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           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.
                  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, 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


                  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, 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 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
                  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.
           -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.
                  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.

    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.
                  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.
                  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
                  The number is the number of copies  of  each  datablock.   2  is
                  normal,  3  can  be useful.  This number can be at most equal to
                  the number of devices in the array.  It does not need to  divide
                  evenly  into  that number (e.g. it is perfectly legal to have an
                  'n2' layout for an array with an odd number of devices).
                  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
                  Set  the  chunksize of the bitmap.  Each bit corresponds to that
                  many Kilobytes of storage.  When using a file based bitmap,  the
                  default  is  to  use  the  smallest  size that is at-least 4 and
                  requires no more than  2^21  chunks.   When  using  an  internal
                  bitmap,  the chunksize defaults to 64Meg, or larger if necessary
                  to fit the bitmap into the available space.
                  A suffix of 'M' or 'G' can be given  to  indicate  Megabytes  or
                  Gigabytes respectively.
           -W, --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
                  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
                  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.
           -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.

    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
                  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,  name,  homehost,  resync, byteorder, devicesize,
                  no-bitmap, 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
                  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  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
                  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 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
                  recovered or may not require any recovery at all.
                  When used on an array that has no metadata (i.e.  it  was  built
                  with  --build)  it will be assumed that bitmap-based recovery is
                  enough to make the device fully consistent with the array.
                  When --re-add can be accompanied  by  --update=devicesize.   See
                  the description of this option when used in Assemble mode for an
                  explanation of its 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.
           -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  con-
                  nected  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 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.

    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.
           -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 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.
           -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.
                  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 (/var/run/mdadm/map)  that  mdadm  uses  to
                  help track which arrays are currently being assembled.
           --run, -R
                  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
           -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/mdadm.conf is used.
           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
           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
           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
           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 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
                  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 /etc/mdadm/mdadm.conf.
                  Having --scan without listing any devices will cause all devices
                  listed in the config file to be examined.
           --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.
           -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 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)
           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 and RAID6.
           ?   convert between RAID1 and RAID5, between RAID5 and  RAID6,  between
               RAID0,  RAID4,  and  RAID5,  and  between  RAID0 and RAID10 (in the
               near-2 mode).
           ?   add a  write-intent  bitmap  to  any  array  which  supports  these
               bitmaps, or remove a write-intent bitmap from such an array.
           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:
           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
           bitmap.   If  an array has a bitmap, it must be removed before the size
           can be changed. Once the change is complete a new bitmap  can  be  cre-
           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
           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
           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
           /var/run/mdadm/map  (or  /var/run/  if  the  directory doesn't
           exist.  Or maybe even  /dev/   If  no  array  exists  which
           matches the metadata on the new device, mdadm must choose a device name
           and unit number.  It does this based on any name given in mdadm.conf or
           any  name  information stored in the metadata.  If this name suggests a
           unit number, that number will be used, otherwise  a  free  unit  number
           will  be  chosen.  Normally mdadm will prefer to create a partitionable
           array, however if the CREATE line in mdadm.conf suggests  that  a  non-
           partitionable array is preferred, that will be honoured.
           If  the array is not found in the config file and its metadata does not
           identify it as belonging to the "homehost", then mdadm  will  choose  a
           name  for  the  array  which  is certain not to conflict with any array
           which does belong to this host.  It does this be adding  an  underscore
           and a small number to the name preferred by the metadata.
           Once  an appropriate array is found or created and the device is added,
           mdadm must decide if the array is ready to be started.   It  will  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
           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.


             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
           /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 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
           then /var/run/ is used instead.  If /var/run is not  available
           (as  may be the case during early boot), /dev/ 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 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 added "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.


           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.


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

    v3.2.3 MDADM(8)


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