# mdadm

## NAME



           mdadm - manage MD devices _aka_ Linux Software RAID
    


## SYNOPSIS



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


## DESCRIPTION



           RAID devices are virtual devices created from two or more real
           block devices.  This allows multiple devices (typically disk
           drives or partitions thereof) to be combined into a single device
           to hold (for example) a single filesystem.  Some RAID levels
           include redundancy and so can survive some degree of device
           failure.
    
           Linux Software RAID devices are implemented through the md
           (Multiple Devices) device driver.
    
           Currently, Linux supports **LINEAR** md devices, **RAID0**(striping),
           **RAID1**(mirroring), **RAID4** , **RAID5** , **RAID6** , **RAID10** , **MULTIPATH** , **FAULTY** ,
           and **CONTAINER**.
    
           **MULTIPATH** is not a Software RAID mechanism, but does involve
           multiple devices: each device is a path to one common physical
           storage device.  New installations should not use md/multipath as
           it is not well supported and has no ongoing development.  Use the
           Device Mapper based multipath-tools instead. It is deprecated and
           support will be removed in the future.
    
           **FAULTY** is also not true RAID, and it only involves one device.  It
           provides a layer over a true device that can be used to inject
           faults. It is deprecated and support will be removed in the
           future.
    
           **CONTAINER** is different again.  A **CONTAINER** is a collection of
           devices that are managed as a set.  This is similar to the set of
           devices connected to a hardware RAID controller.  The set of
           devices may contain a number of different RAID arrays each
           utilising some (or all) of the blocks from a number of the devices
           in the set.  For example, two devices in a 5-device set might form
           a RAID1 using the whole devices.  The remaining three might have a
           RAID5 over the first half of each device, and a RAID0 over the
           second half.
    
           With a **CONTAINER** , there is one set of metadata that describes all
           of the arrays in the container.  So when _mdadm_ creates a **CONTAINER**
           device, the device just represents the metadata.  Other normal
           arrays (RAID1 etc) can be created inside the container.
    


## MODES



           mdadm has several major modes of operation:
    
           **Assemble**
                  Assemble the components of a previously created array into
                  an active array.  Components can be explicitly given or can
                  be searched for.  _mdadm_ checks that the components do form
                  a bona fide array, and can, on request, fiddle superblock
                  information so as to assemble a faulty array.
    
           **Build** Build an array that doesn't have per-device metadata
                  (superblocks).  For these sorts of arrays, _mdadm_ cannot
                  differentiate between initial creation and subsequent
                  assembly of an array.  It also cannot perform any checks
                  that appropriate components have been requested.  Because
                  of this, the **Build** mode should only be used together with a
                  complete understanding of what you are doing.
    
           **Create** Create a new array with per-device metadata (superblocks).
                  Appropriate metadata is written to each device, and then
                  the array comprising those devices is activated.  A
                  'resync' process is started to make sure that the array is
                  consistent (e.g. both sides of a mirror contain the same
                  data) but the content of the device is left otherwise
                  untouched.  The array can be used as soon as it has been
                  created.  There is no need to wait for the initial resync
                  to finish.
    
           **Follow or Monitor**
                  Monitor one or more md devices and act on any state
                  changes.  This is only meaningful for RAID1, 4, 5, 6, 10 or
                  multipath arrays, as only these have interesting state.
                  RAID0 or Linear never have missing, spare, or failed
                  drives, so there is nothing to monitor.
    
           **Grow** Grow (or shrink) an array, or otherwise reshape it in some
                  way.  Currently supported growth options including changing
                  the active size of component devices and changing the
                  number of active devices in Linear and RAID levels
                  0/1/4/5/6, changing the RAID level between 0, 1, 5, and 6,
                  and between 0 and 10, changing the chunk size and layout
                  for RAID 0,4,5,6,10 as well as adding or removing a write-
                  intent bitmap and changing the array's consistency policy.
    
           **Incremental Assembly**
                  Add a single device to an appropriate array.  If the
                  addition of the device makes the array runnable, the array
                  will be started.  This provides a convenient interface to a
                  _hot-plug_ system.  As each device is detected, _mdadm_ has a
                  chance to include it in some array as appropriate.
                  Optionally, when the _--fail_ flag is passed in we will
                  remove the device from any active array instead of adding
                  it.
    
                  If a **CONTAINER** is passed to _mdadm_ in this mode, then any
                  arrays within that container will be assembled and started.
    
           **Manage** This is for doing things to specific components of an array
                  such as adding new spares and removing faulty devices.
    
           **Misc** This is an 'everything else' mode that supports operations
                  on active arrays, operations on component devices such as
                  erasing old superblocks, and information-gathering
                  operations.
    
           **Auto-detect**
                  This mode does not act on a specific device or array, but
                  rather it requests the Linux Kernel to activate any auto-
                  detected arrays.
    


## OPTIONS



    


## Options for selecting a mode are:



           **-A** , **--assemble**
                  Assemble a pre-existing array.
    
           **-B** , **--build**
                  Build a legacy array without superblocks.
    
           **-C** , **--create**
                  Create a new array.
    
           **-F** , **--follow** , **--monitor**
                  Select **Monitor** mode.
    
           **-G** , **--grow**
                  Change the size or shape of an active array.
    
           **-I** , **--incremental**
                  Add/remove a single device to/from an appropriate array,
                  and possibly start the array.
    
           **--auto-detect**
                  Request that the kernel starts any auto-detected arrays.
                  This can only work if _md_ is compiled into the kernel — not
                  if it is a module.  Arrays can be auto-detected by the
                  kernel if all the components are in primary MS-DOS
                  partitions with partition type **FD** , and all use v0.90
                  metadata.  In-kernel autodetect is not recommended for new
                  installations.  Using _mdadm_ to detect and assemble arrays —
                  possibly in an _initrd_ — is substantially more flexible and
                  should be preferred.
    
           If a device is given before any options, or if the first option is
           one of **--add** , **--re-add** , **--add-spare** , **--fail** , **--remove** , or
           **--replace** , then the MANAGE mode is assumed.  Anything other than
           these will cause the **Misc** mode to be assumed.
    


## Options that are not mode-specific are:



           **-h** , **--help**
                  Display a general help message or, after one of the above
                  options, a mode-specific help message.
    
           **--help-options**
                  Display more detailed help about command-line parsing and
                  some commonly used options.
    
           **-V** , **--version**
                  Print version information for mdadm.
    
           **-v** , **--verbose**
                  Be more verbose about what is happening.  This can be used
                  twice to be extra-verbose.  The extra verbosity currently
                  only affects **--detail --scan** and **--examine --scan**.
    
           **-q** , **--quiet**
                  Avoid printing purely informative messages.  With this,
                  _mdadm_ will be silent unless there is something really
                  important to report.
    
           **-f** , **--force**
                  Be more forceful about certain operations.  See the various
                  modes for the exact meaning of this option in different
                  contexts.
    
           **-c** , **--config=**
                  Specify the config file or directory.  If not specified,
                  the default config file and default conf.d directory will
                  be used.  See mdadm.conf(5) for more details.
    
                  If the config file given is **partitions** then nothing will be
                  read, but _mdadm_ will act as though the config file
                  contained exactly
                      **DEVICE partitions containers**
                  and will read **/proc/partitions** to find a list of devices to
                  scan, and **/proc/mdstat** to find a list of containers to
                  examine.  If the word **none** is given for the config file,
                  then _mdadm_ will act as though the config file were empty.
    
                  If the name given is of a directory, then _mdadm_ will
                  collect all the files contained in the directory with a
                  name ending in **.conf** , sort them lexically, and process all
                  of those files as config files.
    
           **-s** , **--scan**
                  Scan config file or **/proc/mdstat** for missing information.
                  In general, this option gives _mdadm_ permission to get any
                  missing information (like component devices, array devices,
                  array identities, and alert destination) from the
                  configuration file (see previous option); one exception is
                  MISC mode when using **--detail** or **--stop,** in which case
                  **--scan** says to get a list of array devices from
                  **/proc/mdstat**.
    
           **-e** , **--metadata=**
                  Declare the style of RAID metadata (superblock) to be used.
                  The default is 1.2 for **--create** , and to guess for other
                  operations.  The default can be overridden by setting the
                  **metadata** value for the **CREATE** keyword in **mdadm.conf**.
    
                  Options are:
    
                  0, 0.90
                         Use the original 0.90 format superblock.  This
                         format limits arrays to 28 component devices and
                         limits component devices of levels 1 and greater to
                         2 terabytes.  It is also possible for there to be
                         confusion about whether the superblock applies to a
                         whole device or just the last partition, if that
                         partition starts on a 64K boundary.
    
                  1, 1.0, 1.1, 1.2 default
                         Use the new version-1 format superblock.  This has
                         fewer restrictions.  It can easily be moved between
                         hosts with different endian-ness, and a recovery
                         operation can be checkpointed and restarted.  The
                         different sub-versions store the superblock at
                         different locations on the device, either at the end
                         (for 1.0), at the start (for 1.1) or 4K from the
                         start (for 1.2).  "1" is equivalent to "1.2" (the
                         commonly preferred 1.x format).  "default" is
                         equivalent to "1.2".
    
                  ddf    Use the "Industry Standard" DDF (Disk Data Format)
                         format defined by SNIA. DDF is deprecated and there
                         is no active development around it.  When creating a
                         DDF array a **CONTAINER** will be created, and normal
                         arrays can be created in that container.
    
                  imsm   Use the Intel(R) Matrix Storage Manager metadata
                         format.  This creates a **CONTAINER** which is managed
                         in a similar manner to DDF, and is supported by an
                         option-rom on some platforms:
    
                         **https://www.intel.com/content/www/us/en/support/products/122484 **
    
           **--homehost=**
                  This will override any **HOMEHOST** setting in the config file
                  and provides the identity of the host which should be
                  considered the home for any arrays.
    
                  When creating an array, the **homehost** will be recorded in
                  the metadata.  For version-1 superblocks, it will be
                  prefixed to the array name.  For version-0.90 superblocks,
                  part of the SHA1 hash of the hostname will be stored in the
                  latter half of the UUID.
    
                  When reporting information about an array, any array which
                  is tagged for the given homehost will be reported as such.
    
                  When using Auto-Assemble, only arrays tagged for the given
                  homehost will be allowed to use 'local' names (i.e. not
                  ending in '_' followed by a digit string).  See below under
                  **Auto-Assembly**.
    
                  The special name "**any** " can be used as a wild card.  If an
                  array is created with **--homehost=any** then the name "**any** "
                  will be stored in the array and it can be assembled in the
                  same way on any host.  If an array is assembled with this
                  option, then the homehost recorded on the array will be
                  ignored.
    
           **--prefer=**
                  When _mdadm_ needs to print the name for a device it normally
                  finds the name in **/dev** which refers to the device and is
                  the shortest.  When a path component is given with **--prefer**
                  _mdadm_ will prefer a longer name if it contains that
                  component.  For example **--prefer=by-uuid** will prefer a name
                  in a subdirectory of **/dev** called **by-uuid**.
    
                  This functionality is currently only provided by **--detail**
                  and **--monitor**.
    
