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Linux 2.6 - man page for mdadm (linux section 8)

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 partitions thereof) to be combined into a  sin-
       gle device to hold (for example) a single filesystem.  Some RAID levels include redundancy
       and so can survive some degree of device failure.

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

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

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

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

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

       With  a	CONTAINER,  there  is one set of metadata that describes all of the arrays in the
       container.  So when mdadm creates a CONTAINER device, the device just represents the meta-
       data.  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.  Compo-
	      nents can be explicitly given or can be searched for.  mdadm checks that the compo-
	      nents do form a bona fide array, and can, on request, fiddle superblock information
	      so as to assemble a faulty array.

       Build  Build an array that doesn't have	per-device  metadata  (superblocks).   For  these
	      sorts of arrays, mdadm cannot differentiate between initial creation and subsequent
	      assembly of an array.  It also cannot perform any checks	that  appropriate  compo-
	      nents  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 oth-
	      erwise 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 meaning-
	      ful 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  sup-
	      ported  growth  options including changing the active size of component devices and
	      changing the number of active devices in RAID levels  1/4/5/6,  changing	the  RAID
	      level  between 1, 5, and 6, changing the chunk size and layout for RAID5 and RAID5,
	      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 inter-
	      face 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  con-
	      tainer 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, opera-
	      tions on component devices such as erasing old superblocks, and information gather-
	      ing operations.

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

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

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

       -C, --create
	      Create a new array.

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

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

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

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

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

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

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

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

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

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

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

       -c, --config=
	      Specify  the  config  file.  Default is to use /etc/mdadm/mdadm.conf, or if that is
	      missing, then /etc/mdadm.conf.  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

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

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

	      Options are:

	      0, 0.90
		     Use  the  original  0.90 format superblock.  This format limits arrays to 28
		     component devices and limits component devices of levels 1 and greater to	2

	      1, 1.0, 1.1, 1.2 default
		     Use  the  new  version-1 format superblock.  This has few restrictions.  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 "1.0".  "default"  is  equivalent  to

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

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


	      This  will  override any HOMEHOST setting in the config file and provides the iden-
	      tity 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  ver-
	      sion-1  superblocks,  it	will  be  prefixed  to	the array name.  For version-0.90
	      superblocks, part of the SHA1 hash of the hostname will be stored in the later half
	      of the UUID.

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

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

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

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

       -z, --size=
	      Amount (in 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 vari-
	      ance among the drives of greater than 1%, a warning is issued.

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

	      This value can not be used with CONTAINER metadata such as DDF and IMSM.

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

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

       -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.

	      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  high-
	      lights  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 round-
	      ing) in later kernels.

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

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

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

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

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

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

	      It is also possibly to cause RAID5 to use a  RAID4-like  layout  by  choosing  par-
	      ity-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 pro-
	      vide a layout which is identical to the corresponding RAID5 layout on the first N-1
	      devices,	and has the 'Q' syndrome (the second 'parity' block used by RAID6) on the
	      last device.  These layouts are: left-symmetric-6, right-symmetric-6, left-asymmet-
	      ric-6, right-asymmetric-6, and parity-first-6.

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

	      Each  failure  mode  can be followed by a number, which is used as a period between
	      fault generation.  Without a number, the fault is generated once on the first rele-
	      vant 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 supported options are:

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

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

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

	      The number is the number of copies of each datablock.  2 is normal, 3 can  be  use-
	      ful.   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 intermediate RAID6 layout is
	      used in which the second parity block (Q) is always on the last device.  To convert
	      a  RAID5 to RAID6 and leave it in this new layout (which does not require re-strip-
	      ing) 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 problems.

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

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

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

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

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

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

       --array-size=, -Z
	      Set the size of the array which is seen by users of the device such as filesystems.
	      This can be less that the real size, but never greater.  The size set this way does
	      not persist across restarts of the array.

	      This  is most useful when reducing the number of devices in a RAID5 or RAID6.  Such
	      arrays require the array-size to be reduced before a reshape can be performed  that
	      reduces the real size.

	      A  value	of  max  restores  the apparent size of the array to be whatever the real
	      amount of available space is.

       -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 sup-

       -f, --force
	      Insist  that mdadm accept the geometry and layout specified without question.  Nor-
	      mally 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.

