Provided by: mdadm_4.2-0ubuntu2_amd64 bug


       mdadm - manage MD devices aka Linux Software RAID


       mdadm [mode] <raiddevice> [options] <component-devices>


       RAID devices are virtual devices created from two or more real block devices.  This allows
       multiple devices (typically disk drives or partitions  thereof)  to  be  combined  into  a
       single  device  to  hold  (for  example)  a  single  filesystem.  Some RAID levels include
       redundancy and so can survive some degree of device failure.

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

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

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

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

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

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


       mdadm has several major modes of operation:

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

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

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

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

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


Options for selecting a mode are:

       -A, --assemble
              Assemble a pre-existing array.

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

       -C, --create
              Create a new array.

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

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

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

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

       -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

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

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

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

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

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

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

              Options are:

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

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

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

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


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

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

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

              When using Auto-Assemble, only arrays tagged for the given homehost will be allowed
              to use 'local' names (i.e. not ending in '_' followed  by  a  digit  string).   See
              below under Auto 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 shortest.  When a path component is given with
              --prefer mdadm will prefer a longer  name  if  it  contains  that  component.   For
              example  --prefer=by-uuid  will  prefer a name in a subdirectory of /dev called by-

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

              specifies the cluster name for the md device. The md device can be  assembled  only
              on  the  cluster  which matches the name specified. 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
              probably  a  mistake and so requires that --force be specified first.  A value of 1
              will then be allowed for linear, multipath, RAID0 and RAID1.  It is  never  allowed
              for RAID4, RAID5 or RAID6.
              This  number  can  only  be  changed using --grow for RAID1, RAID4, RAID5 and RAID6
              arrays, and only on kernels which provide the necessary support.

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

       -z, --size=
              Amount (in 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. When specified as ¸max¸ (as it often  is)
              the  smallest  drive  (or  partition)  sets the size.  In that case, a warning will
              follow if the drives, as a group, have sizes that differ by more than one percent.

              A suffix of 'K', 'M', 'G' or 'T' can be given  to  indicate  Kilobytes,  Megabytes,
              Gigabytes or Terabytes respectively.

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

              This  value can be set with --grow for RAID level 1/4/5/6 though DDF arrays 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

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

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

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

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

              Setting the array-size causes the array to appear smaller to programs  that  access
              the data.  This is particularly needed before reshaping an array so that it will be
              smaller.  As the reshape is not reversible, but setting the size with  --array-size
              is,  it is required that the array size is reduced as appropriate before the number
              of devices in the array is 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', 'G' or 'T' can be given to indicate Kilobytes, Megabytes,
              Gigabytes or Terabytes 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', 'G' or 'T' can be given to indicate Kilobytes, Megabytes,
              Gigabytes or Terabytes 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

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

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

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

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

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

              When  setting  the failure mode for level faulty, the options are: write-transient,
              wt, read-transient,  rt,  write-persistent,  wp,  read-persistent,  rp,  write-all,
              read-fixable, rf, clear, flush, none.

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

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

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

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

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

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

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

              The number is the number of copies of each  datablock.   2  is  normal,  3  can  be
              useful.   This  number  can be at most equal to the number of devices in the array.
              It does not need to divide evenly into that number (e.g. it is perfectly  legal  to
              have an 'n2' layout for an array with an odd number of devices).

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

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

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

              Set the chunksize of the bitmap.  Each bit corresponds to that  many  Kilobytes  of
              storage.   When  using a file based bitmap, the default is to use the smallest size
              that is at-least 4 and requires no more than 2^21 chunks.  When using  an  internal
              bitmap,  the  chunksize defaults to 64Meg, or larger if necessary to fit the bitmap
              into the available space.

              A suffix of 'K', 'M', 'G' or 'T' can be given  to  indicate  Kilobytes,  Megabytes,
              Gigabytes or Terabytes respectively.

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

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

              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 actually write to the array.  It can also be used when creating
              a RAID1 or RAID10 if you want to avoid the initial resync, however this practice  —
              while  normally  safe  — is not recommended.  Use this only if you really know what
              you are doing.

