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
       devices in a 5-device set might form a RAID1 using the  whole  devices.
       The  remaining  three  might  have  a RAID5 over the first half of each
       device, and a RAID0 over the second half.

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

       mdadm has several major modes of operation:

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

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

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

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

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

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

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

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

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

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

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

       -C, --create
              Create a new array.

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

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

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

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

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

       -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
              devices from /proc/mdstat.

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

              Options are:

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

              1, 1.0, 1.1, 1.2 default
                     Use the new version-1 format superblock.  This has  fewer
                     restrictions.   It can easily be moved between hosts with
                     different endian-ness, and a recovery  operation  can  be
                     checkpointed  and  restarted.  The different sub-versions
                     store  the  superblock  at  different  locations  on  the
                     device,  either  at  the end (for 1.0), at the start (for
                     1.1) or 4K from the start (for 1.2).  "1"  is  equivalent
                     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
              array.  Spares can also be added and removed later.  The  number
              of  component  devices listed on the command line must equal the
              number of RAID devices plus the number of spare devices.

       -z, --size=
              Amount (in 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
              requirement.  If there are problems the array can be made bigger
              again with no loss with another --grow --size= command.

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

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

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

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

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

              A  suffix of '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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -b, --bitmap=
              Specify  a  file  to  store  a write-intent bitmap in.  The file
              should not exist unless --force is also given.   The  same  file
              should  be  provided  when  assembling  the  array.  If the word
              internal is given, then the bitmap is stored with  the  metadata
              on  the array, and so is replicated on all devices.  If the word
              none is given with --grow mode, then any bitmap that is  present
              is removed. 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
              requires  no more than 2^21 chunks.  When using an internal bit-
              map, 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

              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
              required --backup-file= option to be  set,  continuation  option
              will require to have exactly the same backup file given as well.

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

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

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

       -f, --force
              Insist that mdadm accept the geometry and layout specified with-
              out question.  Normally mdadm will  not  allow  creation  of  an
              array with only one device, and will try to create a RAID5 array
              with one missing drive (as this makes the  initial  resync  work
              faster).  With --force, mdadm will not try to be so clever.

       -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"
              requires  the  named  md device to have a 'standard' format, and
              the type and minor number will be determined  from  this.   With
              mdadm  3.0,  device creation is normally left up to udev so this
              option is unlikely to be needed.  See DEVICE NAMES below.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -U, --update=
              Update the superblock on each device while assembling the array.
              The argument given to this flag can be  one  of  sparc2.2,  sum-
              maries, uuid, name, nodes, homehost, home-cluster, resync, byte-
              order, devicesize, no-bitmap, bbl, no-bbl,  ppl,  no-ppl,  meta-
              data, or super-minor.

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

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

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

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

              The 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
              updating  the  UUID.   For  version-1 superblocks, this involves
              updating 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
              superblocks  to  have their byteorder reversed, and will correct
              that order before assembling the array.  This is only valid with
              original (Version 0.90) superblocks.

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

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

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

              The old metadata will remain on the  devices,  but  will  appear
              older  than the new metadata and so will usually be ignored. The
              old metadata (or indeed the new metadata) can be removed by 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

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

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

              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.

              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
              requesting an operation on the array.  --test will report  fail-
              ure 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
              device will be added back to the array  in  the  same  position.
              This  will  normally cause the data for that device to be recov-
              ered.  However based on the  event  count  on  the  device,  the
              recovery  may  only  require  sections that are flagged a write-
              intent bitmap to be recovered or may not require any recovery at

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

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

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

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

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

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

              As well as the name of a device file (e.g.  /dev/sda1) the words
              failed, detached and names like set-A can be given to  --remove.
              The  first  causes  all failed device to be removed.  The second
              causes any device which is no longer  connected  to  the  system
              (i.e  an  'open'  returns  ENXIO) to be removed.  The third will
              remove a set as describe below under --fail.

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

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

              same as --fail.

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

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

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

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

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

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

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

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

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

              Print  details  of  the platform's RAID capabilities (firmware /
              hardware topology) for a given metadata format. If used  without
              argument,  mdadm  will  scan  all  controllers looking for their
              capabilities. Otherwise, mdadm will only look at the  controller
              specified  by  the argument in form of an absolute filepath or a
              link, e.g.  /sys/devices/pci0000:00/0000:00:1f.2.