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

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

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

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

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

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

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

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

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

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

       -N, --name=
	      Specify  the  name of the array to assemble.  This must be the name that was speci-
	      fied 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 created.  If an array has
	      an internal bitmap, there is no need to specify this when assembling the array.

	      If --backup-file was used to grow the number of raid-devices in a  RAID5,  and  the
	      system  crashed  during  the  critical section, then the same --backup-file must be
	      presented to --assemble to allow possibly corrupted data to be restored.

       -U, --update=
	      Update the superblock on each device while  assembling  the  array.   The  argument
	      given to this flag can be one of sparc2.2, summaries, uuid, name, homehost, resync,
	      byteorder, devicesize, or super-minor.

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

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

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

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

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

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

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

	      The  devicesize  will rarely be of use.  It applies to version 1.1 and 1.2 metadata
	      only (where the metadata is at the start of the device) and is only useful when the
	      component  device  has changed size (typically become larger).  The version 1 meta-
	      data 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  meta-

For Manage mode:
       -t, --test
	      Unless  a  more  serious	error  occurred, mdadm will exit with a status of 2 if no
	      changes were made to the array and 0 if at least one change was made.  This can  be
	      useful  when  an	indirect specifier such as missing, detached or faulty is used in
	      requesting an operation on the array.  --test will report failure if  these  speci-
	      fiers 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 describe in the
	      next point.  If that fails or the device was never part of the array, the device is
	      added as a hot-spare.  If the array is  degraded,  it  will  immediately	start  to
	      rebuild data onto that spare.

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

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

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

	      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

       -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  and  detached 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.  This will only succeed for
	      devices that are spares or have already been marked as failed.

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

	      same as --fail.

	      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 read-
	      ing from these devices if possible.

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

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

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

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

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

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

	      Print  details  of  the platform's RAID capabilities (firmware / hardware topology)
	      for a given metadata format.

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

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

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

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

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

       -o, --readonly
	      mark array as readonly.

       -w, --readwrite
	      mark array as readwrite.

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

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

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

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

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

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

For Incremental Assembly mode:
       --rebuild-map, -r
	      Rebuild  the  map  file  (/var/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.

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  sec-
	      onds  before  polling again.  The default is 60 seconds.	Since 2.6.16, there is no
	      need to reduce this as the kernel  alerts  mdadm	immediately  when  there  is  any

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

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

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

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

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

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

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

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

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

       In the first usage example (without the --scan) the first device given is the  md  device.
       In  the second usage example, all devices listed are treated as md devices and assembly is
       attempted.  In the third (where no devices are listed) all md devices that are  listed  in
       the  configuration  file  are assembled.  If not arrays are described by the configuration
       file, then any arrays that can be found on unused devices will be assembled.

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

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

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

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

       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 /var/run/mdadm/map which will allow udev to choose the correct

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

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

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

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

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

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

       In  no  array  at  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 meta-
       data, then the minor number as recorded in the superblock is used  to  create  a  name  in
       /dev/md/  so  for  example /dev/md/3.  If the array uses version-1 metadata, then the name
       from the superblock is used to similarly create a name in /dev/md/ (the name will have any
       'host' prefix stripped first).

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

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

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

       The  level  may	raid0,	linear, raid1, raid10, multipath, or faulty, or one of their syn-
       onyms.  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 filesys-
       tems.  They are also checked to see if the variance in device size exceeds 1%.

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

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

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

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

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

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

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

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

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

	      start the array 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 distinct devices.  The
       operations are:

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

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

	      0      The array is functioning normally.

	      1      The array has at least one failed device.

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

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

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

	      0      metadata successfully enumerated its platform components on this system

	      1      metadata is platform independent

	      2      metadata failed to find its platform components on this system

	      If the device is a container and the argument to --update-subarray specifies a sub-
	      array  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 supported.

	      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 multi-
	      ple devices that are components of the one array are grouped together and  reported
	      in a single entry suitable for inclusion in /etc/mdadm/mdadm.conf.