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

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

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

              Arrays  with  1.x  metadata can leave a gap between the start of the device and the
              start of array data.  This gap can be used for various metadata.  The start of data
              is  known  as  the  data-offset.   Normally  an appropriate data offset is computed
              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 offset.

              Setting the offset explicitly over-rides  the  default.   The  value  given  is  in
              Kilobytes  unless  a  suffix of 'K', 'M', 'G' or 'T' is used to explicitly indicate
              Kilobytes, Megabytes, Gigabytes or Terabytes respectively.

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

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

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

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

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

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

       -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

       -f, --force
              Insist  that  mdadm  accept  the  geometry  and  layout specified without question.
              Normally mdadm will not allow creation of an array with only one device,  and  will
              try  to  create  a  RAID5  array  with one missing drive (as this makes the initial
              resync work faster).  With --force, mdadm will not try to be so clever.

       -o, --readonly
              Start the array read only rather than read-write as  normal.   No  writes  will  be
              allowed to the array, and no resync, recovery, or reshape will be started. 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-partitionable array to be used (though
              since Linux 2.6.28, these array devices are in fact partitionable).  "mdp",  "part"
              or "p" causes a partitionable array (2.6 and later) to be used.  "yes" requires the
              named md device to have a 'standard' format, and the type and minor number will  be
              determined  from this.  With mdadm 3.0, device creation is normally left up to udev
              so this option is unlikely to be needed.  See DEVICE NAMES below.

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

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

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

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

              If  the md device name is in a 'standard' format as described in DEVICE NAMES, then
              it will be created, if necessary, with the appropriate device number based on  that
              name.   If  the  device  name  is not in one of these formats, then a unused device
              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 "/dev/md/".

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

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

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

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

              Only  works  when  the array is for clustered environment. It specifies the maximum
              number of nodes in the cluster that will use this  device  simultaneously.  If  not
              specified, this defaults to 4.

              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 unexpected shutdown.  Only
              relevant for RAID levels with redundancy.  Currently supported options are:

              resync Full resync is performed and all redundancy is regenerated when the array is
                     started after unclean shutdown.

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

                     For RAID levels 4/5/6, journal device is used to log transactions and replay
                     after unclean shutdown. Implicitly 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 below.

For assemble:

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

       -m, --super-minor=
              Minor number of device that array was created for.  Devices which don't  have  this
              minor  number  are  excluded.   If  you  create  an  array  as  /dev/md1,  then all
              superblocks will contain the minor number 1, even if the array is  later  assembled
              as /dev/md2.

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

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

       -N, --name=
              Specify the name of the array  to  assemble.   This  must  be  the  name  that  was
              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. This works only for native. For  external
              metadata  it  allows to start dirty degraded RAID 4, 5, 6.  An array which requires
              --force to be started may contain data corruption.  Use it carefully.

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

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

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

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

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

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

       -U, --update=
              Update the superblock on each device while  assembling  the  array.   The  argument
              given  to this flag can be one of sparc2.2, summaries, uuid, name, nodes, homehost,
              home-cluster, resync, byteorder, devicesize, no-bitmap, bbl, no-bbl,  ppl,  no-ppl,
              layout-original, layout-alternate, layout-unspecified, metadata, or super-minor.

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

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

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

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

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

              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 clustered environment.

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

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

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

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

              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 ignored.

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

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

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

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

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

              The  layout-unspecified  option   reverts   the   effect   of   layout-orignal   or
              layout-alternate  and  allows the array to be again used on a kernel prior to Linux
              5.3.  This option should be used with great caution.

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

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

              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  failure if these
              specifiers didn't find any match.

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

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

              re-add  a device that was previously removed from an array.  If the metadata on the
              device reports that it is a member of the array, and the slot that it used is still
              vacant, then the device will be added back to the array in the same position.  This
              will normally cause the data for that device to be recovered.  However based on the
              event  count on the device, the recovery may only require sections that are flagged
              by a write-intent bitmap to be recovered or may not require any recovery at all.

