.

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

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

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

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

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

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

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

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

              The layout options for RAID10 are one of 'n', 'o'  or  'f'  fol-
              lowed  by  a  small number.  The default is 'n2'.  The supported
              options are:

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

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

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

              The number is the number of copies of each datablock.  2 is nor-
              mal, 3 can be useful.  This number can be at most equal  to  the
              number  of  devices  in  the  array.  It does not need to divide
              evenly into that number (e.g. it is perfectly legal to  have  an
              'n2' layout for an array with an odd number of devices).

              A  bug introduced in Linux 3.14 means that RAID0 arrays with de-
              vices of differing sizes started using a different layout.  This
              could  lead  to  data  corruption.  Since Linux 5.4 (and various
              stable releases that received backports), the  kernel  will  not
              accept  such  an array unless a layout is explictly set.  It can
              be set to 'original' or 'alternate'.  When creating a new array,
              mdadm  will select 'original' by default, so the layout does not
              normally need to be set.  An array created for either 'original'
              or  'alternate'  will not be recognized by an (unpatched) kernel
              prior to 5.4.  To create a RAID0 array with devices of differing
              sizes  that can be used on an older kernel, you can set the lay-
              out to 'dangerous'.  This will use whichever layout the  running
              kernel  supports,  so  the  data on the array may become corrupt
              when changing kernel from pre-3.14 to a later kernel.

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

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

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

       -b, --bitmap=
              Specify  a  file  to  store  a write-intent bitmap in.  The file
              should not exist unless --force is also given.   The  same  file
              should  be  provided when assembling the array.  If the word in-
              ternal 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. If the word clustered is given, the array is created
              for a clustered environment. One bitmap is created for each node
              as defined by the --nodes parameter and are stored internally.

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

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

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

              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  re-
              quires 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 'K', 'M' or 'G' can be given to indicate Kilobytes,
              Megabytes or Gigabytes respectively.

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

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

              subsequent devices listed in a --create or --add command will be
              flagged as  'failfast'.  This is  valid  for  RAID1  and  RAID10
              only.   IO  requests to these devices will be encouraged to fail
              quickly rather than cause long delays  due  to  error  handling.
              Also no attempt is made to repair a read error on these devices.

              If  an  array  becomes degraded so that the 'failfast' device is
              the only usable device, the 'failfast' flag will then be ignored
              and extended delays will be preferred to complete failure.

              The 'failfast' flag is appropriate for storage arrays which have
              a low probability of true failure, but which may sometimes cause
              unacceptable delays due to internal maintenance functions.

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

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

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

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

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

              Setting the offset explicitly over-rides the default.  The value
              given is in Kilobytes unless a suffix of 'K', 'M' or 'G' is used
              to explicitly indicate Kilobytes, Megabytes or Gigabytes respec-

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

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

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

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

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

       -f, --force
              Insist that mdadm accept the geometry and layout specified with-
              out question.  Normally mdadm will not allow creation of an  ar-
              ray  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. It works with Create, Assemble, Manage
              and Misc mode.

       -a, --auto{=yes,md,mdp,part,p}{NN}
              Instruct mdadm how to create the device file if needed, possibly
              allocating an unused minor number.  "md" causes a non-partition-
              able array to be used (though since Linux  2.6.28,  these  array
              devices are in fact partitionable).  "mdp", "part" or "p" causes
              a partitionable array (2.6 and later) to  be  used.   "yes"  re-
              quires  the named md device to have a 'standard' format, and the
              type and minor number will be determined from this.  With  mdadm
              3.0,  device creation is normally left up to udev so this option
              is unlikely to be needed.  See DEVICE NAMES below.

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

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

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

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

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

              This is meaningful with --create or --build.

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

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

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

              Only works when the array is for clustered environment. It spec-
              ifies  the  maximum number of nodes in the cluster that will use
              this device simultaneously. If not specified, this  defaults  to

              Specify journal device for the RAID-4/5/6 array. The journal de-
              vice should be a SSD with reasonable lifetime.

              Auto creation of symlinks in /dev to /dev/md, option  --symlinks
              must be 'no' or 'yes' and work with --create and --build.

       -k, --consistency-policy=
              Specify  how  the  array  maintains consistency in case of unex-
              pected shutdown.  Only relevant for RAID levels with redundancy.
              Currently supported options are:

              resync Full  resync  is performed and all redundancy is regener-
                     ated when the array is started after unclean shutdown.

              bitmap Resync assisted by a write-intent bitmap. Implicitly  se-
                     lected when using --bitmap.