           **--home-cluster=**
                  specifies the cluster name for the md device. The md device
                  can be assembled only on the cluster which matches the name
                  specified. If this option is not provided, mdadm tries to
                  detect the cluster name automatically.
    


## For create, build, or grow:



           **-n** , **--raid-devices=**
                  Specify the number of active devices in the array.  This,
                  plus the number of spare devices (see below) must equal the
                  number of _component-devices_ (including "**missing** " devices)
                  that are listed on the command line for **--create**.  Setting
                  a value of 1 is probably a mistake and so requires that
                  **--force** be specified first.  A value of 1 will then be
                  allowed for linear, multipath, RAID0 and RAID1.  It is
                  never allowed for RAID4, RAID5 or RAID6.
                  This number can only be changed using **--grow** for RAID1,
                  RAID4, RAID5 and RAID6 arrays.
    
           **-x** , **--spare-devices=**
                  Specify the number of spare (eXtra) devices in the initial
                  array.  Spares can also be added and removed later.  The
                  number of component devices listed on the command line must
                  equal the number of RAID devices plus the number of spare
                  devices.
    
           **-z** , **--size=**
                  Amount (in Kilobytes) of space to use from each drive in
                  RAID levels 1/4/5/6/10 and for RAID 0 on external metadata.
                  This must be a multiple of the chunk size, and must leave
                  about 128Kb of space at the end of the drive for the RAID
                  superblock.  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 warning is issued.
    
                  A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
                  Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
    
                  Sometimes a replacement drive can be a little smaller than
                  the original drives though this should be minimised by
                  IDEMA standards.  Such a replacement drive will be rejected
                  by _md_.  To guard against this it can be useful to set the
                  initial size slightly smaller than the smaller device with
                  the aim that it will still be larger than any replacement.
    
                  This option can be used with **--create** for determining the
                  initial size of an array. For external metadata, it can be
                  used on a volume, but not on a container itself.  Setting
                  the initial size of **RAID 0** array is only valid for external
                  metadata.
    
                  This value can be set with **--grow** for RAID level 1/4/5/6/10
                  though DDF arrays may not be able to support this.  RAID 0
                  array size cannot be changed.  If the array was created
                  with a size smaller than the currently active drives, the
                  extra space can be accessed using **--grow**.  The size can be
                  given as **max** which means to choose the largest size that
                  fits on all current drives.
    
                  Before reducing the size of the array (with **--grow --size=**)
                  you should make sure that space isn't needed.  If the
                  device holds a filesystem, you would need to resize the
                  filesystem to use less space.
    
                  After reducing the array size you should check that the
                  data stored in the device is still available.  If the
                  device holds a filesystem, then an 'fsck' of the filesystem
                  is a minimum requirement.  If there are problems the array
                  can be made bigger again with no loss with another **--grow**
                  **--size=** command.
    
           **-Z** , **--array-size=**
                  This is only meaningful with **--grow** and its effect is not
                  persistent: when the array is stopped and restarted the
                  default array size will be restored.
    
                  Setting the array-size causes the array to appear smaller
                  to programs that access the data.  This is particularly
                  needed before reshaping an array so that it will be
                  smaller.  As the reshape is not reversible, but setting the
                  size with **--array-size** is, it is required that the array
                  size is reduced as appropriate before the number of devices
                  in the array is reduced.
    
                  Before reducing the size of the array you should make sure
                  that space isn't needed.  If the device holds a filesystem,
                  you would need to resize the filesystem to use less space.
    
                  After reducing the array size you should check that the
                  data stored in the device is still available.  If the
                  device holds a filesystem, then an 'fsck' of the filesystem
                  is a minimum requirement.  If there are problems the array
                  can be made bigger again with no loss with another **--grow**
                  **--array-size=** command.
    
                  A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
                  Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
                  A value of **max** restores the apparent size of the array to
                  be whatever the real amount of available space is.
    
                  Clustered arrays do not support this parameter yet.
    
           **-c** , **--chunk=**
                  Specify chunk size in kilobytes.  The default when creating
                  an array is 512KB.  To ensure compatibility with earlier
                  versions, the default when building an array with no
                  persistent metadata is 64KB.  This is only meaningful for
                  RAID0, RAID4, RAID5, RAID6, and RAID10.
    
                  RAID4, RAID5, RAID6, and RAID10 require the chunk size to
                  be a power of 2, with minimal chunk size being 4KB.
    
                  A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
                  Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
    
           **--rounding=**
                  Specify the rounding factor for a Linear array.  The size
                  of each component will be rounded down to a multiple of
                  this size.  This is a synonym for **--chunk** but highlights
                  the different meaning for Linear as compared to other RAID
                  levels.  The default is 0K (i.e. no rounding).
    
           **-l** , **--level=**
                  Set RAID level.  When used with **--create** , options are:
                  linear, raid0, 0, stripe, raid1, 1, mirror, raid4, 4,
                  raid5, 5, raid6, 6, raid10, 10, multipath, mp, faulty,
                  container.  Obviously some of these are synonymous.
    
                  When a **CONTAINER** metadata type is requested, only the
                  **container** level is permitted, and it does not need to be
                  explicitly given.
    
                  When used with **--build** , only linear, stripe, raid0, 0,
                  raid1, multipath, mp, and faulty are valid.
    
                  Can be used with **--grow** to change the RAID level in some
                  cases.  See LEVEL CHANGES below.
    
           **-p** , **--layout=**
                  This option configures the fine details of data layout for
                  RAID5, RAID6, and RAID10 arrays, and controls the failure
                  modes for _faulty_.  It can also be used for working around a
                  kernel bug with RAID0, but generally doesn't need to be
                  used explicitly.
    
                  The layout of the RAID5 parity block can be one of
                  **left-asymmetric** , **left-symmetric** , **right-asymmetric** ,
                  **right-symmetric** , **la** , **ra** , **ls** , **rs**.  The default is
                  **left-symmetric**.
    
                  It is also possible to cause RAID5 to use a RAID4-like
                  layout by choosing **parity-first** , or **parity-last**.
    
                  Finally for RAID5 there are DDF-compatible layouts,
                  **ddf-zero-restart** , **ddf-N-restart** , and **ddf-N-continue**.
    
                  These same layouts are available for RAID6.  There are also
                  4 layouts that will provide an intermediate stage for
                  converting between RAID5 and RAID6.  These provide a layout
                  which is identical to the corresponding RAID5 layout on the
                  first N-1 devices, and has the 'Q' syndrome (the second
                  'parity' block used by RAID6) on the last device.  These
                  layouts are: **left-symmetric-6** , **right-symmetric-6** ,
                  **left-asymmetric-6** , **right-asymmetric-6** , and **parity-first-6**.
    
                  When setting the failure mode for level _faulty,_ the options
                  are: **write-transient** , **wt** , **read-transient** , **rt** ,
                  **write-persistent** , **wp** , **read-persistent** , **rp** , **write-all** ,
                  **read-fixable** , **rf** , **clear** , **flush** , **none**.
    
                  Each failure mode can be followed by a number, which is
                  used as a period between fault generation.  Without a
                  number, the fault is generated once on the first relevant
                  request.  With a number, the fault will be generated after
                  that many requests, and will continue to be generated every
                  time the period elapses.
    
                  Multiple failure modes can be current simultaneously by
                  using the **--grow** option to set subsequent failure modes.
    
                  "clear" or "none" will remove any pending or periodic
                  failure modes, and "flush" will clear any persistent
                  faults.
    
                  The layout options for RAID10 are one of 'n', 'o' or 'f'
                  followed by a small number signifying the number of copies
                  of each datablock.  The default is 'n2'.  The supported
                  options are:
    
                  _'n'_ signals 'near' copies.  Multiple copies of one data
                  block are at similar offsets in different devices.
    
                  _'o'_ signals 'offset' copies.  Rather than the chunks being
                  duplicated within a stripe, whole stripes are duplicated
                  but are rotated by one device so duplicate blocks are on
                  different devices.  Thus subsequent copies of a block are
                  in the next drive, and are one chunk further down.
    
                  _'f'_ signals 'far' copies (multiple copies have very
                  different offsets).  See md(4) for more detail about
                  'near', 'offset', and 'far'.
    
                  As for the number of copies of each data block, 2 is
                  normal, 3 can be useful.  This number can be at most equal
                  to the number of devices in the array.  It does not need to
                  divide evenly into that number (e.g. it is perfectly legal
                  to have an 'n2' layout for an array with an odd number of
                  devices).
    
                  A bug introduced in Linux 3.14 means that RAID0 arrays **with**
                  **devices of differing sizes** started using a different
                  layout.  This could lead to data corruption.  Since Linux
                  5.4 (and various stable releases that received backports),
                  the kernel will not accept such an array unless a layout is
                  explicitly set.  It can be set to '**original** ' or
                  '**alternate** '.  When creating a new array, _mdadm_ will select
                  '**original** ' by default, so the layout does not normally need
                  to be set.  An array created for either '**original** ' or
                  '**alternate** ' will not be recognized by an (unpatched) kernel
                  prior to 5.4.  To create a RAID0 array with devices of
                  differing sizes that can be used on an older kernel, you
                  can set the layout to '**dangerous** '.  This will use whichever
                  layout the running kernel supports, so the data on the
                  array may become corrupt when changing kernel from pre-3.14
                  to a later kernel.
    
                  When an array is converted between RAID5 and RAID6 an
                  intermediate RAID6 layout is used in which the second
                  parity block (Q) is always on the last device.  To convert
                  a RAID5 to RAID6 and leave it in this new layout (which
                  does not require re-striping) use **--layout=preserve**.  This
                  will try to avoid any restriping.
    
                  The converse of this is **--layout=normalise** which will
                  change a non-standard RAID6 layout into a more standard
                  arrangement.
    
           **--parity=**
                  same as **--layout**(thus explaining the p of **-p**).
    
           **-b** , **--bitmap=**
                  Specify how to store a write-intent bitmap.  Following
                  values are supported:
    
                  **internal** - the bitmap is stored with the metadata on the
                  array and so is replicated on all devices.
    
                  **clustered** - the array is created for a clustered
                  environment. One bitmap is created for each node as defined
                  by the **--nodes** parameter and are stored internally.
    
                  **none** - create array with no bitmap or remove any present
                  bitmap (grow mode).
    
           **--bitmap-chunk=**
                  Set the chunk size of the bitmap.  Each bit corresponds to
                  that many Kilobytes of storage.
    
                  **internal** bitmap, the chunk size defaults to 64Meg, or
                  larger if necessary to fit the bitmap into the available
                  space.
    
                  A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
                  Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
    
           **-W** , **--write-mostly**
                  subsequent devices listed in a **--build** , **--create** , or **--add**
                  command will be flagged as 'write-mostly'.  This is valid
                  for RAID1 only and means that the 'md' driver will avoid
                  reading from these devices if at all possible.  This can be
                  useful if mirroring over a slow link.
    
           **--write-behind=**
                  Specify that write-behind mode should be enabled (valid for
                  RAID1 only).  If an argument is specified, it will set the
                  maximum number of outstanding writes allowed.  The default
                  value is 256.  A write-intent bitmap is required in order
                  to use write-behind mode, and write-behind is only
                  attempted on drives marked as _write-mostly_.
    
