Mdadm/man

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

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 lev‐ els 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. It is deprecated and support will be removed in the future.

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. It is deprecated and support will be removed in the future.

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 con‐ tainer.

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  su‐
      perblock information so as to assemble a faulty array.

Build Build an array that doesn't have per-device metadata (superblocks). For these sorts of arrays, mdadm cannot differ‐

      entiate between initial creation and subsequent assembly of an array.  It also cannot perform any checks that  appro‐
      priate  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 con‐
      sistent (e.g. both sides of a mirror contain the same data) but the content of  the  device  is  left  otherwise  un‐
      touched.   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  mul‐
      tipath 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 chang‐

      ing 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 pre‐
      ferred.

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 impor‐
      tant 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 con‐
      tained 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 config‐
      uration  file  (see  previous option); one exception is MISC mode when using --detail or --stop, in which case --scan
      says to get a list of array devices from /proc/mdstat.

-e, --metadata=

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

      Options are:

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

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

      ddf    Use  the  "Industry Standard" DDF (Disk Data Format) format defined by SNIA. DDF is deprecated and there is no
             active development around it.  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 simi‐
             lar 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 con‐
      sidered the home for any arrays.

      When creating an array, the homehost will be recorded in the metadata.  For version-1 superblocks, it  will  be  pre‐
      fixed  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  com‐
      ponent.  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 al‐
      lowed 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.

-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 de‐
      vice 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 de‐
      fault 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  filesys‐
      tem, 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 0K (i.e. no rounding).

-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  ex‐
      plicitly 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-symmet‐
      ric, 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-per‐
      sistent, 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 lay‐
      out.  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 cor‐
      rupt 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).

--bitmap-chunk=

      Set the chunk size of the bitmap.  Each bit corresponds to that many Kilobytes of storage.

      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.  --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 successful, 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  par‐
      allel 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.

      --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 af‐
      ter 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 in‐
      terrupted 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= op‐
      tion 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 ef‐
      fective 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 ques‐
      tion to be suppressed.

-f, --force

      Insist that mdadm accept the geometry and layout specified without question.  Normally mdadm will not allow the  cre‐
      ation  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, --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 sim‐
      ply 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  shut‐
             down.

      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 assem‐
      bled 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 be‐
      ing 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 work‐
      ing 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.   Nor‐
      mally  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.

--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  sum‐
      maries,  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 super-minor option will update the preferred minor field on each superblock to match the minor number of the  ar‐
      ray  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  automati‐
      cally at the first write to an array with redundancy (RAID level 1 or greater).

      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 re‐
      versed, and will correct that order before assembling the array.  This is only valid with original (Version 0.90) su‐
      perblocks.

      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  us‐
      able 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  ig‐
      nored.  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 con‐
      tains 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 ker‐
      nel, --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 re‐
      stored. This happens before the file system pivot operation and avoids loss of filesystem context.  Losing file  sys‐
      tem 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  re‐
      moved)  the device is re-added as described in the next point.  If that fails or the device was never part of the ar‐
      ray, 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 re‐
      covery 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 re‐
      covery 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  acknowl‐
      edge  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 persis‐ tent 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 re‐ move 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/de‐
      vices/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 rea‐
      son (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  cur‐
      rently active.

-X, --examine-bitmap

      Report  information  about  a  bitmap.  The argument is an array component. 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 dri‐
      ves.   See --zero-superblock for completely removing a superblock.  Note that some formats depend on the subarray in‐
      dex 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  at‐
      tempt 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.

--udev-rules=

      it  generates  the  udev  rules  to  the  file  that  handles hot-plug bare devices.  Given the POLICYs defined under
      /etc/mdadm.conf (or/etc/mdadm/mdadm.conf)

      See mdadm.conf(5) for more details and usage examples about POLICY.

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.

--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 an mail address to send alerts to. Can be configured in mdadm.conf as MAILADDR.

-p, --program, --alert

      Give a program to be run whenever an event is detected. Can be configured in mdadm.conf as PROGRAM.

-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. The default is 60 seconds.  mdadm polls the md arrays and then waits this many seconds be‐
      fore polling again if no event happened.  Can be configured in mdadm.conf as MONITORDELAY.

-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  pro‐
      gram.  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 de‐ vice, 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 de‐ vices 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 informa‐ tion 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 lat‐ ter 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.

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 pos‐ sible 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 ac‐ tive.

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 cau‐ tion.

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= set‐ ting.

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 func‐ tional.

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 op‐ erations 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 re‐ moved, or if the array has a write-intent bitmap which has recorded whatever changes there were, then the device will imme‐ diately 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  at‐
      tempt  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 to‐
      gether 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 pro‐
      vided 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/md‐

      stat.  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 speci‐ fied) 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 nei‐ ther 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: On systems where mdadm monitoring is managed through systemd, the mdmonitor.service should be present. This service is designed to be the primary solution for array monitoring. It is configured to operate in system-wide mode. It is initiated by udev when start criteria are met, e.g. mdadm.conf exists and necessary configuration parameters are set. It is kept alive as long as a redundant RAID array is active; it stops otherwise. User should customize MAILADDR in mdadm.conf to re‐ ceive mail notifications. MONITORDELAY, MAILFROM and PROGRAM are optional. See mdadm.conf(5) for detailed description of these options. Use systemctl status mdmonitor.service to verify status or determine if additional configuration is needed.

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 pri‐
          ority: 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: Criti‐
          cal)

   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 SpareAc‐ tive 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.

The following changes are supported:

• 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 con‐ tainer 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 re‐ duce 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.

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 va‐ cant. 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. Linux Kernel is able to increase or decrease the number of devices in a RAID5 and RAID6 safely, including restarting an interrupted "reshape".

The Linux Kernel is able to convert a RAID0 into a RAID4 or RAID5. mdadm uses this functionality and the ability to add de‐ vices 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 re‐ shaped, and takes a backup of the data that is in that section. For grows, this backup may be stored in any spare devices that the array has, however it can also be stored in a separate file specified with the --backup-file option, and is re‐ quired to be specified for shrinks, RAID level changes and layout changes. If this option is used, and the system does crash during the critical period, the same file must be passed to --assemble to restore the backup and reassemble the array. When shrinking rather than growing the array, the reshape is done from the end towards the beginning, so the "critical sec‐ tion" 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-stan‐ dard 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.

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 ar‐ ray. 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 filesys‐
      tem, 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{DE‐
      VLINKS}.

+ 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 meta‐ data 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 ar‐ ray 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 (particu‐
      larly mdmon) rather than forking and running them in the background.  This can be suppressed by setting MDADM_NO_SYS‐
      TEMCTL=1.

IMSM_NO_PLATFORM

      A key value of IMSM metadata is that it allows interoperability with boot ROMs on Intel platforms, and with other ma‐
      jor 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 be‐
      ing 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  environ‐
      ment.

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 de‐ vice.

 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 ar‐ rays 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 in‐ formation 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 su‐ perblock 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 de‐ vices. 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/md‐ stat.

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 deter‐ mine 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 names for arrays are of the form:

      /dev/mdNN

where NN is a number.

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  multi‐
      plying  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).

v4.4 MDADM(8)