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

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

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

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

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

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

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

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

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

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

       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 moni-

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

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

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

       The different events are:

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

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

		  An md array started reconstruction. (syslog priority: Warning)

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

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

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

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

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

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

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

		  A spare drive has been moved from one array in  a  spare-group  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 cer-
		  tain 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  prior-
		  ity: 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 in the configuration file.  The spare-group name can  be
       any string; it is only necessary that different spare groups use different names.

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

       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, including restructuring a RAID5 array to have more active

       Currently the only support available is to

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

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

	   change the chunk-size and layout of RAID5 and RAID6.

	   convert between RAID1 and RAID5, and between RAID5 and RAID6.

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

       GROW mode is not currently supported for CONTAINERS or arrays inside containers.

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

       Note that when an array changes size, any filesystem that may be stored in the array  will
       not  automatically  grow to use the space.  The filesystem will need to be explicitly told
       to use the extra space.

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

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

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

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

       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.  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 inac-
       cessible.  The integrity of any data can then be checked before the non-reversible  reduc-
       tion in the number of devices is request.

       When  relocating  the first few stripes on a RAID5, it is not possible to keep the data on
       disk completely consistent and crash-proof.  To provide the required  safety,  mdadm  dis-
       ables writes to the array while this "critical section" is reshaped, and takes a backup of
       the data that is in that section.  This backup is normally 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.  If this option is used, and the system does crash during the criti-
       cal  period, the same file must be passed to --assemble to restore the backup and reassem-
       ble the array.

       Changing the RAID level of any array happens instantaneously.   However	in  the  RAID  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  accomplish.   So
       while  the  level  change is instant, the accompanying layout change can take quite a long

       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.

       If  the	reshape is interrupted for any reason, this backup file must be make 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  bit-
       map  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 condition-
       ally 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  meta-
       data 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 only add devices to an array which were previously working (active or
       spare) parts of that array.  It does not currently support automatic inclusion  of  a  new
       drive as a spare in some array.

       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.  Other-
	      wise 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 metadata version is
	      request with --metadata or -e then only that style of metadata is accepted,  other-
	      wise  mdadm  finds  any known version of metadata.  If no md metadata is found, the
	      device is rejected.

	      mdadm keeps a list of arrays that it has partially assembled in  /var/run/mdadm/map
	      (or   /var/run/mdadm.map	 if   the   directory	doesn't  exist.   Or  maybe  even
	      /dev/.mdadm.map).  If no array exists which matches the metadata on the new device,
	      mdadm  must  choose  a device name and unit number.  It does this based on any name
	      given in mdadm.conf or any name information stored in the metadata.  If  this  name
	      suggests a unit number, that number will be used, otherwise a free unit number will
	      be chosen.  Normally mdadm will prefer to create a partitionable array, however  if
	      the CREATE line in mdadm.conf suggests that a non-partitionable array is preferred,
	      that will be honoured.

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

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

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

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

       This section describes environment variables that affect how mdadm operates.

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

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

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

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

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

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

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

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

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

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

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

	 mdadm --monitor --scan --daemonise > /var/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 /var/run/mdadm.

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

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

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

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

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

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

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

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

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

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

	 mdadm --help
       Provide general help.

       If you're using the /proc filesystem, /proc/mdstat lists all active md devices with infor-
       mation 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 contain MD super block,
       and gives identifying information (e.g. UUID) about known MD  arrays.   See  mdadm.conf(5)
       for more details.

       When  --incremental mode is used, this file gets a list of arrays currently being created.
       If /var/run/mdadm does not exist as a directory, then /var/run/mdadm.map is used  instead.
       If  /var/run  is  not available (as may be the case during early boot), /dev/.mdadm.map is
       used on the basis that /dev is usually available very early in boot.

       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 /etc/mdadm.conf, then it will leave off the suffix if possible.  Also if  the
       homehost is specified as <ignore> mdadm will only use a suffix if a different array of the
       same name already exists or is listed in the config file.

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


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


       Partition numbers should be indicated by added "pMM" to these, thus "/dev/md/d1p2".

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

       mdadm was previously known as mdctl.

       mdadm is completely separate from the raidtools package, and does not use the /etc/raidtab
       configuration file at all.

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


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

       The latest version of mdadm should always be available from


       Related man pages:

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

       raidtab(5), raid0run(8), raidstop(8), mkraid(8).

v3.1.4											 MDADM(8)

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