           **--failfast**
                  subsequent devices listed in a **--create** or **--add** command
                  will be flagged as  'failfast'.  This is valid for RAID1
                  and RAID10 only.  IO requests to these devices will be
                  encouraged to fail quickly rather than cause long delays
                  due to error handling.  Also no attempt is made to repair a
                  read error on these devices.
    
                  If an array becomes degraded so that the 'failfast' device
                  is the only usable device, the 'failfast' flag will then be
                  ignored and extended delays will be preferred to complete
                  failure.
    
                  The 'failfast' flag is appropriate for storage arrays which
                  have a low probability of true failure, but which may
                  sometimes cause unacceptable delays due to internal
                  maintenance functions.
    
           **--assume-clean**
                  Tell _mdadm_ that the array pre-existed and is known to be
                  clean.  It can be useful when trying to recover from a
                  major failure as you can be sure that no data will be
                  affected unless you actually write to the array.  It can
                  also be used when creating a RAID1 or RAID10 if you want to
                  avoid the initial resync, however this practice — while
                  normally safe — is not recommended.  Use this only if you
                  really know what you are doing.
    
                  When the devices that will be part of a new array were
                  filled with zeros before creation the operator knows the
                  array is actually clean. If that is the case, such as after
                  running badblocks, this argument can be used to tell mdadm
                  the facts the operator knows.
    
                  When an array is resized to a larger size with **--grow**
                  **--size=** the new space is normally resynced in that same way
                  that the whole array is resynced at creation.
                  **--assume-clean** can be used with that command to avoid the
                  automatic resync.
    
           **--write-zeroes**
                  When creating an array, send write zeroes requests to all
                  the block devices.  This should zero the data area on all
                  disks such that the initial sync is not necessary and, if
                  successful, will behave as if **--assume-clean** was specified.
    
                  This is intended for use with devices that have hardware
                  offload for zeroing, but despite this zeroing can still
                  take several minutes for large disks.  Thus a message is
                  printed before and after zeroing and each disk is zeroed in
                  parallel with the others.
    
                  This is only meaningful with --create.
    
           **--backup-file=**
                  This is needed when **--grow** is used to increase the number
                  of raid devices in a RAID5 or RAID6 if there are no spare
                  devices available, or to shrink, change RAID level or
                  layout.  See the GROW MODE section below on RAID-DEVICES
                  CHANGES.  The file must be stored on a separate device, not
                  on the RAID array being reshaped.
    
           **--data-offset=**
                  Arrays with 1.x metadata can leave a gap between the start
                  of the device and the start of array data.  This gap can be
                  used for various metadata.  The start of data is known as
                  the _data-offset_.  Normally an appropriate data offset is
                  computed automatically.  However it can be useful to set it
                  explicitly such as when re-creating an array which was
                  originally created using a different version of _mdadm_ which
                  computed a different offset.
    
                  Setting the offset explicitly over-rides the default.  The
                  value given is in Kilobytes unless a suffix of 'K', 'M',
                  'G' or 'T' is used to explicitly indicate Kilobytes,
                  Megabytes, Gigabytes or Terabytes respectively.
    
                  **--data-offset** can also be used with **--grow** for some RAID
                  levels (initially on RAID10).  This allows the data-offset
                  to be changed as part of the reshape process.  When the
                  data offset is changed, no backup file is required as the
                  difference in offsets is used to provide the same
                  functionality.
    
                  When the new offset is earlier than the old offset, the
                  number of devices in the array cannot shrink.  When it is
                  after the old offset, the number of devices in the array
                  cannot increase.
    
                  When creating an array, **--data-offset** can be specified as
                  **variable**.  In the case each member device is expected to
                  have an offset appended to the name, separated by a colon.
                  This makes it possible to recreate exactly an array which
                  has varying data offsets (as can happen when different
                  versions of _mdadm_ are used to add different devices).
    
           **-N** , **--name=**
                  Set a **name** for the array. It cannot be longer than 32
                  chars. This is effective when creating an array with a v1
                  metadata, or an external array.
    
                  If name is needed but not specified, it is taken from the
                  basename of the device that is being created. See **DEVICE**
                  **NAMES**
    
           **-R** , **--run**
                  Insist that _mdadm_ run the array, even if some of the
                  components appear to be active in another array or
                  filesystem.  Normally _mdadm_ will ask for confirmation
                  before including such components in an array.  This option
                  causes that question to be suppressed.
    
           **-f** , **--force**
                  Insist that _mdadm_ accept the geometry and layout specified
                  without question.  Normally _mdadm_ will not allow the
                  creation of an array with only one device, and will try to
                  create a RAID5 array with one missing drive (as this makes
                  the initial resync work faster).  With **--force** , _mdadm_ will
                  not try to be so clever.
    
           **-o** , **--readonly**
                  Start the array **read only** rather than read-write as normal.
                  No writes will be allowed to the array, and no resync,
                  recovery, or reshape will be started. It works with Create,
                  Assemble, Manage and Misc mode.
    
           **-a** , **--add**
                  This option can be used in Grow mode in two cases.
    
                  If the target array is a Linear array, then **--add** can be
                  used to add one or more devices to the array.  They are
                  simply catenated on to the end of the array.  Once added,
                  the devices cannot be removed.
    
                  If the **--raid-disks** option is being used to increase the
                  number of devices in an array, then **--add** can be used to
                  add some extra devices to be included in the array.  In
                  most cases this is not needed as the extra devices can be
                  added as spares first, and then the number of raid disks
                  can be changed.  However, for RAID0 it is not possible to
                  add spares.  So to increase the number of devices in a
                  RAID0, it is necessary to set the new number of devices,
                  and to add the new devices, in the same command.
    
           **--nodes**
                  Only works when the array is created for a clustered
                  environment. It specifies the maximum number of nodes in
                  the cluster that will use this device simultaneously. If
                  not specified, this defaults to 4.
    
           **--write-journal**
                  Specify journal device for the RAID-4/5/6 array. The
                  journal device should be an SSD with a reasonable lifetime.
    
           **-k** , **--consistency-policy=**
                  Specify how the array maintains consistency in the case of
                  an unexpected shutdown.  Only relevant for RAID levels with
                  redundancy.  Currently supported options are:
    
                  **resync** Full resync is performed and all redundancy is
                         regenerated when the array is started after an
                         unclean shutdown.
    
                  **bitmap** Resync assisted by a write-intent bitmap. Implicitly
                         selected when using **--bitmap**.
    
                  **journal**
                         For RAID levels 4/5/6, the journal device is used to
                         log transactions and replay after an unclean
                         shutdown. Implicitly selected when using
                         **--write-journal**.
    
                  **ppl** For RAID5 only, Partial Parity Log is used to close
                         the write hole and eliminate resync. PPL is stored
                         in the metadata region of RAID member drives, no
                         additional journal drive is needed.
    
                  Can be used with --grow to change the consistency policy of
                  an active array in some cases. See CONSISTENCY POLICY
                  CHANGES below.
    


## For assemble:



           **-u** , **--uuid=**
                  uuid of array to assemble.  Devices which don't have this
                  uuid are excluded
    
           **-m** , **--super-minor=**
                  Minor number of device that array was created for.  Devices
                  which don't have this minor number are excluded.  If you
                  create an array as /dev/md1, then all superblocks will
                  contain the minor number 1, even if the array is later
                  assembled as /dev/md2.
    
                  Giving the literal word "dev" for **--super-minor** will cause
                  _mdadm_ to use the minor number of the md device that is
                  being assembled.  e.g. when assembling **/dev/md0** ,
                  **--super-minor=dev** will look for super blocks with a minor
                  number of 0.
    
                  **--super-minor** is only relevant for v0.90 metadata, and
                  should not normally be used.  Using **--uuid** is much safer.
    
           **-N** , **--name=**
                  Specify the name of the array to assemble. It cannot be
                  longer than 32 chars.  This must be the name that was
                  specified when creating the array. It must either match the
                  name stored in the superblock exactly, or it must match
                  with the current _homehost_ prefixed to the start of the
                  given name.
    
           **-f** , **--force**
                  Assemble the array even if the metadata on some devices
                  appears to be out-of-date.  If _mdadm_ cannot find enough
                  working devices to start the array, but can find some
                  devices that are recorded as having failed, then it will
                  mark those devices as working so that the array can be
                  started. This works only for native. For external metadata
                  it allows to start dirty degraded RAID 4, 5, 6.  An array
                  which requires **--force** to be started may contain data
                  corruption.  Use it carefully.
    
           **-R** , **--run**
                  Attempt to start the array even if fewer drives were given
                  than were present last time the array was active.  Normally
                  if not all the expected drives are found and **--scan** is not
                  used, then the array will be assembled but not started.
                  With **--run** an attempt will be made to start it anyway.
    
           **--no-degraded**
                  This is the reverse of **--run** in that it inhibits the
                  startup of array unless all expected drives are present.
                  This is only needed with **--scan,** and can be used if the
                  physical connections to devices are not as reliable as you
                  would like.
    
           **--backup-file=**
                  If **--backup-file** was used while reshaping an array (e.g.
                  changing number of devices or chunk size) and the system
                  crashed during the critical section, then the same
                  **--backup-file** must be presented to **--assemble** to allow
                  possibly corrupted data to be restored, and the reshape to
                  be completed.
    
           **--invalid-backup**
                  If the file needed for the above option is not available
                  for any reason an empty file can be given together with
                  this option to indicate that the backup file is invalid.
                  In this case the data that was being rearranged at the time
                  of the crash could be irrecoverably lost, but the rest of
                  the array may still be recoverable.  This option should
                  only be used as a last resort if there is no way to recover
                  the backup file.
    
           **-U** , **--update=**
                  Update the superblock on each device while assembling the
                  array.  The argument given to this flag can be one of
                  **summaries** , **uuid** , **name** , **nodes** , **homehost** , **home-cluster** ,
                  **resync** , **byteorder** , **devicesize** , **no-bitmap** , **bbl** , **no-bbl** , **ppl** ,
                  **no-ppl** , **layout-original** , **layout-alternate** ,
                  **layout-unspecified** , **metadata** , or **super-minor**.
    
                  The **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).
    
                  The **uuid** option will change the uuid of the array.  If a
                  UUID is given with the **--uuid** option that UUID will be used
                  as a new UUID and will **NOT** be used to help identify the
                  devices in the array.  If no **--uuid** is given, a random UUID
                  is chosen.
    
                  The **name** option will change the _name_ of the array as stored
                  in the superblock.  This is only supported for version-1
                  superblocks.
    
                  The **nodes** option will change the _nodes_ of the array as
                  stored in the bitmap superblock. This option only works for
                  a clustered environment.
    
                  The **homehost** option will change the _homehost_ as recorded in
                  the superblock.  For version-0 superblocks, this is the
                  same as updating the UUID.  For version-1 superblocks, this
                  involves updating the name.
    
                  The **home-cluster** option will change the cluster name as
                  recorded in the superblock and bitmap. This option only
                  works for a clustered environment.
    
                  The **resync** option will cause the array to be marked _dirty_
                  meaning that any redundancy in the array (e.g. parity for
                  RAID5, copies for RAID1) may be incorrect.  This will cause
                  the RAID system to perform a "resync" pass to make sure
                  that all redundant information is correct.
    