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

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

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

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

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

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

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

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

              For RAID10 arrays where the number of copies evenly divides the number of  devices,
              the  devices can be conceptually divided into sets where each set contains a single
              complete copy of the  data  on  the  array.   Sometimes  a  RAID10  array  will  be
              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 --detail.

              same as --fail.

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

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

              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-confirm.   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  interrupting 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

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

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

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

For Misc mode:

       -Q, --query
              Examine a device to see (1) if it is an md device and (2) if it is a  component  of
              an md array.  Information about what is discovered is presented.

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

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

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

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

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

              If  an  array  was  created on a SPARC machine with a 2.2 Linux kernel patched with
              RAID support, the superblock will  have  been  created  incorrectly,  or  at  least
              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

       -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 and IMSM metadata support bad-blocks lists.


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

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

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

       -o, --readonly
              mark array as readonly.

       -w, --readwrite
              mark array as readwrite.

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

              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 subarray is deleted.  Deleting all
              subarrays will leave an 'empty-container' or spare superblock on the  drives.   See
              --zero-superblock  for  completely  removing  a superblock.  Note that some formats
              depend on the subarray index for generating a UUID, this command will  fail  if  it
              would change the UUID of an active subarray.

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

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

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

              For each md device given, or each  device  in  /proc/mdstat  if  --scan  is  given,
              arrange  for  the  array to be marked clean as soon as possible.  mdadm will return
              with success if the array uses external metadata and we successfully  waited.   For
              native   arrays   this  returns  immediately  as  the  kernel  handles  dirty-clean
              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 SCRUBBING AND MISMATCHES.

For Incremental Assembly mode:

       --rebuild-map, -r
              Rebuild  the  map  file (/run/mdadm/map) that mdadm uses to help track which arrays
              are currently being assembled.

       --run, -R
              Run any array assembled as soon as a  minimal  number  of  devices  are  available,
              rather than waiting until all expected devices are present.

       --scan, -s
              Only  meaningful  with  -R  this  will  scan the map file for arrays that are being
              incrementally assembled and will try to start any that are not already started.  If
              any such array is listed in mdadm.conf as requiring an external bitmap, that bitmap
              will be attached first.

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

              Only  used  with --fail.  The 'path' given will be recorded so that if a new device
              appears at the same location it can be automatically added to the same array.  This
              allows  the  failed  device  to  be  automatically replaced by a new device without
              metadata if it appears at specified path.   This option is normally only set  by  a
              udev script.

For Monitor mode:

       -m, --mail
              Give a mail address to send alerts to.

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

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

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

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

       -f, --daemonise
              Tell mdadm to run as a background daemon if it decides to monitor  anything.   This
              causes  it  to fork and run in the child, and to disconnect from the terminal.  The
              process id of the child is written to stdout.  This is  useful  with  --scan  which
              will  only  continue  monitoring if a mail address or alert program is found in the
              config file.

       -i, --pid-file
              When mdadm is running in daemon mode, write the pid of the daemon  process  to  the
              specified file, instead of printing it on standard output.

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

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

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


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

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

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

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

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

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

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

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

       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

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

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

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

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

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

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

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

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

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

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

       If mdadm finds a consistent set of devices that look like they should comprise  an  array,
       and if the superblock is tagged as belonging to the given home host, it will automatically
       choose a device name and try to assemble  the  array.   If  the  array  uses  version-0.90
       metadata,  then the minor number as recorded in the superblock is used to create a name in
       /dev/md/ so for example /dev/md/3.  If the array uses version-1 metadata,  then  the  name
       from the superblock is used to similarly create a name in /dev/md/ (the name will have any
       'host' prefix stripped first).

       This behaviour can be modified by the AUTO line  in  the  mdadm.conf  configuration  file.
       This line can indicate that specific metadata type should, or should not, be automatically
       assembled.  If an array is found which is not listed in  mdadm.conf  and  has  a  metadata
       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  auto-


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

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

       The level may raid0, linear,  raid1,  raid10,  multipath,  or  faulty,  or  one  of  their
       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.