                     For  RAID  levels  4/5/6,  journal  device is used to log
                     transactions and replay after unclean  shutdown.  Implic-
                     itly selected when using --write-journal.

              ppl    For  RAID5  only, Partial Parity Log is used to close the
                     write hole and eliminate resync. PPL  is  stored  in  the
                     metadata  region  of  RAID  member  drives, no additional
                     journal drive is needed.

              Can be used with --grow to change the consistency policy  of  an
              active  array  in some cases. See CONSISTENCY POLICY CHANGES be-

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 mi-
              nor 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 assem-
              bled.  e.g. when  assembling  /dev/md0,  --super-minor=dev  will
              look for super blocks with a minor number of 0.

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

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

       -f, --force
              Assemble the array even if the metadata on some devices  appears
              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  at-
              tempt will be made to start it anyway.

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

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

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

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

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

       -U, --update=
              Update the superblock on each device while assembling the array.
              The  argument  given  to  this flag can be one of sparc2.2, sum-
              maries, uuid, name, nodes, homehost, home-cluster, resync, byte-
              order,  devicesize,  no-bitmap,  bbl,  no-bbl, ppl, no-ppl, lay-
              out-original, layout-alternate, 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 ker-
              nel.  This kernel got the alignment of part  of  the  superblock
              wrong.   You can use the --examine --sparc2.2 option to mdadm to
              see what effect this would have.

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

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

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

              The nodes option will change the nodes of the array as stored in
              the  bitmap  superblock.  This option only works for a clustered

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

              The home-cluster option will change the cluster name as recorded
              in the superblock and bitmap. This option only works  for  clus-
              tered environment.

              The  resync option will cause the array to be marked dirty mean-
              ing that any redundancy in the array  (e.g.  parity  for  RAID5,
              copies  for  RAID1)  may be incorrect.  This will cause the RAID
              system to perform a "resync" pass to make sure that  all  redun-
              dant information is correct.

              The  byteorder option allows arrays to be moved between machines
              with different byte-order, such as  from  a  big-endian  machine
              like  a  Sparc  or some MIPS machines, to a little-endian x86_64
              machine.  When assembling such an array for the first time after
              a move, giving --update=byteorder will cause mdadm to expect su-
              perblocks 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  su-
              perblock.  That is the counts of total, working, active, failed,
              and spare devices.

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

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

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

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

              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  en-
              tries,  this  will  fail,  as removing the list could cause data

              The ppl option will enable PPL for a  RAID5  array  and  reserve
              space  for  PPL  on each device. There must be enough free space
              between the data and superblock and  a  write-intent  bitmap  or
              journal must not be used.

              The no-ppl option will disable PPL in the superblock.

              The  layout-original  and layout-alternate options are for RAID0
              arrays in use before Linux 5.4.  If the  array  was  being  used
              with  Linux 3.13 or earlier, then to assemble the array on a new
              kernel, --update=layout-original must be given.   If  the  array
              was created and used with a kernel from Linux 3.14 to Linux 5.3,
              then --update=layout-alternate must be given.  This  only  needs
              to  be given once.  Subsequent assembly of the array will happen
              normally.  For more information, see md(4).

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

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

              See this option under Create and Build options.

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 re-
              questing an operation on the array.  --test will report  failure
              if these specifiers didn't find any match.

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

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

              re-add  a  device that was 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 de-
              vice 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 devices.

              If  the device name given is faulty then mdadm will find all de-
              vices in the array that are marked faulty, remove them  and  at-
              tempt 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 re-
              move a set as describe below under --fail.

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

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

              same as --fail.

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

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

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

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

              Confirm the existence of the device. This is issued in  response
              to  an  --add request by a node in a cluster. When a node adds a
              device it sends a message to all nodes in the  cluster  to  look
              for a device with a UUID. This translates to a udev notification
              with the UUID of the device to be added and the slot number. The
              receiving node must acknowledge this message with --cluster-con-
              firm. Valid arguments are <slot>:<devicename> in case the device
              is found or <slot>:missing in case the device is not found.

              Add  journal  to  an  existing  array,  or  recreate journal for
              RAID-4/5/6 array that lost a journal device. To avoid interrupt-
              ing on-going write opertions, --add-journal only works for array
              in Read-Only state.

              Subsequent devices that are added  or  re-added  will  have  the
              'failfast'  flag  set.   This is only valid for RAID1 and RAID10
              and means that the 'md' driver will avoid long timeouts on error
              handling where possible.