                  The **byteorder** option allows arrays to be moved between
                  machines with different byte-order, such as from a big-
                  endian machine like a Sparc or some MIPS machines, to a
                  little-endian x86_64 machine.  When assembling such an
                  array for the first time after a move, giving
                  **--update=byteorder** will cause _mdadm_ to expect superblocks
                  to have their byteorder reversed, and will correct that
                  order before assembling the array.  This is only valid with
                  original (Version 0.90) superblocks.
    
                  The **summaries** option will correct the summaries in the
                  superblock.  That is the counts of total, working, active,
                  failed, and spare devices.
    
                  The **devicesize** option will rarely be of use.  It applies to
                  version 1.1 and 1.2 metadata only (where the metadata is at
                  the start of the device) and is only useful when the
                  component device has changed size (typically become
                  larger).  The version 1 metadata records the amount of the
                  device that can be used to store data, so if a device in a
                  version 1.1 or 1.2 array becomes larger, the metadata will
                  still be visible, but the extra space will not.  In this
                  case it might be useful to assemble the array with
                  **--update=devicesize**.  This will cause _mdadm_ to determine
                  the maximum usable amount of space on each device and
                  update the relevant field in the metadata.
    
                  The **metadata** option only works on v0.90 metadata arrays and
                  will convert them to v1.0 metadata.  The array must not be
                  dirty (i.e. it must not need a sync) and it must not have a
                  write-intent bitmap.
    
                  The old metadata will remain on the devices, but will
                  appear older than the new metadata and so will usually be
                  ignored. The old metadata (or indeed the new metadata) can
                  be removed by giving the appropriate **--metadata=** option to
                  **--zero-superblock**.
    
                  The **no-bitmap** option can be used when an array has an
                  internal bitmap which is corrupt in some way so that
                  assembling the array normally fails.  It will cause any
                  internal bitmap to be ignored.
    
                  The **bbl** option will reserve space in each device for a bad
                  block list.  This will be 4K in size and positioned near
                  the end of any free space between the superblock and the
                  data.
    
                  The **no-bbl** option will cause any reservation of space for a
                  bad block list to be removed.  If the bad block list
                  contains entries, this will fail, as removing the list
                  could cause data corruption.
    
                  The **ppl** option will enable PPL for a RAID5 array and
                  reserve space for PPL on each device. There must be enough
                  free space between the data and superblock and a write-
                  intent bitmap or journal must not be used.
    
                  The **no-ppl** option will disable PPL in the superblock.
    
                  The **layout-original** and **layout-alternate** options are for
                  RAID0 arrays with non-uniform devices size that were in use
                  before Linux 5.4.  If the array was being used with Linux
                  3.13 or earlier, then to assemble the array on a new
                  kernel, **--update=layout-original** must be given.  If the
                  array was created and used with a kernel from Linux 3.14 to
                  Linux 5.3, then **--update=layout-alternate** must be given.
                  This only needs to be given once.  Subsequent assembly of
                  the array will happen normally.  For more information, see
                  md(4).
    
                  The **layout-unspecified** option reverts the effect of
                  **layout-orignal** or **layout-alternate** and allows the array to
                  be again used on a kernel prior to Linux 5.3.  This option
                  should be used with great caution.
    


## For Manage mode:



           **-t** , **--test**
                  Unless a more serious error occurred, _mdadm_ will exit with
                  a status of 2 if no changes were made to the array and 0 if
                  at least one change was made.  This can be useful when an
                  indirect specifier such as **missing** , **detached** or **faulty** is
                  used in requesting an operation on the array.  **--test** will
                  report failure if these specifiers didn't find any match.
    
           **-a** , **--add**
                  hot-add listed devices.  If a device appears to have
                  recently been part of the array (possibly it failed or was
                  removed) the device is re-added as described in the next
                  point.  If that fails or the device was never part of the
                  array, the device is added as a hot-spare.  If the array is
                  degraded, it will immediately start to rebuild data onto
                  that spare.
    
                  Note that this and the following options are only
                  meaningful on array with redundancy.  They don't apply to
                  RAID0 or Linear.
    
           **--re-add**
                  re-add a device that was previously removed from an array.
                  If the metadata on the device reports that it is a member
                  of the array, and the slot that it used is still vacant,
                  then the device will be added back to the array in the same
                  position.  This will normally cause the data for that
                  device to be recovered.  However, based on the event count
                  on the device, the recovery may only require sections that
                  are flagged by a write-intent bitmap to be recovered or may
                  not require any recovery at all.
    
                  When used on an array that has no metadata (i.e. it was
                  built with **--build)** it will be assumed that bitmap-based
                  recovery is enough to make the device fully consistent with
                  the array.
    
                  **--re-add** can also be accompanied by **--update=devicesize** ,
                  **--update=bbl** , or **--update=no-bbl**.  See descriptions of
                  these options when used in Assemble mode for an explanation
                  of their use.
    
                  If the device name given is **missing** then _mdadm_ will try to
                  find any device that looks like it should be part of the
                  array but isn't and will try to re-add all such devices.
    
                  If the device name given is **faulty** then _mdadm_ will find all
                  devices in the array that are marked **faulty** , remove them
                  and attempt to immediately re-add them.  This can be useful
                  if you are certain that the reason for failure has been
                  resolved.
    
           **--add-spare**
                  Add a device as a spare.  This is similar to **--add** except
                  that it does not attempt **--re-add** first.  The device will
                  be added as a spare even if it looks like it could be a
                  recent member of the array.
    
           **-r** , **--remove**
                  remove listed devices.  They must not be active.  i.e. they
                  should be failed or spare devices.
    
                  As well as the name of a device file (e.g.  **/dev/sda1**) the
                  words **failed** , **detached** and names like **set-A** can be given to
                  **--remove**.  The first causes all failed devices to be
                  removed.  The second causes any device which is no longer
                  connected to the system (i.e an 'open' returns **ENXIO**) to be
                  removed.  The third will remove a set as described below
                  under **--fail**.
    
           **-f** , **--fail**
                  Mark listed devices as faulty.  As well as the name of a
                  device file, the word **detached** or a set name like **set-A** can
                  be given.  The former will cause any device that has been
                  detached from the system to be marked as failed.  It can
                  then be removed.
    
                  For RAID10 arrays where the number of copies evenly divides
                  the number of devices, the devices can be conceptually
                  divided into sets where each set contains a single complete
                  copy of the data on the array.  Sometimes a RAID10 array
                  will be configured so that these sets are on separate
                  controllers.  In this case, all the devices in one set can
                  be failed by giving a name like **set-A** or **set-B** to **--fail**.
                  The appropriate set names are reported by **--detail**.
    
           **--set-faulty**
                  same as **--fail**.
    
           **--replace**
                  Mark listed devices as requiring replacement.  As soon as a
                  spare is available, it will be rebuilt and will replace the
                  marked device.  This is similar to marking a device as
                  faulty, but the device remains in service during the
                  recovery process to increase resilience against multiple
                  failures.  When the replacement process finishes, the
                  replaced device will be marked as faulty.
    
           **--with** This can follow a list of **--replace** devices.  The devices
                  listed after **--with** will preferentially be used to replace
                  the devices listed after **--replace**.  These devices must
                  already be spare devices in the array.
    
           **--write-mostly**
                  Subsequent devices that are added or re-added will have the
                  'write-mostly' flag set.  This is only valid for RAID1 and
                  means that the 'md' driver will avoid reading from these
                  devices if possible.
    
           **--readwrite**
                  Subsequent devices that are added or re-added will have the
                  'write-mostly' flag cleared.
    
           **--cluster-confirm**
                  Confirm the existence of the device. This is issued in
                  response to an --add request by a node in a cluster. When a
                  node adds a device it sends a message to all nodes in the
                  cluster to look for a device with a UUID. This translates
                  to a udev notification with the UUID of the device to be
                  added and the slot number. The receiving node must
                  acknowledge this message with --cluster-confirm. Valid
                  arguments are <slot>:<devicename> in case the device is
                  found or <slot>:missing in case the device is not found.
    
           **--add-journal**
                  Add a journal to an existing array, or recreate journal for
                  a RAID-4/5/6 array that lost a journal device. To avoid
                  interrupting ongoing write operations, **--add-journal** only
                  works for array in Read-Only state.
    
           **--failfast**
                  Subsequent devices that are added or re-added will have the
                  'failfast' flag set.  This is only valid for RAID1 and
                  RAID10 and means that the 'md' driver will avoid long
                  timeouts on error handling where possible.
    
           **--nofailfast**
                  Subsequent devices that are re-added will be re-added
                  without the 'failfast' flag set.
    
           Each of these options requires that the first device listed is the
           array to be acted upon, and the remainder are component devices to
           be added, removed, marked as faulty, etc.  Several different
           operations can be specified for different devices, e.g.
                mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove
                /dev/sdb1
           Each operation applies to all devices listed until the next
           operation.
    
           If an array is using a write-intent bitmap, then devices which
           have been removed can be re-added in a way that avoids a full
           reconstruction but instead just updates the blocks that have
           changed since the device was removed.  For arrays with persistent
           metadata (superblocks) this is done automatically.  For arrays
           created with **--build** mdadm needs to be told that this device we
           removed recently with **--re-add**.
    
           Devices can only be removed from an array if they are not in
           active use, i.e. that must be spares or failed devices.  To remove
           an active device, it must first be marked as **faulty.**
    


## For Misc mode:



           **-Q** , **--query**
                  Examine a device to see (1) if it is an md device and (2)
                  if it is a component of an md array.  Information about
                  what is discovered is presented.
    
           **-D** , **--detail**
                  Print details of one or more md devices.
    
           **--detail-platform**
                  Print details of the platform's RAID capabilities (firmware
                  / hardware topology) for a given metadata format. If used
                  without an argument, mdadm will scan all controllers
                  looking for their capabilities. Otherwise, mdadm will only
                  look at the controller specified by the argument in the
                  form of an absolute filepath or a link, e.g.
                  _/sys/devices/pci0000:00/0000:00:1f.2_.
    
           **-Y** , **--export**
                  When used with **--detail** , **--detail-platform** , **--examine** , or
                  **--incremental** output will be formatted as **key=value** pairs
                  for easy import into the environment.
    
                  With **--incremental** The value **MD_STARTED** indicates whether
                  an array was started (**yes**) or not, which may include a
                  reason (**unsafe** , **nothing** , **no**).  Also the value **MD_FOREIGN**
                  indicates if the array is expected on this host (**no**), or
                  seems to be from elsewhere (**yes**).
    
           **-E** , **--examine**
                  Print contents of the metadata stored on the named
                  device(s).  Note the contrast between **--examine** and
                  **--detail**.  **--examine** applies to devices which are
                  components of an array, while **--detail** applies to a whole
                  array which is currently active.
    
           **-X** , **--examine-bitmap**
                  Report information about a bitmap.  The argument is an
                  array component. Note that running this on an array device
                  (e.g.  **/dev/md0**) does not report the bitmap for that array.
    
           **--examine-badblocks**
                  List the bad-blocks recorded for the device, if a bad-
                  blocks list has been configured. Currently only **1.x** and
                  **IMSM** metadata support bad-blocks lists.
    