       The max length md-device name is limited to 32 characters.  Different metadata types  have
       more strict limitation (like IMSM where only 16 characters are allowed).  For that reason,
       long name could be truncated or rejected, it depends on metadata policy.

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

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

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

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

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

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

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

       If  the  array type supports a write-intent bitmap, and if the devices in the array exceed
       100G is size, an internal write-intent bitmap will  automatically  be  added  unless  some
       other  option  is explicitly requested with the --bitmap option 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 --bitmap=internal.

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

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

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

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

              start the array 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 distinct devices.   The
       operations are:

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

              The device  should  be  an  active  md  device.   mdadm  will  display  a  detailed
              description  of  the  array.   --brief  or  --scan will cause the output to be less
              detailed and the format to be suitable  for  inclusion  in  mdadm.conf.   The  exit
              status  of  mdadm  will normally be 0 unless mdadm failed to get useful information
              about the device(s); however, if the --test option is given, then the  exit  status
              will be:

              0      The array is functioning normally.

              1      The array has at least one failed device.

              2      The array has multiple failed devices such that it is unusable.

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

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

              0      metadata successfully enumerated its platform components on this system

              1      metadata is platform independent

              2      metadata failed to find its platform components on this system

              If the device is a container and the  argument  to  --update-subarray  specifies  a
              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, no-ppl,
              bitmap and no-bitmap.

              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 metadata. Currently
              supported only for IMSM subarrays.

              The bitmap and no-bitmap options enable and  disable  write-intent  bitmap  in  the
              metadata. 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 written in the file at the same locate that it exists in the
              device.  However the file will be "sparse" so that  only  those  blocks  containing
              metadata will be allocated. The total space used will be small.

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

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

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

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

       --stop The  devices  should be active md arrays which will be deactivated, as long as they
              are not currently in use.

       --run  This will fully activate a partially assembled md array.

              This will mark an active array as read-only, providing that  it  is  not  currently
              being used.

              This will change a readonly array back to being read/write.

       --scan For  all operations except --examine, --scan will cause the operation to be applied
              to all arrays listed in /proc/mdstat.  For --examine,  --scan  causes  all  devices
              listed in the config file to be examined.

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


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

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

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

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

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

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

       If  --scan  is given, then a program or an E-mail address must be specified on the command
       line or in the config file.  If  neither  are  available,  then  mdadm  will  not  monitor
       anything.   Without  --scan, mdadm will continue monitoring as long as something was found
       to monitor.  If no program or email is given, then each event is reported to stdout.

       The different events are:

                  An md array which previously was configured appears to no longer be configured.
                  (syslog priority: Critical)

                  If  mdadm  was  told to monitor an array which is RAID0 or Linear, then it will
                  report DeviceDisappeared with  the  extra  information  Wrong-Level.   This  is
                  because RAID0 and Linear do not support the device-failed, hot-spare and resync
                  operations which are monitored.

                  An md array started reconstruction  (e.g.  recovery,  resync,  reshape,  check,
                  repair). (syslog priority: Warning)

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

                  An md array that was rebuilding, isn't any more,  either  because  it  finished
                  normally or was aborted. (syslog priority: Warning)

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

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

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

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

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

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

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

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

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

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


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

       Currently the supported changes include

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

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

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

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

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

       •   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 supported by the container, and the conversion is on of those listed above.


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

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

       •   When growing a raid0 device, the new component disk size  (or  external  backup  size)
           should  be larger than LCM(old, new) * chunk-size * 2, where LCM() is the least common
           multiple of the old and new count of component disks, and "* 2" comes  from  the  fact
           that mdadm refuses to use more than half of a spare device for backup space.

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

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

       Also  the  size  of an array cannot be changed while it has an active bitmap.  If an array
       has a bitmap, it must be removed before the size  can  be  changed.  Once  the  change  is
       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 devices.

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

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

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

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

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

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

       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  "critical
       section" but throughout the reshape operation, as described below under LAYOUT CHANGES.

