oracular (8) mdadm.8.gz

Provided by: mdadm_4.3+20240723-2ubuntu1_amd64 bug

NAME

       mdadm - manage MD devices aka Linux Software RAID

SYNOPSIS

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

DESCRIPTION

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

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

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

       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.

MODES

       mdadm has several major modes of operation:

       Assemble
              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.

       Auto-detect
              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

Options for selecting a mode are:

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

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

       -C, --create
              Create a new array.

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

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

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

       --auto-detect
              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 a general help message or, after one of the above options, a mode-specific help message.

       --help-options
              Display more detailed help about command-line parsing and some commonly used options.

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

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

       -q, --quiet
              Avoid printing purely informative messages.  With this, mdadm  will  be  silent  unless  there  is
              something really important to 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.  If not specified,  the  default  config  file  and  default
              conf.d directory will be used.  See mdadm.conf(5) for more details.

              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:

                     https://www.intel.com/content/www/us/en/support/products/122484

       --homehost=
              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 latter 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.

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

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

       --home-cluster=
              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 1/4/5/6/10 and for RAID 0  on
              external metadata.  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 option can be used with --create for determining the initial size of an array.  For  external
              metadata,  it can be used on a volume, but not on a container itself.  Setting the initial size of
              RAID 0 array is only valid for external metadata.

              This value can be set with --grow for RAID level 1/4/5/6/10 though DDF arrays may not be  able  to
              support  this.  RAID 0 array size cannot be changed.  If the array was created with a size smaller
              than the currently active drives, the extra space can be accessed using --grow.  The size  can  be
              given as max which means to choose the largest size that fits on all current drives.

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

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

       -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 in 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, with minimal chunk size
              being 4KB.

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

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

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

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

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

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

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

              The  layout  of  the  RAID5  parity  block  can  be  one   of   left-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 signifying the
              number of copies of each datablock.  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'.

              As  for the number of copies of each data block, 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.

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

       -b, --bitmap=
              Specify how to store a write-intent bitmap.  Following values are supported:

              internal  -  the  bitmap  is  stored  with  the  metadata on the array and so is replicated on all
              devices.

              clustered - 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.

              none - create array with no bitmap or remove any present bitmap (grow mode).

              Setting  bitmap  for  file  is deprecated and should not be used. The file should not exist unless
              --force is also given. The same file should be provided when assembling the array. The  file  name
              must  contain  at  least  one  slash  ('/'). Bitmap files 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 --consistency-policy.

       --bitmap-chunk=
              Set the chunk size 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 chunk size  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.

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

       --failfast
              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.

       --assume-clean
              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.

       --write-zeroes
              When creating an array, send write zeroes requests to all the block devices.  This should zero the
              data  area  on  all  disks  such  that the initial sync is not necessary and, if successfull, will
              behave as if --assume-clean was specified.

              This is intended for use with devices that have hardware offload for  zeroing,  but  despite  this
              zeroing  can  still  take  several  minutes for large disks.  Thus a message is printed before and
              after zeroing and each disk is zeroed in parallel with the others.

              This is only meaningful with --create.

       --backup-file=
              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.

       --data-offset=
              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 an 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).

       --continue
              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. It must be POSIX PORTABLE NAME compatible and cannot be longer than 32
              chars. This is effective when creating an array with a v1 metadata, or an external array.

              If name is needed but not specified, it is taken from the basename of the  device  that  is  being
              created. See DEVICE NAMES

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

       -f, --force
              Insist  that mdadm accept the geometry and layout specified without question.  Normally mdadm will
              not allow the 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.  /dev/md/scratch3.

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

       --nodes
              Only  works when the array is created for a 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.

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

       -k, --consistency-policy=
              Specify how the array maintains consistency in the case of an 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
                     an unclean shutdown.

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

              journal
                     For RAID levels 4/5/6, the journal device is used to log transactions and replay  after  an
                     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. It must be POSIX PORTABLE NAME compatible and cannot be
              longer than 32 chars. 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 one 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.

       --no-degraded
              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.

       --backup-file=
              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.

       --invalid-backup
              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 a 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 metadata.

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

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

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

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

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

For Manage mode:

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

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

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

       --re-add
              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.

              --re-add  can  also  be accompanied by --update=devicesize, --update=bbl, or --update=no-bbl.  See
              descriptions of these options when used in Assemble mode for an explanation of their use.