           **--dump=**_directory_
    
           **--restore=**_directory_
                  Save metadata from lists devices, or restore metadata to
                  listed devices.
    
           **-R** , **--run**
                  start a partially assembled array.  If **--assemble** did not
                  find enough devices to fully start the array, it might
                  leaving it partially assembled.  If you wish, you can then
                  use **--run** to start the array in degraded mode.
    
           **-S** , **--stop**
                  deactivate array, releasing all resources.
    
           **-o** , **--readonly**
                  mark array as readonly.
    
           **-w** , **--readwrite**
                  mark array as readwrite.
    
           **--zero-superblock**
                  If the device contains a valid md superblock, the block is
                  overwritten with zeros.  With **--force** the block where the
                  superblock would be is overwritten even if it doesn't
                  appear to be valid.
    
                  **Note:** Be careful when calling --zero-superblock with
                  clustered raid. Make sure the array isn't used or assembled
                  in another cluster node before executing it.
    
           **--kill-subarray=**
                  If the device is a container and the argument to
                  --kill-subarray specifies an inactive subarray in the
                  container, then the subarray is deleted.  Deleting all
                  subarrays will leave an 'empty-container' or spare
                  superblock on the drives.  See **--zero-superblock** for
                  completely removing a superblock.  Note that some formats
                  depend on the subarray index for generating a UUID, this
                  command will fail if it would change the UUID of an active
                  subarray.
    
           **--update-subarray=**
                  If the device is a container and the argument to
                  --update-subarray specifies a subarray in the container,
                  then attempt to update the given superblock field in the
                  subarray. See below in **MISC MODE** for details.
    
           **-t** , **--test**
                  When used with **--detail** , the exit status of _mdadm_ is set to
                  reflect the status of the device.  See below in **MISC MODE**
                  for details.
    
           **-W** , **--wait**
                  For each md device given, wait for any resync, recovery, or
                  reshape activity to finish before returning.  _mdadm_ will
                  return with success if it actually waited for every device
                  listed, otherwise it will return failure.
    
           **--wait-clean**
                  For each md device given, or each device in /proc/mdstat if
                  **--scan** is given, arrange for the array to be marked clean
                  as soon as possible.  _mdadm_ will return with success if the
                  array uses external metadata and we successfully waited.
                  For native arrays, this returns immediately as the kernel
                  handles dirty-clean transitions at shutdown.  No action is
                  taken if safe-mode handling is disabled.
    
           **--action=**
                  Set the "sync_action" for all md devices given to one of
                  **idle** , **frozen** , **check** , **repair**.  Setting to **idle** will abort
                  any currently running action though some actions will
                  automatically restart.  Setting to **frozen** will abort any
                  current action and ensure no other action starts
                  automatically.
    
                  Details of **check** and **repair** can be found it md(4) under
                  **SCRUBBING AND MISMATCHES**.
    
           **--udev-rules=**
                  it generates the udev rules to the file that handles hot-
                  plug bare devices.  Given the POLICYs defined under
                  _/etc/mdadm.conf (or_ /etc/mdadm/mdadm.conf)
    
                  See mdadm.conf(5) for more details and usage examples about
                  POLICY.
    


## For Incremental Assembly mode:



           **--rebuild-map** , **-r**
                  Rebuild the map file (**/run/mdadm/map**) that _mdadm_ uses to
                  help track which arrays are currently being assembled.
    
           **--run** , **-R**
                  Run any array assembled as soon as a minimal number of
                  devices is available, rather than waiting until all
                  expected devices are present.
    
           **--scan** , **-s**
                  Only meaningful with **-R** this will scan the **map** file for
                  arrays that are being incrementally assembled and will try
                  to start any that are not already started.
    
           **--fail** , **-f**
                  This allows the hot-plug system to remove devices that have
                  fully disappeared from the kernel.  It will first fail and
                  then remove the device from any array it belongs to.  The
                  device name given should be a kernel device name such as
                  "sda", not a name in _/dev_.
    
           **--path=**
                  Only used with --fail.  The 'path' given will be recorded
                  so that if a new device appears at the same location it can
                  be automatically added to the same array.  This allows the
                  failed device to be automatically replaced by a new device
                  without metadata if it appears at specified path.   This
                  option is normally only set by an _udev_ script.
    


## For Monitor mode:



           **-m** , **--mail**
                  Give an mail address to send alerts to. Can be configured
                  in **mdadm.conf** as MAILADDR.
    
           **-p** , **--program** , **--alert**
                  Give a program to be run whenever an event is detected. Can
                  be configured in **mdadm.conf** as PROGRAM.
    
           **-y** , **--syslog**
                  Cause all events to be reported through 'syslog'.  The
                  messages have facility of 'daemon' and varying priorities.
    
           **-d** , **--delay**
                  Give a delay in seconds. The default is 60 seconds.  _mdadm_
                  polls the md arrays and then waits this many seconds before
                  polling again if no event happened.  Can be configured in
                  **mdadm.conf** as MONITORDELAY.
    
           **-r** , **--increment**
                  Give a percentage increment.  _mdadm_ will generate RebuildNN
                  events with the given percentage increment.
    
           **-f** , **--daemonise**
                  Tell _mdadm_ to run as a background daemon if it decides to
                  monitor anything.  This causes it to fork and run in the
                  child, and to disconnect from the terminal.  The process id
                  of the child is written to stdout.  This is useful with
                  **--scan** which will only continue monitoring if a mail
                  address or alert program is found in the config file.
    
           **-i** , **--pid-file**
                  When _mdadm_ is running in daemon mode, write the pid of the
                  daemon process to the specified file, instead of printing
                  it on standard output.
    
           **-1** , **--oneshot**
                  Check arrays only once.  This will generate **NewArray** events
                  and more significantly **DegradedArray** and **SparesMissing**
                  events.  Running
                          **mdadm --monitor --scan -1**
                  from a cron script will ensure regular notification of any
                  degraded arrays.
    
           **-t** , **--test**
                  Generate a **TestMessage** alert for every array found at
                  startup.  This alert gets mailed and passed to the alert
                  program.  This can be used for testing that alert message
                  do get through successfully.
    
           **--no-sharing**
                  This inhibits the functionality for moving spares between
                  arrays.  Only one monitoring process started with **--scan**
                  but without this flag is allowed, otherwise the two could
                  interfere with each other.
    


## ASSEMBLE MODE



    
           Usage: **mdadm --assemble** _md-device options-and-component-devices..._
    
           Usage: **mdadm --assemble --scan** _md-devices-and-options..._
    
           Usage: **mdadm --assemble --scan** _options..._
    
           This usage assembles one or more RAID arrays from pre-existing
           components.  For each array, mdadm needs to know the md device,
           the identity of the array, and the number of component devices.
           These can be found in a number of ways.
    
           In the first usage example (without the **--scan**) the first device
           given is the md device.  In the second usage example, all devices
           listed are treated as md devices and assembly is attempted.  In
           the third (where no devices are listed) all md devices that are
           listed in the configuration file are assembled.  If no arrays are
           described by the configuration file, then any arrays that can be
           found on unused devices will be assembled.
    
           If precisely one device is listed, but **--scan** is not given, then
           _mdadm_ acts as though **--scan** was given and identity information is
           extracted from the configuration file.
    
           The identity can be given with the **--uuid** option, the **--name**
           option, or the **--super-minor** option, will be taken from the md-
           device record in the config file, or will be taken from the super
           block of the first component-device listed on the command line.
    
           Devices can be given on the **--assemble** command line or in the
           config file.  Only devices which have an md superblock which
           contains the right identity will be considered for any array.
    
           The config file is only used if explicitly named with **--config** or
           requested with (a possibly implicit) **--scan**.  In the latter case,
           the default config file is used.  See mdadm.conf(5) for more
           details.
    
           If **--scan** is not given, then the config file will only be used to
           find the identity of md arrays.
    
           Normally the array will be started after it is assembled.  However
           if **--scan** is not given and not all expected drives were listed,
           then the array is not started (to guard against usage errors).  To
           insist that the array be started in this case (as may work for
           RAID1, 4, 5, 6, or 10), give the **--run** flag.
    
           If _udev_ is active, _mdadm_ does not create any entries in **/dev** but
           leaves that to _udev_.  It does record information in **/run/mdadm/map**
           which will allow _udev_ to choose the correct name.
    
           If _mdadm_ detects that udev is not configured, it will create the
           devices in **/dev** itself.
    
       **Auto-Assembly**
           When **--assemble** is used with **--scan** and no devices are listed,
           _mdadm_ will first attempt to assemble all the arrays listed in the
           config file.
    
           If no arrays are listed in the config (other than those marked
           **< ignore>**) it will look through the available devices for possible
           arrays and will try to assemble anything that it finds.  Arrays
           which are tagged as belonging to the given homehost will be
           assembled and started normally.  Arrays which do not obviously
           belong to this host are given names that are expected not to
           conflict with anything local, and are started "read-auto" so that
           nothing is written to any device until the array is written to.
           i.e.  automatic resync etc is delayed.
    
           If _mdadm_ finds a consistent set of devices that look like they
           should comprise an array, and if the superblock is tagged as
           belonging to the given home host, it will automatically choose a
           device name and try to assemble the array.  If the array uses
           version-0.90 metadata, then the **minor** number as recorded in the
           superblock is used to create a name in **/dev/md/** so for example
           **/dev/md/3**.  If the array uses version-1 metadata, then the **name**
           from the superblock is used to similarly create a name in **/dev/md/**
           (the name will have any 'host' prefix stripped first).
    
           This behaviour can be modified by the _AUTO_ line in the _mdadm.conf_
           configuration file.  This line can indicate that specific metadata
           type should, or should not, be automatically assembled.  If an
           array is found which is not listed in _mdadm.conf_ and has a
           metadata format that is denied by the _AUTO_ line, then it will not
           be assembled.  The _AUTO_ line can also request that all arrays
           identified as being for this homehost should be assembled
           regardless of their metadata type.  See mdadm.conf(5) for further
           details.
    
           Note: Auto-assembly cannot be used for assembling and activating
           some arrays which are undergoing reshape.  In particular as the
           **backup-file** cannot be given, any reshape which requires a backup
           file to continue cannot be started by auto-assembly.  An array
           which is growing to more devices and has passed the critical
           section can be assembled using auto-assembly.
    


## BUILD MODE



    
           Usage: **mdadm --build** _md-device_ **--chunk=**_X_ **--level=**_Y_
                       **--raid-devices=**_Z devices_
    
           This usage is similar to **--create**.  The difference is that it
           creates an array without a superblock.  With these arrays there is
           no difference between initially creating the array and
           subsequently assembling the array, except that hopefully there is
           useful data there in the second case.
    
           The level may raid0, linear, raid1, raid10, multipath, or faulty,
           or one of their synonyms.  All devices must be listed and the
           array will be started once complete.  It will often be appropriate
           to use **--assume-clean** with levels raid1 or raid10.
    


## CREATE MODE



    
           Usage: **mdadm --create** _md-device_ **--chunk=**_X_ **--level=**_Y_
                       **--raid-devices=**_Z devices_
    
           This usage will initialize a new md array, associate some devices
           with it, and activate the array.
    
