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MDADM(8)							      MDADM(8)

       mdadm - manage MD devices aka Linux Software RAID

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

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

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

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

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

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

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

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

       mdadm has several major modes of operation:

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

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

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

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

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

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

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

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

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

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

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

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

       -C, --create
	      Create a new array.

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

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

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

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

       If a device is given before any options, or  if	the  first  option  is
       --add, --fail, --remove, or --replace, then the MANAGE mode is assumed.
       Anything other than these will cause the Misc mode to be assumed.

Options that are not mode-specific are:
       -h, --help
	      Display general help message or, after one of the above options,
	      a mode-specific help message.

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

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

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

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

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

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

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

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

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

	      Options are:

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

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

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

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

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

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

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

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

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

	      This  functionality  is  currently only provided by --detail and

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -c, --chunk=
	      Specify  chunk  size of kibibytes.  The default when creating an
	      array is 512KB.  To ensure compatibility with earlier  versions,
	      the  default when Building and array with no persistent metadata
	      is 64KB.	This is	 only  meaningful  for	RAID0,	RAID4,	RAID5,
	      RAID6, and RAID10.

	      RAID4,  RAID5,  RAID6, and RAID10 require the chunk size to be a
	      power of 2.  In any case it must be a multiple of 4KB.

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

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

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

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

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

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

       -p, --layout=
	      This option configures the  fine	details	 of  data  layout  for
	      RAID5,  RAID6, and RAID10 arrays, and controls the failure modes
	      for faulty.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -b, --bitmap=
	      Specify a file to store a	 write-intent  bitmap  in.   The  file
	      should  not  exist  unless --force is also given.	 The same file
	      should be provided when  assembling  the	array.	 If  the  word
	      internal	is  given, then the bitmap is stored with the metadata
	      on the array, and so is replicated on all devices.  If the  word
	      none  is given with --grow mode, then any bitmap that is present
	      is removed.

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

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

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

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

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

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

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

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

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

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

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

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

	      Setting the offset explicitly over-rides the default.  The value
	      given is in Kilobytes unless an 'M' or 'G' suffix is given.

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

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

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

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

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

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

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

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

       -o, --readonly
	      Start the array read only rather than read-write as normal.   No
	      writes will be allowed to the array, and no resync, recovery, or
	      reshape will be started.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       -U, --update=
	      Update the superblock on each device while assembling the array.
	      The argument given to this flag can be  one  of  sparc2.2,  sum‐
	      maries,  uuid,  naml,  homehost,	resync, byteorder, devicesize,
	      no-bitmap, bbl, no-, metadata, or super-minor.

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

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

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

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

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

	      The  byteorder option allows arrays to be moved between machines
	      with different byte-order.  When assembling such	an  array  for
	      the  first  time	after  a  move, giving --update=byteorder will
	      cause mdadm  to  expect  superblocks  to	have  their  byteorder
	      reversed,	 and  will  correct  that  order before assembling the
	      array.   This  is	 only  valid  with  original  (Version	 0.90)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	      same as --fail.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


	      Save metadata from lists devices, or restore metadata to	listed

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

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

       -o, --readonly
	      mark array as readonly.

       -w, --readwrite
	      mark array as readwrite.

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

	      If the device is a container and the argument to --kill-subarray
	      specifies an inactive subarray in the container, then the subar‐
	      ray is deleted.  Deleting all subarrays will  leave  an  'empty-
	      container'   or	spare	superblock   on	  the	drives.	   See
	      --zero-superblock for completely removing	 a  superblock.	  Note
	      that  some formats depend on the subarray index for generating a
	      UUID, this command will fail if it would change the UUID	of  an
	      active subarray.