              Subsequent  devices  that  are re-added will be re-added without
              the 'failfast' flag set.

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

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

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

For Misc mode:
       -Q, --query
              Examine a device to see (1) if it is an md device and (2) if  it
              is  a  component of an md array.  Information about what is dis-
              covered is presented.

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

              Print details of the platform's RAID  capabilities  (firmware  /
              hardware  topology) for a given metadata format. If used without
              argument, mdadm will scan all controllers looking for their  ca-
              pabilities.  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 --in-
              cremental output will be formatted as key=value pairs  for  easy
              import into the environment.

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

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

              If an array was created on a SPARC machine with a 2.2 Linux ker-
              nel  patched  with  RAID  support, the superblock will have been
              created incorrectly, or at least incompatibly with 2.4 and later
              kernels.   Using the --sparc2.2 flag with --examine will fix the
              superblock before displaying it.  If  this  appears  to  do  the
              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 in-
              ternal bitmap.  Note that running this on an array device  (e.g.
              /dev/md0) does not report the bitmap for that array.

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


              Save  metadata from lists devices, or restore metadata to listed

       -R, --run
              start a partially assembled array.  If --assemble did  not  find
              enough  devices  to  fully  start the array, it might leaving it
              partially assembled.  If you wish, you can  then  use  --run  to
              start the array in degraded mode.

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

       -o, --readonly
              mark array as readonly.

       -w, --readwrite
              mark array as readwrite.

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

              Note:  Be careful to call --zero-superblock with clustered raid,
              make sure array isn't used or assembled in  other  cluster  node
              before execute it.

              If the device is a container and the argument to --kill-subarray
              specifies an inactive subarray in the container, then the subar-
              ray  is  deleted.   Deleting all subarrays will leave an 'empty-
              container' or spare superblock on the  drives.   See  --zero-su-
              perblock  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 sub-

              If the device is a container and the argument to --update-subar-
              ray  specifies  a subarray in the container, then attempt to up-
              date 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 re-
              flect the status of the device.  See below in MISC MODE for  de-

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

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

              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  SCRUB-
              BING 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 au-
              tomatically added to the same array.  This allows the failed de-
              vice  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 script.

For Monitor mode:
       -m, --mail
              Give a mail address to send alerts to.

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

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

       -d, --delay
              Give a delay in seconds.  mdadm polls the  md  arrays  and  then
              waits this many seconds before polling again.  The default is 60
              seconds.  Since 2.6.16, there is no need to reduce this  as  the
              kernel alerts mdadm immediately when there is any change.

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

       -f, --daemonise
              Tell mdadm to run as a background daemon if it decides to  moni-
              tor  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 dae-
              mon process to the specified file, instead  of  printing  it  on
              standard output.

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

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

              This  inhibits  the  functionality for moving spares between ar-
              rays.  Only one monitoring process started with --scan but with-
              out this flag is allowed, otherwise the two could interfere with
              each other.

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

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

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

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

       In  the first usage example (without the --scan) the first device given
       is the md device.  In the second usage example, all devices listed  are
       treated  as  md devices and assembly is attempted.  In the third (where
       no devices are listed) all md devices that are listed in the configura-
       tion  file are assembled.  If no arrays are described by the configura-
       tion file, then any arrays that can be found on unused devices will  be

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

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

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

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

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

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

       If udev is active, mdadm does not create any entries in /dev but leaves
       that to udev.  It does record information in /run/mdadm/map which  will
       allow udev to choose the correct name.

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

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

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

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

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

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

       If no arrays are listed in the config (other  than  those  marked  <ig-
       nore>)  it  will look through the available devices for possible arrays
       and will try to assemble anything that  it  finds.   Arrays  which  are
       tagged as belonging to the given homehost will be assembled and started
       normally.  Arrays which do not obviously belong to this host are  given
       names  that  are  expected not to conflict with anything local, and are
       started "read-auto" so that nothing is written to any device until  the
       array is written to. i.e.  automatic resync etc is delayed.

       If  mdadm  finds a consistent set of devices that look like they should
       comprise an array, and if the superblock is tagged as belonging to  the
       given  home host, it will automatically choose a device name and try to
       assemble the array.  If the array uses version-0.90 metadata, then  the
       minor  number as recorded in the superblock is used to create a name in
       /dev/md/ so for example /dev/md/3.  If the array uses  version-1  meta-
       data,  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  con-
       figuration  file.   This  line can indicate that specific metadata type
       should, or should not, be automatically  assembled.   If  an  array  is
       found  which is not listed in mdadm.conf and has a metadata format that
       is denied by the AUTO line, then it will not be  assembled.   The  AUTO
       line  can  also  request  that  all arrays identified as being for this
       homehost should be assembled regardless of their  metadata  type.   See
       mdadm.conf(5) for further details.