           _md-device_ is a new device. This could be standard name or chosen
           name. For details see: **DEVICE NAMES**
    
           The named device will normally not exist when _mdadm --create_ is
           run, but will be created by _udev_ once the array becomes active.
    
           The max length md-device name is limited to 32 characters.
           Different metadata types have more strict limitation (like IMSM
           where only 16 characters are allowed).  For that reason, long name
           could be truncated or rejected, it depends on metadata policy.
    
           As devices are added, they are checked to see if they contain RAID
           superblocks or filesystems.  They are also checked to see if the
           variance in device size exceeds 1%.
    
           If any discrepancy is found, the array will not automatically be
           run, though the presence of a **--run** can override this caution.
    
           To create a "degraded" array in which some devices are missing,
           simply give the word "**missing** " in place of a device name.  This
           will cause _mdadm_ to leave the corresponding slot in the array
           empty.  For a RAID4 or RAID5 array at most one slot can be
           "**missing** "; for a RAID6 array at most two slots.  For a RAID1
           array, only one real device needs to be given.  All of the others
           can be "**missing** ".
    
           When creating a RAID5 array, _mdadm_ will automatically create a
           degraded array with an extra spare drive.  This is because
           building the spare into a degraded array is in general faster than
           resyncing the parity on a non-degraded, but not clean, array.
           This feature can be overridden with the **--force** option.
    
           When creating a partition based array, using _mdadm_ with
           version-1.x metadata, the partition type should be set to **0xDA**
           (non fs-data).  This type of selection allows for greater
           precision since using any other [RAID auto-detect (0xFD) or a
           GNU/Linux partition (0x83)], might create problems in the event of
           array recovery through a live cdrom.
    
           A new array will normally get a randomly assigned 128bit UUID
           which is very likely to be unique.  If you have a specific need,
           you can choose a UUID for the array by giving the **--uuid=** option.
           Be warned that creating two arrays with the same UUID is a recipe
           for disaster.  Also, using **--uuid=** when creating a v0.90 array
           will silently override any **--homehost=** setting.
    
           Space for a bitmap will be reserved so that one can be added later
           with **--grow --bitmap=internal**.
    
           If the metadata type supports it (currently only 1.x and IMSM
           metadata), space will be allocated to store a bad block list.
           This allows a modest number of bad blocks to be recorded, allowing
           the drive to remain in service while only partially functional.
    
           When creating an array within a **CONTAINER** _mdadm_ can be given
           either the list of devices to use, or simply the name of the
           container.  The former case gives control over which devices in
           the container will be used for the array.  The latter case allows
           _mdadm_ to automatically choose which devices to use based on how
           much spare space is available.
    
           The General Management options that are valid with **--create** are:
    
           **--run** insist on running the array even if some devices look like
                  they might be in use.
    
           **--readonly**
                  start the array in readonly mode.
    


## MANAGE MODE



    
           Usage: **mdadm** _device options... devices..._
    
           This usage will allow individual devices in an array to be failed,
           removed or added.  It is possible to perform multiple operations
           with on command.  For example:
             **mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1**
           will firstly mark **/dev/hda1** as faulty in **/dev/md0** and will then
           remove it from the array and finally add it back in as a spare.
           However, only one md array can be affected by a single command.
    
           When a device is added to an active array, mdadm checks to see if
           it has metadata on it which suggests that it was recently a member
           of the array.  If it does, it tries to "re-add" the device.  If
           there have been no changes since the device was removed, or if the
           array has a write-intent bitmap which has recorded whatever
           changes there were, then the device will immediately become a full
           member of the array and those differences recorded in the bitmap
           will be resolved.
    


## MISC MODE



    
           Usage: **mdadm** _options ..._  _devices ..._
    
           MISC mode includes a number of distinct operations that operate on
           distinct devices.  The operations are:
    
           **--query**
                  The device is examined to see if it is (1) an active md
                  array, or (2) a component of an md array.  The information
                  discovered is reported.
    
           **--detail**
                  The device should be an active md device.  **mdadm** will
                  display a detailed description of the array.  **--brief** or
                  **--scan** will cause the output to be less detailed and the
                  format to be suitable for inclusion in **mdadm.conf**.  The
                  exit status of _mdadm_ will normally be 0 unless _mdadm_ failed
                  to get useful information about the device(s); however, if
                  the **--test** option is given, then the exit status will be:
    
                  0      The array is functioning normally.
    
                  1      The array has at least one failed device.
    
                  2      The array has multiple failed devices such that it
                         is unusable.
    
                  4      There was an error while trying to get information
                         about the device.
    
           **--detail-platform**
                  Print detail of the platform's RAID capabilities (firmware
                  / hardware topology).  If the metadata is specified with **-e**
                  or **--metadata=** then the return status will be:
    
                  0      metadata successfully enumerated its platform
                         components on this system
    
                  1      metadata is platform independent
    
                  2      metadata failed to find its platform components on
                         this system
    
           **--update-subarray=**
                  If the device is a container and the argument to
                  --update-subarray specifies a subarray in the container,
                  then attempt to update the given superblock field in the
                  subarray.  Similar to updating an array in "assemble" mode,
                  the field to update is selected by **-U** or **--update=** option.
                  The supported options are **name** , **ppl** , **no-ppl** , **bitmap** and
                  **no-bitmap**.
    
                  The **name** option updates the subarray name in the metadata.
                  It cannot be longer than 32 chars. If successes, new value
                  will be respected after next assembly.
    
                  The **ppl** and **no-ppl** options enable and disable PPL in the
                  metadata. Currently supported only for IMSM subarrays.
    
                  The **bitmap** and **no-bitmap** options enable and disable write-
                  intent bitmap in the metadata. Currently supported only for
                  IMSM subarrays.
    
           **--examine**
                  The device should be a component of an md array.  _mdadm_
                  will read the md superblock of the device and display the
                  contents.  If **--brief** or **--scan** is given, then multiple
                  devices that are components of the one array are grouped
                  together and reported in a single entry suitable for
                  inclusion in **mdadm.conf**.
    
                  Having **--scan** without listing any devices will cause all
                  devices listed in the config file to be examined.
    
           **--dump=**_directory_
                  If the device contains RAID metadata, a file will be
                  created in the _directory_ and the metadata will be written
                  to it.  The file will be the same size as the device and
                  will have the metadata written at the same location as it
                  exists in the device.  However, the file will be "sparse"
                  so that only those blocks containing metadata will be
                  allocated. The total space used will be small.
    
                  The filename used in the _directory_ will be the base name of
                  the device.   Further, if any links appear in _/dev/disk/by-_
                  _id_ which point to the device, then hard links to the file
                  will be created in _directory_ based on these _by-id_ names.
    
                  Multiple devices can be listed and their metadata will all
                  be stored in the one directory.
    
           **--restore=**_directory_
                  This is the reverse of **--dump**.  _mdadm_ will locate a file in
                  the directory that has a name appropriate for the given
                  device and will restore metadata from it.  Names that match
                  _/dev/disk/by-id_ names are preferred, however if two of
                  those refer to different files, _mdadm_ will not choose
                  between them but will abort the operation.
    
                  If a file name is given instead of a _directory_ then _mdadm_
                  will restore from that file to a single device, always
                  provided the size of the file matches that of the device,
                  and the file contains valid metadata.
    
           **--stop** The devices should be active md arrays which will be
                  deactivated, as long as they are not currently in use.
    
           **--run** This will fully activate a partially assembled md array.
    
           **--readonly**
                  This will mark an active array as read-only, providing that
                  it is not currently being used.
    
           **--readwrite**
                  This will change a **readonly** array back to being read/write.
    
           **--scan** For all operations except **--examine** , **--scan** will cause the
                  operation to be applied to all arrays listed in
                  **/proc/mdstat**.  For **--examine, --scan** causes all devices
                  listed in the config file to be examined.
    
           **-b** , **--brief**
                  Be less verbose.  This is used with **--detail** and **--examine**.
                  Using **--brief** with **--verbose** gives an intermediate level of
                  verbosity.
    


## MONITOR MODE



    
           Usage: **mdadm --monitor** _options... devices..._
    
           Monitor option can work in two modes:
    
           •   system wide mode, follow all md devices based on **/proc/mdstat,**
    
           •   follow only specified MD devices in command line.
    
           **--scan -** indicates system wide mode. Option causes the _monitor_ to
           track all md devices that appear in **/proc/mdstat.** If it is not
           set, then at least one **device** must be specified.
    
           Monitor usage causes _mdadm_ to periodically poll a number of md
           arrays and to report on any events noticed.
    
           In both modes, _monitor_ will work as long as there is an active
           array with redundancy and it is defined to follow (for **--scan**
           every array is followed).
    
           As well as reporting events, _mdadm_ may move a spare drive from one
           array to another if they are in the same **spare-group** or **domain** and
           if the destination array has a failed drive but no spares.
    
           The result of monitoring the arrays is the generation of events.
           These events are passed to a separate program (if specified) and
           may be mailed to a given E-mail address.
    
           When passing events to a program, the program is run once for each
           event, and is given 2 or 3 command-line arguments: the first is
           the name of the event (see below), the second is the name of the
           md device which is affected, and the third is the name of a
           related device if relevant (such as a component device that has
           failed).
    
           If **--scan** is given, then a **program** or an **e-mail** address must be
           specified on the command line or in the config file. If neither
           are available, then _mdadm_ will not monitor anything. For devices
           given directly in command line, without **program** or **email**
           specified, each event is reported to **stdout.**
    
           Note: On systems where mdadm monitoring is managed through
           systemd, the mdmonitor.service should be present. This service is
           designed to be the primary solution for array monitoring.  It is
           configured to operate in system-wide mode. It is initiated by udev
           when start criteria are met, e.g.  **mdadm.conf** exists and necessary
           configuration parameters are set.  It is kept alive as long as a
           redundant RAID array is active; it stops otherwise. User should
           customize MAILADDR in **mdadm.conf** to receive mail notifications.
           MONITORDELAY, MAILFROM and PROGRAM are optional. See mdadm.conf(5)
           for detailed description of these options.  Use systemctl status
           mdmonitor.service to verify status or determine if additional
           configuration is needed.
    
           The different events are:
    
               **DeviceDisappeared**
                      An md array which previously was configured appears to
                      no longer be configured. (syslog priority: Critical)
    
                      If _mdadm_ was told to monitor an array which is RAID0 or
                      Linear, then it will report **DeviceDisappeared** with the
                      extra information **Wrong-Level**.  This is because RAID0
                      and Linear do not support the device-failed, hot-spare
                      and resync operations which are monitored.
    