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

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

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

              If an array was created on a SPARC machine with a 2.2 Linux 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
              internal  bitmap.   Note  that  running  this on an array device
              (e.g.  /dev/md0) does not report the bitmap for that array.

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


              Save metadata from lists devices, or restore metadata to  listed

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

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

       -o, --readonly
              mark array as readonly.

       -w, --readwrite
              mark array as readwrite.

              If the device contains a valid md superblock, the block is 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-superblock for completely removing  a  superblock.   Note
              that  some formats depend on the subarray index for generating a
              UUID, this command will fail if it would change the UUID  of  an
              active subarray.

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

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

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

              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
              automatically  added  to the same array.  This allows the failed
              device to be automatically replaced  by  a  new  device  without
              metadata  if it appears at specified path.   This option is nor-
              mally only set by a udev script.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       If  no  arrays  are  listed  in  the  config  (other  than those marked
       <ignore>) it will look  through  the  available  devices  for  possible
       arrays  and  will try to assemble anything that it finds.  Arrays which
       are tagged as belonging to the given homehost  will  be  assembled  and
       started  normally.   Arrays  which do not obviously belong to this host
       are given names that are expected not to conflict with anything  local,
       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

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

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

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

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

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

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

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

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

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

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

       When creating a partition based array,  using  mdadm  with  version-1.x
       metadata, the partition type should be set to 0xDA (non fs-data).  This
       type selection allows for greater precision since using any other [RAID
       auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create 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
       automatically 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
       remain in service while only partially functional.

       When creating an array within a CONTAINER mdadm can be given either the
       list  of devices to use, or simply the name of the container.  The 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,
       removed or added.  It is possible to perform multiple  operations  with
       on command.  For example:
         mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
       will  firstly mark /dev/hda1 as faulty in /dev/md0 and will then remove
       it from the array and finally add it back in as a spare.  However  only
       one md array can be affected by a single command.

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

       Usage: mdadm options ...  devices ...

       MISC mode includes a number of distinct operations that operate on 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
              device(s); however, if the --test option is given, then the exit
              status will be:

              0      The array is functioning normally.

              1      The array has at least one failed device.

              2      The array has multiple failed devices  such  that  it  is

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

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

              0      metadata successfully enumerated its platform  components
                     on this system

              1      metadata is platform independent

              2      metadata  failed  to find its platform components on this

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

              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.
              Using --brief with --verbose gives an intermediate level of ver-

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       For mdadm to move spares from  one  array  to  another,  the  different
       arrays  need to be labeled with the same spare-group or the spares must
       be allowed to migrate through matching POLICY domains in the configura-
       tion  file.   The spare-group name can be any string; it is only neces-
       sary that different spare groups use different names.

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

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

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

       Currently the supported changes include

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

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

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

       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
       required to be specified for shrinks, RAID  level  changes  and  layout
       changes.   If this option is used, and the system does crash during the
       critical period, the same file must be passed to --assemble to  restore
       the  backup and reassemble the array.  When shrinking rather than grow-
       ing the array, the reshape is done from the end towards the  beginning,
       so the "critical section" is at the end of the reshape.

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

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

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

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

       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

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

       Usage: mdadm --incremental --rebuild-map

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

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

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

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

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

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

       The tests that mdadm makes are as follow:

       +      Is the device permitted by mdadm.conf?  That is, is it listed in
              a DEVICES line in that file.  If  DEVICES  is  absent  then  the
              default  it  to allow any device.  Similarly if DEVICES contains
              the special word partitions then any device is allowed.   Other-
              wise  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
       array.  For a clean RAID5, the array will be started as soon as all but
       one drive is present.

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

       This  section  describes  environment  variables  that affect how mdadm

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

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

              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
       active  arrays that are known to be made from partitions of IDE or SCSI
       drives.  This file should be reviewed before being used as it may  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

         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,
       either full path name such as /dev/md0 or /dev/md/home can be given, or
       just the suffix of the second sort of name, such as home can be given.

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

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


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


       Partition numbers should be indicated by 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-rc1                                                              MDADM(8)
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