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

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

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

       The  level  may  raid0, linear, raid1, raid10, multipath, or faulty, or
       one of their synonyms.  All devices must be listed and the  array  will
       be  started  once  complete.  It will often be appropriate to use --as-
       sume-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  su-
       perblocks or filesystems.  They are also checked to see if the variance
       in device size exceeds 1%.

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

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

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

       When creating an array with version-1 metadata a name for the array  is
       required.   If  this  is  not  given with the --name option, mdadm will
       choose a name based on the last component of the name of the device be-
       ing  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

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

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

       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 au-
       tomatically be added unless some other option is  explicitly  requested
       with  the --bitmap option or a different consistency policy is selected
       with the --consistency-policy option. In any case space  for  a  bitmap
       will  be  reserved  so  that  one can be added later with --grow --bit-

       If the metadata type supports it (currently only  1.x  and  IMSM  meta-
       data),  space will be allocated to store a bad block list.  This allows
       a modest number of bad blocks to be recorded, allowing the drive to re-
       main 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  for-
       mer case gives control over which devices in the container will be used
       for the array.  The latter case allows mdadm  to  automatically  choose
       which devices to use based on how much spare space is available.

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

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

              start the array in readonly mode.

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

       This usage will allow individual devices in an array to be failed,  re-
       moved  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 dis-
       tinct devices.  The operations are:

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

              The  device should be an active md device.  mdadm will display a
              detailed description of the array.  --brief or --scan will cause
              the output to be less detailed and the format to be suitable for
              inclusion in mdadm.conf.  The exit status of mdadm will normally
              be 0 unless mdadm failed to get useful information about the de-
              vice(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 un-

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

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

              0      metadata  successfully enumerated its platform components
                     on this system

              1      metadata is platform independent

              2      metadata failed to find its platform components  on  this

              If the device is a container and the argument to --update-subar-
              ray specifies a subarray in the container, then attempt  to  up-
              date the given superblock field in the subarray.  Similar to up-
              dating an array in "assemble" mode, the field to update  is  se-
              lected  by  -U  or  --update=  option. The supported options are
              name, ppl and no-ppl.

              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 ppl and no-ppl options enable and disable PPL in  the  meta-
              data. Currently supported only for IMSM subarrays.

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

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

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

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

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

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

              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 con-
              tains valid metadata.

       --stop The devices should be active md arrays  which  will  be  deacti-
              vated, 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 oper-
              ation  to  be applied to all arrays listed in /proc/mdstat.  For
              --examine, --scan causes all devices listed in the  config  file
              to be examined.

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

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

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

       As well as reporting events, mdadm may move a spare drive from one  ar-
       ray 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  de-
       vices that appear in /proc/mdstat will also be monitored.

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

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

       If  --scan is given, then a program or an E-mail address must be speci-
       fied on the command line or in the config file.  If neither are  avail-
       able, then mdadm will not monitor anything.  Without --scan, mdadm will
       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 Lin-
                  ear,  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 oper-
                  ations 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 be-
                  cause it finished normally or was aborted. (syslog priority:

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

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

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

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

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

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

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

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

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

       Each event has an associated array device (e.g.  /dev/md1) and possibly
       a  second  device.  For Fail, FailSpare, and SpareActive the second de-
       vice 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 ar-
       rays need to be labeled with the same spare-group or the spares must be
       allowed to migrate through matching POLICY domains in the configuration
       file.  The spare-group name can be any string;  it  is  only  necessary
       that different spare groups use different names.

       When  mdadm detects that an array in a spare group has fewer active de-
       vices 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  re-
       move  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 ar-
       ray.  For this to work, the kernel must support the  necessary  change.
       Various types of growth are being added during 2.6 development.

       Currently the supported changes include

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

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

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

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

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

       o   change the array's consistency policy.

       Using GROW on containers is currently supported only for  Intel's  IMSM
       container  format.   The  number  of  devices in a container can be in-
       creased - which affects all arrays in the container - or an array in  a
       container  can  be converted between levels where those levels are sup-
       ported 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.


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

       o   Roaming  between  Windows(R) and Linux systems for IMSM metadata is
           not supported during grow process.

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

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

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

       Note: --grow --size is not yet supported for external file bitmap.