               **RebuildStarted**
                      An md array started reconstruction (e.g. recovery,
                      resync, reshape, check, repair). (syslog priority:
                      Warning)
    
               **Rebuild** _NN_
                      Where _NN_ is a two-digit number (eg. 05, 48). This
                      indicates that the rebuild has reached that percentage
                      of the total. The events are generated at a fixed
                      increment from 0. The increment size may be specified
                      with a command-line option (the default is 20). (syslog
                      priority: Warning)
    
               **RebuildFinished**
                      An md array that was rebuilding, isn't any more, either
                      because it finished normally or was aborted. (syslog
                      priority: Warning)
    
               **Fail** An active component device of an array has been marked
                      as faulty. (syslog priority: Critical)
    
               **FailSpare**
                      A spare component device which was being rebuilt to
                      replace a faulty device has failed. (syslog priority:
                      Critical)
    
               **SpareActive**
                      A spare component device which was being rebuilt to
                      replace a faulty device has been successfully rebuilt
                      and has been made active.  (syslog priority: Info)
    
               **NewArray**
                      A new md array has been detected in the **/proc/mdstat**
                      file.  (syslog priority: Info)
    
               **DegradedArray**
                      A newly noticed array appears to be degraded.  This
                      message is not generated when _mdadm_ notices a drive
                      failure which causes degradation, but only when _mdadm_
                      notices that an array is degraded when it first sees
                      the array.  (syslog priority: Critical)
    
               **MoveSpare**
                      A spare drive has been moved from one array in a **spare-**
                      **group** or **domain** to another to allow a failed drive to
                      be replaced.  (syslog priority: Info)
    
               **SparesMissing**
                      If _mdadm_ has been told, via the config file, that an
                      array should have a certain number of spare devices,
                      and _mdadm_ detects that it has fewer than this number
                      when it first sees the array, it will report a
                      **SparesMissing** message.  (syslog priority: Warning)
    
               **TestMessage**
                      An array was found at startup, and the **--test** flag was
                      given.  (syslog priority: Info)
    
           Only **Fail, FailSpare, DegradedArray, SparesMissing** and **TestMessage**
           cause Email to be sent.  All events cause the program to be run.
           The program is run with two or three arguments: the event name,
           the array device and possibly a second device.
    
           Each event has an associated array device (e.g.  **/dev/md1**) and
           possibly a second device.  For **Fail** , **FailSpare** , and **SpareActive**
           the second device is the relevant component device.  For **MoveSpare**
           the second device is the array that the spare was moved from.
    
           For _mdadm_ to move spares from one array to another, the different
           arrays need to be labeled with the same **spare-group** or the spares
           must be allowed to migrate through matching POLICY domains in the
           configuration file.  The **spare-group** name can be any string; it is
           only necessary that different spare groups use different names.
    
           When _mdadm_ detects that an array in a spare group has fewer active
           devices than necessary for the complete array, and has no spare
           devices, it will look for another array in the same spare group
           that has a full complement of working drives and a spare.  It will
           then attempt to remove the spare from the second array and add it
           to the first.  If the removal succeeds but the adding fails, then
           it is added back to the original array.
    
           If the spare group for a degraded array is not defined, _mdadm_ will
           look at the rules of spare migration specified by POLICY lines in
           **mdadm.conf** and then follow similar steps as above if a matching
           spare is found.
    


## GROW MODE



           The GROW mode is used for changing the size or shape of an active
           array.
    
           The following changes are supported:
    
           •   change the "size" attribute for RAID1, RAID4, RAID5 and RAID6.
    
           •   increase or decrease the "raid-devices" attribute of RAID0,
               RAID1, RAID4, RAID5, and RAID6.
    
           •   change the chunk-size and layout of RAID0, RAID4, RAID5, RAID6
               and RAID10.
    
           •   convert between RAID1 and RAID5, between RAID5 and RAID6,
               between RAID0, RAID4, and RAID5, and between RAID0 and RAID10
               (in the near-2 mode).
    
           •   add a write-intent bitmap to any array which supports these
               bitmaps, or remove a write-intent bitmap from such an array.
    
           •   change the array's consistency policy.
    
           Using GROW on containers is currently supported only for Intel's
           IMSM container format.  The number of devices in a container can
           be increased - which affects all arrays in the container - or an
           array in a container can be converted between levels where those
           levels are supported by the container, and the conversion is on of
           those listed above.
    
           Notes:
    
           •   Intel's native checkpointing doesn't use **--backup-file** option
               and it is transparent for assembly feature.
    
           •   Roaming between Windows(R) and Linux systems for IMSM metadata
               is not supported during grow process.
    
           •   When growing a raid0 device, the new component disk size (or
               external backup size) should be larger than LCM(old, new) *
               chunk-size * 2, where LCM() is the least common multiple of
               the old and new count of component disks, and "* 2" comes from
               the fact that mdadm refuses to use more than half of a spare
               device for backup space.
    
       **SIZE CHANGES**
           Normally when an array is built the "size" is taken from the
           smallest of the drives.  If all the small drives in an arrays are,
           over time, removed and replaced with larger drives, then you could
           have an array of large drives with only a small amount used.  In
           this situation, changing the "size" with "GROW" mode will allow
           the extra space to start being used.  If the size is increased in
           this way, a "resync" process will start to make sure the new parts
           of the array are synchronised.
    
           Note that when an array changes size, any filesystem that may be
           stored in the array will not automatically grow or shrink to use
           or vacate the space.  The filesystem will need to be explicitly
           told to use the extra space after growing, or to reduce its size
           **prior** to shrinking the array.
    
           Also, the size of an array cannot be changed while it has an
           active bitmap.  If an array has a bitmap, it must be removed
           before the size can be changed. Once the change is complete a new
           bitmap can be created.
    
       **RAID-DEVICES CHANGES**
           A RAID1 array can work with any number of devices from 1 upwards
           (though 1 is not very useful).  There may be times which you want
           to increase or decrease the number of active devices.  Note that
           this is different to hot-add or hot-remove which changes the
           number of inactive devices.
    
           When reducing the number of devices in a RAID1 array, the slots
           which are to be removed from the array must already be vacant.
           That is, the devices which were in those slots must be failed and
           removed.
    
           When the number of devices is increased, any hot spares that are
           present will be activated immediately.
    
           Changing the number of active devices in a RAID5 or RAID6 is much
           more effort. Every block in the array will need to be read and
           written back to a new location. Linux Kernel is able to increase
           or decrease the number of devices in a RAID5 and RAID6 safely,
           including restarting an interrupted "reshape".
    
           The Linux Kernel is able to convert a RAID0 into a RAID4 or RAID5.
           _mdadm_ uses this functionality and the ability to add devices to a
           RAID4 to allow devices to be added to a RAID0.  When requested to
           do this, _mdadm_ will convert the RAID0 to a RAID4, add the
           necessary disks and make the reshape happen, and then convert the
           RAID4 back to RAID0.
    
           When decreasing the number of devices, the size of the array will
           also decrease.  If there was data in the array, it could get
           destroyed and this is not reversible, so you should firstly shrink
           the filesystem on the array to fit within the new size.  To help
           prevent accidents, _mdadm_ requires that the size of the array be
           decreased first with **mdadm --grow --array-size**.  This is a
           reversible change which simply makes the end of the array
           inaccessible.  The integrity of any data can then be checked
           before the non-reversible reduction in the number of devices is
           request.
    
           When relocating the first few stripes on a RAID5 or RAID6, it is
           not possible to keep the data on disk completely consistent and
           crash-proof.  To provide the required safety, mdadm disables
           writes to the array while this "critical section" is reshaped, and
           takes a backup of the data that is in that section.  For grows,
           this backup may be stored in any spare devices that the array has,
           however it can also be stored in a separate file specified with
           the **--backup-file** option, and is required to be specified for
           shrinks, RAID level changes and layout changes.  If this option is
           used, and the system does crash during the critical period, the
           same file must be passed to **--assemble** to restore the backup and
           reassemble the array.  When shrinking rather than growing the
           array, the reshape is done from the end towards the beginning, so
           the "critical section" is at the end of the reshape.
    
       **LEVEL CHANGES**
           Changing the RAID level of any array happens instantaneously.
           However in the RAID5 to RAID6 case this requires a non-standard
           layout of the RAID6 data, and in the RAID6 to RAID5 case that non-
           standard layout is required before the change can be accomplished.
           So while the level change is instant, the accompanying layout
           change can take quite a long time.  A **--backup-file** is required.
           If the array is not simultaneously being grown or shrunk, so that
           the array size will remain the same - for example, reshaping a
           3-drive RAID5 into a 4-drive RAID6 - the backup file will be used
           not just for a "critical section" but throughout the reshape
           operation, as described below under LAYOUT CHANGES.
    
       **CHUNK-SIZE AND LAYOUT CHANGES**
           Changing the chunk-size or layout without also changing the number
           of devices as the same time will involve re-writing all blocks in-
           place.  To ensure against data loss in the case of a crash, a
           **--backup-file** must be provided for these changes.  Small sections
           of the array will be copied to the backup file while they are
           being rearranged.  This means that all the data is copied twice,
           once to the backup and once to the new layout on the array, so
           this type of reshape will go very slowly.
    
           If the reshape is interrupted for any reason, this backup file
           must be made available to **mdadm --assemble** so the array can be
           reassembled.  Consequently, the file cannot be stored on the
           device being reshaped.
    
       **BITMAP CHANGES**
           A write-intent bitmap can be added to, or removed from, an active
           array.
    
       **CONSISTENCY POLICY CHANGES**
           The consistency policy of an active array can be changed by using
           the **--consistency-policy** option in Grow mode. Currently this works
           only for the **ppl** and **resync** policies and allows to enable or
           disable the RAID5 Partial Parity Log (PPL).
    


## INCREMENTAL MODE



    
           Usage: **mdadm --incremental**[**--run**] [**--quiet**] _component-device_
                       [_optional-aliases-for-device_]
    
           Usage: **mdadm --incremental --fail** _component-device_
    
           Usage: **mdadm --incremental --rebuild-map**
    
           Usage: **mdadm --incremental --run --scan**
    
           This mode is designed to be used in conjunction with a device
           discovery system.  As devices are found in a system, they can be
           passed to **mdadm --incremental** to be conditionally added to an
           appropriate array.
    
           Conversely, it can also be used with the **--fail** flag to do just
           the opposite and find whatever array a particular device is part
           of and remove the device from that array.
    
           If the device passed is a **CONTAINER** device created by a previous
           call to _mdadm_ , then rather than trying to add that device to an
           array, all the arrays described by the metadata of the container
           will be started.
    
           _mdadm_ performs a number of tests to determine if the device is
           part of an array, and which array it should be part of.  If an
           appropriate array is found, or can be created, _mdadm_ adds the
           device to the array and conditionally starts the array.
    
           Note that _mdadm_ will normally only add devices to an array which
           were previously working (active or spare) parts of that array.
           The support for automatic inclusion of a new drive as a spare in
           some array requires a configuration through POLICY in config file.
    
           The tests that _mdadm_ makes are as follow:
    
           +      Is the device permitted by **mdadm.conf**?  That is, is it
                  listed in a **DEVICES** line in that file.  If **DEVICES** is
                  absent then the default it to allow any device.  Similarly
                  if **DEVICES** contains the special word **partitions** then any
                  device is allowed.  Otherwise the device name given to
                  _mdadm_ , or one of the aliases given, or an alias found in
                  the filesystem, must match one of the names or patterns in
                  a **DEVICES** line.
    
                  This is the only context where the aliases are used.  They
                  are usually provided by a _udev_ rules mentioning
                  **$env{DEVLINKS}**.
    