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

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

       --add-spare
              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  a  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 devices 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 described below under --fail.

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

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

       --set-faulty
              same as --fail.

       --replace
              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 preferentially
              be used to replace the devices listed after  --replace.   These  devices  must  already  be  spare
              devices in the array.

       --write-mostly
              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.

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

       --cluster-confirm
              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
              Add a journal to an existing array, or recreate journal for a RAID-4/5/6 array that lost a journal
              device. To avoid interrupting ongoing write operations, --add-journal  only  works  for  array  in
              Read-Only state.

       --failfast
              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.

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

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

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

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

For Misc mode:

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

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

       --detail-platform
              Print details of the platform's RAID capabilities (firmware  /  hardware  topology)  for  a  given
              metadata  format.  If  used without an argument, mdadm will scan all controllers looking for their
              capabilities. Otherwise, mdadm will only look at the controller specified by the argument  in  the
              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.

       --sparc2.2
              If an array was created on a SPARC machine with a 2.2 Linux kernel patched with RAID support,  the
              superblock  will  have  been  created  incorrectly,  or  at  least incompatibly with 2.4 and later
              kernels.  Using the --sparc2.2 flag with --examine will fix the superblock before  displaying  it.
              If  this  appears  to  do  the  right  thing,  then  the array can be successfully assembled using
              --assemble --update=sparc2.2.

       -X, --examine-bitmap
              Report information about a bitmap file.  The argument is either an  external  bitmap  file  or  an
              array  component  in  case of an internal bitmap.  Note that running this on an array device (e.g.
              /dev/md0) does not report the bitmap for that array.

       --examine-badblocks
              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.

       --dump=directory

       --restore=directory
              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.

       --zero-superblock
              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 when calling --zero-superblock with clustered raid. Make  sure  the  array  isn't
              used or assembled in another cluster node before executing it.

       --kill-subarray=
              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.

       --update-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.

       --wait-clean
              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.

       --action=
              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 in 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 is 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.

       --path=
              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 an udev script.

For Monitor mode:

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

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

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

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

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

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

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

       -1, --oneshot
              Check  arrays  only once.  This will generate NewArray events and more significantly DegradedArray
              and SparesMissing events.  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.

       --no-sharing
              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.

ASSEMBLE MODE

       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 the number of component devices.  These can be
       found in a number of ways.

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

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

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

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

       The config file is only used if explicitly named with --config or requested with  (a  possibly  implicit)
       --scan.  In the latter case, the default config file is used.  See mdadm.conf(5) for more details.

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

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

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

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

       In Linux kernels prior to version 2.6.28 there were two distinct  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 types 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-assembly.

BUILD MODE

       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.

CREATE MODE

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

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

       md-device is a new device. This could be standard name or chosen name. For details see: DEVICE NAMES

       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 a partition based array, using mdadm with version-1.x metadata, the partition  type  should
       be  set to 0xDA (non fs-data).  This type of 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.

       --readonly
              start the array in readonly mode.

MANAGE 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.

MISC MODE

       Usage: mdadm options ...  devices ...

       MISC mode includes a number of distinct operations that operate on distinct devices.  The operations are:

       --query
              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.

       --detail
              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.

       --detail-platform
              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

       --update-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.  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  must  be  POSIX  PORTABLE  NAME
              compatible  and  cannot  be  longer than 32 chars. If successes, new value will be respected after
              next assembly.

              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.

       --examine
              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.

       --dump=directory
              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 will have the metadata
              written at the same location as 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  filename  used  in the directory will be the base name of the device.   Further, if any links
              appear in /dev/disk/by-id which point to the device, then hard links to the file will  be  created
              in directory based on these by-id names.

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

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

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

       --readwrite
              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.

MONITOR MODE

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

       Monitor option can work in two modes:

       •   system wide mode, follow all md devices based on /proc/mdstat,

       •   follow only specified MD devices in command line.

       --scan - indicates system wide mode. Option causes the monitor to track all md  devices  that  appear  in
       /proc/mdstat.  If it is not set, then at least one device must be specified.

       Monitor  usage  causes  mdadm  to  periodically  poll  a  number of md arrays and to report on any events
       noticed.

       In both modes, monitor will work as long as there is an active array with redundancy and it is defined to
       follow (for --scan every array is followed).