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

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

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

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

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

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

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

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

	      Only used with --fail.  The 'path' given	will  be  recorded  so
	      that  if	a  new	device	appears at the same location it can be
	      automatically added to the same array.  This allows  the	failed
	      device  to  be  automatically  replaced  by a new device without
	      metadata if it appears at specified path.	  This option is  nor‐
	      mally only set by a udev script.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       This behaviour can be modified by the AUTO line in the mdadm.conf  con‐
       figuration  file.   This	 line can indicate that specific metadata type
       should, or should not, be automatically	assembled.   If	 an  array  is
       found  which is not listed in mdadm.conf and has a metadata format that
       is denied by the AUTO line, then it will not be	assembled.   The  AUTO
       line  can  also	request	 that  all arrays identified as being for this
       homehost should be assembled regardless of their	 metadata  type.   See
       mdadm.conf(5) for further details.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       If the array type supports a write-intent bitmap, and if the devices in
       the  array  exceed  100G	 is size, an internal write-intent bitmap will
       automatically be added unless some other option is explicitly requested
       with  the  --bitmap  option.   In  any  case space for a bitmap will be
       reserved so that one can be added layer with --grow --bitmap=internal.

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

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

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

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

	      start the array readonly — not supported yet.

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

       This usage will allow individual devices in  an	array  to  be  failed,
       removed	or  added.  It is possible to perform multiple operations with
       on command.  For example:
	 mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
       will firstly mark /dev/hda1 as faulty in /dev/md0 and will then	remove
       it  from the array and finally add it back in as a spare.  However only
       one md array can be affected by a single command.

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

       Usage: mdadm options ...	 devices ...

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

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

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

	      0	     The array is functioning normally.

	      1	     The array has at least one failed device.

	      2	     The  array	 has  multiple	failed devices such that it is

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

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

	      0	     metadata  successfully enumerated its platform components
		     on this system

	      1	     metadata is platform independent

	      2	     metadata failed to find its platform components  on  this

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

	      The name option updates the subarray name in  the	 metadata,  it
	      may  not	affect the device node name or the device node symlink
	      until the subarray is  re-assembled.   If	 updating  name	 would
	      change the UUID of an active subarray this operation is blocked,
	      and the command will end in an error.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The different events are:

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

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

		  An  md array started reconstruction. (syslog priority: Warn‐

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Currently the supported changes include

       ·   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

       ·   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  bit‐
	   maps, or remove a write-intent bitmap from such an array.

       Using  GROW  on containers is currently supported only for Intel's IMSM
       container format.   The	number	of  devices  in	 a  container  can  be
       increased  - which affects all arrays in the container - or an array in
       a container can be converted between levels where those levels are sup‐
       ported  by  the	container,  and	 the  conversion is on of those listed
       above.  Resizing arrays in an IMSM container with --grow --size is  not
       yet supported.

       Grow  functionality  (e.g. expand a number of raid devices) for Intel's
       IMSM container format has an experimental status. It is guarded by  the
       MDADM_EXPERIMENTAL  environment variable which must be set to '1' for a
       GROW command to succeed.	 This is for the following reasons:

       1.     Intel's native IMSM check-pointing  is  not  fully  tested  yet.
	      This can causes IMSM incompatibility during the grow process: an
	      array which is growing cannot roam between Microsoft  Windows(R)
	      and Linux systems.

       2.     Interrupting a grow operation is not recommended, because it has
	      not been fully tested for Intel's IMSM container format yet.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       Usage: mdadm --incremental --rebuild-map

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

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

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

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

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

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

       The tests that mdadm makes are as follow:

       +      Is the device permitted by mdadm.conf?  That is, is it listed in
	      a DEVICES line in that file.  If	DEVICES	 is  absent  then  the
	      default it to allow any device.  Similar if DEVICES contains the
	      special word partitions then any device is  allowed.   Otherwise
	      the  device  name	 given to mdadm must match one of the names or
	      patterns in a DEVICES line.

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

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

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

       Once an appropriate array is found or created and the device is	added,
       mdadm  must  decide  if the array is ready to be started.  It will nor‐
       mally compare the number of available (non-spare) devices to the number
       of  devices that the metadata suggests need to be active.  If there are
       at least that many, the array will be started.  This means that if  any
       devices are missing the array will not be restarted.

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

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

       This  section  describes	 environment  variables	 that affect how mdadm

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

	 mdadm --help
       Provide general help.

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

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

       A directory containing configuration files which are  read  in  lexical

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

       mdadm understand two sorts of names for array devices.

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

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

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

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


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


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

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

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


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

       mdadm was previously known as mdctl.

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

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

       The latest version of mdadm should always be available from

       Related man pages:

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

v3.3								      MDADM(8)

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