           +      Does the device have a valid md superblock?  If a specific
                  metadata version is requested with **--metadata** or **-e** then
                  only that style of metadata is accepted, otherwise _mdadm_
                  finds any known version of metadata.  If no _md_ metadata is
                  found, the device may be still added to an array as a spare
                  if POLICY allows.
    
           _mdadm_ keeps a list of arrays that it has partially assembled in
           **/run/mdadm/map**.  If no array exists which matches the metadata on
           the new device, _mdadm_ must choose a device name and unit number.
           It does this based on any name given in **mdadm.conf** or any name
           information stored in the metadata.  If this name suggests a unit
           number, that number will be used, otherwise a free unit number
           will be chosen.  Normally _mdadm_ will prefer to create a
           partitionable array, however if the **CREATE** line in **mdadm.conf**
           suggests that a non-partitionable array is preferred, that will be
           honoured.
    
           If the array is not found in the config file and its metadata does
           not identify it as belonging to the "homehost", then _mdadm_ will
           choose a name for the array which is certain not to conflict with
           any array which does belong to this host.  It does this be adding
           an underscore and a small number to the name preferred by the
           metadata.
    
           Once an appropriate array is found or created and the device is
           added, _mdadm_ must decide if the array is ready to be started.  It
           will normally compare the number of available (non-spare) devices
           to the number of devices that the metadata suggests need to be
           active.  If there are at least that many, the array will be
           started.  This means that if any devices are missing the array
           will not be restarted.
    
           As an alternative, **--run** may be passed to _mdadm_ in which case the
           array will be run as soon as there are enough devices present for
           the data to be accessible.  For a RAID1, that means one device
           will start the array.  For a clean RAID5, the array will be
           started as soon as all but one drive is present.
    
           Note that neither of these approaches is really ideal.  If it can
           be known that all device discovery has completed, then
              **mdadm -IRs**
           can be run which will try to start all arrays that are being
           incrementally assembled.  They are started in "read-auto" mode in
           which they are read-only until the first write request.  This
           means that no metadata updates are made and no attempt at resync
           or recovery happens.  Further devices that are found before the
           first write can still be added safely.
    


## ENVIRONMENT



           This section describes environment variables that affect how mdadm
           operates.
    
           **MDADM_NO_MDMON**
                  Setting this value to 1 will prevent mdadm from
                  automatically launching mdmon.  This variable is intended
                  primarily for debugging mdadm/mdmon.
    
           **MDADM_NO_UDEV**
                  Normally, _mdadm_ does not create any device nodes in /dev,
                  but leaves that task to _udev_.  If _udev_ appears not to be
                  configured, or if this environment variable is set to '1',
                  the _mdadm_ will create and devices that are needed.
    
           **MDADM_NO_SYSTEMCTL**
                  If _mdadm_ detects that _systemd_ is in use it will normally
                  request _systemd_ to start various background tasks
                  (particularly _mdmon_) rather than forking and running them
                  in the background.  This can be suppressed by setting
                  **MDADM_NO_SYSTEMCTL=1**.
    
           **IMSM_NO_PLATFORM**
                  A key value of IMSM metadata is that it allows
                  interoperability with boot ROMs on Intel platforms, and
                  with other major operating systems.  Consequently, _mdadm_
                  will only allow an IMSM array to be created or modified if
                  detects that it is running on an Intel platform which
                  supports IMSM, and supports the particular configuration of
                  IMSM that is being requested (some functionality requires
                  newer OROM support).
    
                  These checks can be suppressed by setting
                  IMSM_NO_PLATFORM=1 in the environment.  This can be useful
                  for testing or for disaster recovery.  You should be aware
                  that interoperability may be compromised by setting this
                  value.
    
                  These change can also be suppressed by adding
                  **mdadm.imsm.test=1** to the kernel command line. This makes it
                  easy to test IMSM code in a virtual machine that doesn't
                  have IMSM virtual hardware.
    
           **MDADM_GROW_ALLOW_OLD**
                  If an array is stopped while it is performing a reshape and
                  that reshape was making use of a backup file, then when the
                  array is re-assembled _mdadm_ will sometimes complain that
                  the backup file is too old.  If this happens and you are
                  certain it is the right backup file, you can over-ride this
                  check by setting **MDADM_GROW_ALLOW_OLD=1** in the environment.
    
           **MDADM_CONF_AUTO**
                  Any string given in this variable is added to the start of
                  the **AUTO** line in the config file, or treated as the whole
                  **AUTO** line if none is given.  It can be used to disable
                  certain metadata types when _mdadm_ is called from a boot
                  script.  For example
                      **export MDADM_CONF_AUTO='-ddf -imsm'**
                  will make sure that _mdadm_ does not automatically assemble
                  any DDF or IMSM arrays that are found.  This can be useful
                  on systems configured to manage such arrays with **dmraid**.
    


## EXAMPLES



             **mdadm --query /dev/name-of-device**
           This will find out if a given device is a RAID array, or is part
           of one, and will provide brief information about the device.
    
             **mdadm --assemble --scan**
           This will assemble and start all arrays listed in the standard
           config file.  This command will typically go in a system startup
           file.
    
             **mdadm --stop --scan**
           This will shut down all arrays that can be shut down (i.e. are not
           currently in use).  This will typically go in a system shutdown
           script.
    
             **mdadm --follow --scan --delay=120**
           If (and only if) there is an Email address or program given in the
           standard config file, then monitor the status of all arrays listed
           in that file by polling them ever 2 minutes.
    
             **mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1**
           Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and
           /dev/hdc1.
    
             **echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf**
             **mdadm --detail --scan >> mdadm.conf**
           This will create a prototype config file that describes currently
           active arrays that are known to be made from partitions of IDE or
           SCSI drives.  This file should be reviewed before being used as it
           may contain unwanted detail.
    
             **echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf**
             **mdadm --examine --scan --config=mdadm.conf >> mdadm.conf**
           This will find arrays which could be assembled from existing IDE
           and SCSI whole drives (not partitions), and store the information
           in the format of a config file.  This file is very likely to
           contain unwanted detail, particularly the **devices=** entries.  It
           should be reviewed and edited before being used as an actual
           config file.
    
             **mdadm --examine --brief --scan --config=partitions**
             **mdadm -Ebsc partitions**
           Create a list of devices by reading **/proc/partitions** , scan these
           for RAID superblocks, and printout a brief listing of all that
           were found.
    
             **mdadm -Ac partitions -m 0 /dev/md0**
           Scan all partitions and devices listed in **/proc/partitions** and
           assemble **/dev/md0** out of all such devices with a RAID superblock
           with a minor number of 0.
    
             **mdadm --monitor --scan --daemonise > /run/mdadm/mon.pid**
           If config file contains a mail address or alert program, run mdadm
           in the background in monitor mode monitoring all md devices.  Also
           write pid of mdadm daemon to **/run/mdadm/mon.pid**.
    
             **mdadm -Iq /dev/somedevice**
           Try to incorporate newly discovered device into some array as
           appropriate.
    
             **mdadm --incremental --rebuild-map --run --scan**
           Rebuild the array map from any current arrays, and then start any
           that can be started.
    
             **mdadm /dev/md4 --fail detached --remove detached**
           Any devices which are components of /dev/md4 will be marked as
           faulty and then remove from the array.
    
             **mdadm --grow /dev/md4 --level=6 --backup-file=/root/backup-md4**
           The array **/dev/md4** which is currently a RAID5 array will be
           converted to RAID6.  There should normally already be a spare
           drive attached to the array as a RAID6 needs one more drive than a
           matching RAID5.
    
             **mdadm --create /dev/md/ddf --metadata=ddf --raid-disks 6**
           **/dev/sd[a-f]**
           Create a DDF array over 6 devices.
    
             **mdadm --create /dev/md/home -n3 -l5 -z 30000000 /dev/md/ddf**
           Create a RAID5 array over any 3 devices in the given DDF set.  Use
           only 30 gigabytes of each device.
    
             **mdadm -A /dev/md/ddf1 /dev/sd[a-f]**
           Assemble a pre-exist ddf array.
    
             **mdadm -I /dev/md/ddf1**
           Assemble all arrays contained in the ddf array, assigning names as
           appropriate.
    
             **mdadm --create --help**
           Provide help about the Create mode.
    
             **mdadm --config --help**
           Provide help about the format of the config file.
    
             **mdadm --help**
           Provide general help.
    


## FILES



       **/proc/mdstat**
           If you're using the **/proc** filesystem, **/proc/mdstat** lists all
           active md devices with information about them.  _mdadm_ uses this to
           find arrays when **--scan** is given in Misc mode, and to monitor
           array reconstruction on Monitor mode.
    
       **/etc/mdadm.conf (or /etc/mdadm/mdadm.conf)**
           Default config file.  See mdadm.conf(5) for more details.
    
       **/etc/mdadm.conf.d (or /etc/mdadm/mdadm.conf.d)**
           Default directory containing configuration files.  See
           mdadm.conf(5) for more details.
    
       **/run/mdadm/map**
           When **--incremental** mode is used, this file gets a list of arrays
           currently being created.
    


## POSIX PORTABLE NAME



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


## DEVICE NAMES



           _mdadm_ understand two sorts of names for array devices.
    
           The first is the so-called 'standard' format name, which matches
           the names used by the kernel and which appear in _/proc/mdstat_.
    
           The second sort can be freely chosen, but must reside in _/dev/md/_.
           When giving a device name to _mdadm_ to create or assemble an array,
           either full path name such as _/dev/md0_ or _/dev/md/home_ can be
           given, or just the suffix of the second sort of name, such as _home_
           can be given.
    
           In every style, raw name has to be no longer than 32 chars.
    
           When _mdadm_ chooses device names during auto-assembly or
           incremental assembly, it will sometimes add a small sequence
           number to the end of the name to avoid conflicted between multiple
           arrays that have the same name.  If _mdadm_ can reasonably determine
           that the array really is meant for this host, either by a hostname
           in the metadata, or by the presence of the array in **mdadm.conf** ,
           then it will leave off the suffix if possible.  Also if the
           homehost is specified as **< ignore> **_mdadm_ will only use a suffix if
           a different array of the same name already exists or is listed in
           the config file.
    
           The names for arrays are of the form:
    
                  /dev/md**NN**
    
           where NN is a number.
    
           Names can be non-numeric following the form:
    
                  /dev/md_**XXX**
    
           where **XXX** is any string.  These names are supported by _mdadm_ since
           version 3.3 provided they are enabled in _mdadm.conf_.
    


## UNDERSTANDING OUTPUT



           EXAMINE
    
           **checkpoint**
                  Checkpoint value is reported when array is performing some
                  action including resync, recovery or reshape. Checkpoints
                  allow resuming action from certain point if it was
                  interrupted.
    
                  Checkpoint is reported as combination of two values:
                  current migration unit and number of blocks per unit. By
                  multiplying those values and dividing by array size
                  checkpoint progress percentage can be obtained in relation
                  to current progress reported in /proc/mdstat. Checkpoint is
                  also related to (and sometimes based on) sysfs entry
                  sync_completed but depending on action units may differ.
                  Even if units are the same, it should not be expected that
                  checkpoint and sync_completed will be exact match nor
                  updated simultaneously.
    


## NOTE



           _mdadm_ was previously known as _mdctl_.
    


* * *