       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.

       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.  For devices given directly
       in command line, without program or email specified, each event is reported to stdout.

       Note: For systems where is configured via systemd, mdmonitor(mdmonitor.service) should be configured. The
       service  is designed to be primary solution for array monitoring, it is configured to work in system wide
       mode.  It is automatically started and stopped according to current state  and  types  of  MD  arrays  in
       system.   The service may require additional configuration, like e-mail or delay.  That should be done in
       mdadm.conf.

       The different events are:

           DeviceDisappeared
                  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.

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

           RebuildNN
                  Where NN is a two-digit number (eg. 05, 48). This indicates that the rebuild has reached  that
                  percentage  of  the total. The events are generated at a fixed increment from 0. The increment
                  size may be specified with a command-line  option  (the  default  is  20).  (syslog  priority:
                  Warning)

           RebuildFinished
                  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)

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

           SpareActive
                  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)

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

           DegradedArray
                  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)

           MoveSpare
                  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)

           SparesMissing
                  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)

           TestMessage
                  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 drives and a spare.  It will then attempt  to  remove  the  spare  from  the
       second  array  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.

GROW MODE

       The GROW mode is used for changing the size or shape of an active array.

       During the kernel 2.6 era the following changes were added:

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

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

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

       Notes:

       •   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.

   SIZE CHANGES
       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, over 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.

   RAID-DEVICES CHANGES
       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 immediately.

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

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

   LEVEL CHANGES
       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.

   CHUNK-SIZE AND 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.

   BITMAP CHANGES
       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.

   CONSISTENCY POLICY CHANGES
       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).

INCREMENTAL MODE

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

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

       Usage: mdadm --incremental --rebuild-map

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

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

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

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

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

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

       The tests that mdadm makes are as follow:

       +      Is the device permitted by mdadm.conf?  That is, is it listed in a DEVICES line in that file.   If
              DEVICES  is  absent  then  the  default it to allow any device.  Similarly if DEVICES contains the
              special word partitions then any device is allowed.  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.

ENVIRONMENT

       This section describes environment variables that affect how mdadm operates.

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

       MDADM_NO_UDEV
              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.

       MDADM_NO_SYSTEMCTL
              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.

       IMSM_NO_PLATFORM
              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.

              These change can also be suppressed by adding mdadm.imsm.test=1 to the kernel command  line.  This
              makes it easy to test IMSM code in a virtual machine that doesn't have IMSM virtual hardware.

       MDADM_GROW_ALLOW_OLD
              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.

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

EXAMPLES

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

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

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

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

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

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

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

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

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

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

         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.

FILES

   /proc/mdstat
       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.conf (or /etc/mdadm/mdadm.conf)
       Default config file.  See mdadm.conf(5) for more details.

   /etc/mdadm.conf.d (or /etc/mdadm/mdadm.conf.d)
       Default directory containing configuration files.  See mdadm.conf(5) for more details.

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

POSIX PORTABLE NAME

       A valid name can only consist of characters "A-Za-z0-9.-_".  The name cannot start with a leading "-" and
       cannot exceed 255 chars.

DEVICE NAMES

       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.

       In every style, raw name must be compatible with POSIX PORTABLE NAME and has to  be  no  longer  than  32
       chars.

       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

              /dev/mdNN

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

              /dev/md_dNN

       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 possible.

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

              /dev/md_XXX

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

UNDERSTANDING OUTPUT

       EXAMINE

       checkpoint
              Checkpoint value is reported when array is performing some action including  resync,  recovery  or
              reshape. Checkpoints allow resuming action from certain point if it was interrupted.

              Checkpoint  is  reported as combination of two values: current migration unit and number of blocks
              per unit. By multiplying those values and dividing by array size  checkpoint  progress  percentage
              can  be  obtained  in  relation  to  current progress reported in /proc/mdstat. Checkpoint is also
              related to (and sometimes based on) sysfs entry sync_completed but depending on action  units  may
              differ.  Even  if units are the same, it should not be expected that checkpoint and sync_completed
              will be exact match nor updated simultaneously.

NOTE

       mdadm was previously known as mdctl.

SEE ALSO

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

              https://raid.wiki.kernel.org/

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

       The latest version of mdadm should always be available from

              https://www.kernel.org/pub/linux/utils/raid/mdadm/

       Related man pages:

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