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

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

       A write-intent bitmap can be added to, or removed from, an active array.  Either  internal
       bitmaps,  or  bitmaps  stored  in  a  separate file, can be added.  Note that if you add a
       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
       one to enable or disable the RAID5 Partial Parity Log (PPL).


       Usage:  mdadm  --incremental  [--run]  [--quiet]  component-device  [optional-aliases-for-

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

       Usage: mdadm --incremental --rebuild-map

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

       This mode is designed to be used in  conjunction  with  a  device  discovery  system.   As
       devices  are  found  in  a  system,  they  can  be  passed  to  mdadm  --incremental to be
       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.  Otherwise the device name given to mdadm, or one of the aliases given, or
              an  alias  found  in  the  filesystem, must match one of the names or patterns in a
              DEVICES line.

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

       +      Does  the  device  have  a  valid md superblock?  If a specific metadata version is
              requested with --metadata or -e then only  that  style  of  metadata  is  accepted,
              otherwise  mdadm  finds any known version of metadata.  If no md metadata is found,
              the device may be still added to an array as a spare if POLICY 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 number will
       be used, otherwise a free unit number will be  chosen.   Normally  mdadm  will  prefer  to
       create  a  partitionable  array,  however if the CREATE line in mdadm.conf suggests that a
       non-partitionable array is preferred, that will be honoured.

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

       Once  an  appropriate array is found or created and the device is added, mdadm must decide
       if the array is ready to be started.  It will normally compare  the  number  of  available
       (non-spare) devices to the number of devices that the metadata suggests need to be active.
       If there are at least that many, the array will  be  started.   This  means  that  if  any
       devices are missing the array will not be restarted.

       As  an  alternative,  --run  may be passed to mdadm in which case the array will be run as
       soon as there are enough devices present for the data to be accessible.  For a RAID1, that
       means  one  device  will start the array.  For a clean RAID5, the array will be started as
       soon as all but one drive is present.

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


       This section describes environment variables that affect how mdadm operates.

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

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

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

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

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

              If an array is stopped while it is performing a reshape and that reshape was making
              use  of  a  backup  file,  then when the array is re-assembled mdadm will sometimes
              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
                  export MDADM_CONF_AUTO='-ddf -imsm'
              will  make  sure  that mdadm does not automatically assemble any DDF or IMSM arrays
              that are found.  This can be useful on systems configured  to  manage  such  arrays
              with dmraid.


         mdadm --query /dev/name-of-device
       This  will find out if a given device is a RAID array, or is part of one, and will provide
       brief information about the device.

         mdadm --assemble --scan
       This will assemble and start all arrays listed in the standard config file.  This  command
       will typically go in a system startup file.

         mdadm --stop --scan
       This  will  shut  down  all  arrays that can be shut down (i.e. are not currently in use).
       This will typically go in a system shutdown script.

         mdadm --follow --scan --delay=120
       If (and only if) there is an Email address or program given in the standard  config  file,
       then monitor the status of all arrays listed in that file by polling them ever 2 minutes.

         mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1
       Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and /dev/hdc1.

         echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf
         mdadm --detail --scan >> mdadm.conf
       This  will  create a prototype config file that describes currently active arrays that are
       known to be made from partitions of IDE or SCSI drives.   This  file  should  be  reviewed
       before being used as it may contain unwanted detail.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

         mdadm --help
       Provide general help.


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

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

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

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


       mdadm understand two sorts of names for array devices.

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

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

       When mdadm chooses device names during auto-assembly  or  incremental  assembly,  it  will
       sometimes  add  a small sequence number to the end of the name to avoid conflicted between
       multiple arrays that have the same name.  If mdadm can reasonably determine that the array
       really is meant for this host, either by a hostname in the metadata, or by the presence of
       the array in mdadm.conf, then it will leave off the  suffix  if  possible.   Also  if  the
       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 "/dev/md/d1p2".

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

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


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


       mdadm was previously known as mdctl.


       For further information on mdadm usage, MD and the various levels of RAID, see:


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

       The latest version of mdadm should always be available from


       Related man pages:

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