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

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

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

       Changing the number of active devices in a RAID5 or RAID6 is much  more
       effort.  Every block in the array will need to be read and written back
       to a new location.  From 2.6.17, the Linux Kernel is able  to  increase
       the number of devices in a RAID5 safely, including restarting an inter-
       rupted "reshape".  From 2.6.31, the Linux Kernel is able to increase or
       decrease the number of devices in a RAID5 or RAID6.

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

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

       When relocating the first few stripes on a RAID5 or RAID6,  it  is  not
       possible  to  keep  the  data  on disk completely consistent and crash-
       proof.  To provide the required safety, mdadm disables  writes  to  the
       array  while this "critical section" is reshaped, and takes a backup of
       the data that is in that section.  For grows, this backup may be stored
       in  any spare devices that the array has, however it can also be stored
       in a separate file specified with the --backup-file option, and is  re-
       quired  to  be  specified  for  shrinks,  RAID level changes and layout
       changes.  If this option is used, and the system does crash during  the
       critical  period, the same file must be passed to --assemble to restore
       the backup and reassemble the array.  When shrinking rather than  grow-
       ing  the array, the reshape is done from the end towards the beginning,
       so the "critical section" is at the end of the reshape.

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

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

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

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

       The consistency policy of an active array can be changed by  using  the
       --consistency-policy option in Grow mode. Currently this works only for
       the ppl and resync policies and allows to enable or disable  the  RAID5
       Partial Parity Log (PPL).

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

       Usage: mdadm --incremental --fail component-device

       Usage: mdadm --incremental --rebuild-map

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

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

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

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

       mdadm performs a number of tests to determine if the device is part  of
       an  array, and which array it should be part of.  If an appropriate ar-
       ray 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 re-
       quires 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 de-
              fault it to allow any device.  Similarly 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 $env{DEVLINKS}.

       +      Does the device have a valid md superblock?  If a specific meta-
              data version is requested with --metadata or -e then  only  that
              style  of  metadata is accepted, otherwise mdadm finds any known
              version of metadata.  If no md metadata is found, the device may
              be still added to an array as a spare if POLICY allows.

       mdadm  keeps  a  list  of  arrays  that  it  has partially assembled in
       /run/mdadm/map.  If no array exists which matches the metadata  on  the
       new  device,  mdadm must choose a device name and unit number.  It does
       this based on any name given in  mdadm.conf  or  any  name  information
       stored in the metadata.  If this name suggests a unit number, that num-
       ber will be used, otherwise a free unit number will  be  chosen.   Nor-
       mally mdadm will prefer to create a partitionable array, however if the
       CREATE line in mdadm.conf suggests that a  non-partitionable  array  is
       preferred, that will be honoured.

       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  nor-
       mally 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  ar-
       ray.   For  a clean RAID5, the array will be started as soon as all but
       one drive is present.

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

       This section describes environment variables that affect how mdadm  op-

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

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

              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  operat-
              ing  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  functional-
              ity 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 com-
              promised 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 complain 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 example
                  export MDADM_CONF_AUTO='-ddf -imsm'
              will make sure that mdadm does not  automatically  assemble  any
              DDF  or  IMSM arrays that are found.  This can be useful on sys-
              tems 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 cur-
       rently in use).  This will typically go in a system shutdown script.

         mdadm --follow --scan --delay=120

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

         mdadm --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 ac-
       tive 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 con-
       tain unwanted detail.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

         mdadm --help
       Provide general help.

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

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

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

       When --incremental mode is used, this file gets a list of  arrays  cur-
       rently being created.

       mdadm understand two sorts of names for array devices.

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

       The second sort can be freely chosen,  but  must  reside  in  /dev/md/.
       When  giving a device name to mdadm to create or assemble an array, ei-
       ther 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 as-
       sembly, it will sometimes add a small sequence number to the end of the
       name  to  avoid  conflicted  between multiple arrays that have the same
       name.  If mdadm can reasonably determine that the array really is meant
       for this host, either by a hostname in the metadata, or by the presence
       of the array in mdadm.conf, then it will leave off the suffix if possi-
       ble.  Also if the homehost is specified as <ignore> mdadm will only use
       a suffix if a different array of the same name  already  exists  or  is
       listed in the config file.

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


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


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

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

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


       where XXX is any string.  These names are supported by mdadm since ver-
       sion 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 Ostergaard's Software-RAID.HOWTO)

       The latest version of mdadm should always be available from


       Related man pages:

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

v4.1-rc2                                                              MDADM(8)
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