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 RAID arrays
 ===========
 
 Boot time assembly of RAID arrays
 ---------------------------------
 
 Tools that manage md devices can be found at
    https://www.kernel.org/pub/linux/utils/raid/
 
 
 You can boot with your md device with the following kernel command
 lines:
 
 for old raid arrays without persistent superblocks::
 
   md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
 
 for raid arrays with persistent superblocks::
 
   md=<md device no.>,dev0,dev1,...,devn
 
 or, to assemble a partitionable array::
 
   md=d<md device no.>,dev0,dev1,...,devn
 
 ``md device no.``
 +++++++++++++++++
 
 The number of the md device
 
 ================= =========
 ``md device no.`` device
 ================= =========
               0         md0
               1         md1
               2         md2
               3         md3
               4         md4
 ================= =========
 
 ``raid level``
 ++++++++++++++
 
 level of the RAID array
 
 =============== =============
 ``raid level``  level
 =============== =============
 -1              linear mode
 0               striped mode
 =============== =============
 
 other modes are only supported with persistent super blocks
 
 ``chunk size factor``
 +++++++++++++++++++++
 
 (raid-0 and raid-1 only)
 
 Set  the chunk size as 4k << n.
 
 ``fault level``
 +++++++++++++++
 
 Totally ignored
 
 ``dev0`` to ``devn``
 ++++++++++++++++++++
 
 e.g. ``/dev/hda1``, ``/dev/hdc1``, ``/dev/sda1``, ``/dev/sdb1``
 
 A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this::
 
         e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
 
 
 Boot time autodetection of RAID arrays
 --------------------------------------
 
 When md is compiled into the kernel (not as module), partitions of
 type 0xfd are scanned and automatically assembled into RAID arrays.
 This autodetection may be suppressed with the kernel parameter
 ``raid=noautodetect``.  As of kernel 2.6.9, only drives with a type 0
 superblock can be autodetected and run at boot time.
 
 The kernel parameter ``raid=partitionable`` (or ``raid=part``) means
 that all auto-detected arrays are assembled as partitionable.
 
 Boot time assembly of degraded/dirty arrays
 -------------------------------------------
 
 If a raid5 or raid6 array is both dirty and degraded, it could have
 undetectable data corruption.  This is because the fact that it is
 ``dirty`` means that the parity cannot be trusted, and the fact that it
 is degraded means that some datablocks are missing and cannot reliably
 be reconstructed (due to no parity).
 
 For this reason, md will normally refuse to start such an array.  This
 requires the sysadmin to take action to explicitly start the array
 despite possible corruption.  This is normally done with::
 
    mdadm --assemble --force ....
 
 This option is not really available if the array has the root
 filesystem on it.  In order to support this booting from such an
 array, md supports a module parameter ``start_dirty_degraded`` which,
 when set to 1, bypassed the checks and will allows dirty degraded
 arrays to be started.
 
 So, to boot with a root filesystem of a dirty degraded raid 5 or 6, use::
 
    md-mod.start_dirty_degraded=1
 
 
 Superblock formats
 ------------------
 
 The md driver can support a variety of different superblock formats.
 Currently, it supports superblock formats ``0.90.0`` and the ``md-1`` format
 introduced in the 2.5 development series.
 
 The kernel will autodetect which format superblock is being used.
 
 Superblock format ``0`` is treated differently to others for legacy
 reasons - it is the original superblock format.
 
 
 General Rules - apply for all superblock formats
 ------------------------------------------------
 
 An array is ``created`` by writing appropriate superblocks to all
 devices.
 
 It is ``assembled`` by associating each of these devices with an
 particular md virtual device.  Once it is completely assembled, it can
 be accessed.
 
 An array should be created by a user-space tool.  This will write
 superblocks to all devices.  It will usually mark the array as
 ``unclean``, or with some devices missing so that the kernel md driver
 can create appropriate redundancy (copying in raid 1, parity
 calculation in raid 4/5).
 
 When an array is assembled, it is first initialized with the
 SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
 version number.  The major version number selects which superblock
 format is to be used.  The minor number might be used to tune handling
 of the format, such as suggesting where on each device to look for the
 superblock.
 
 Then each device is added using the ADD_NEW_DISK ioctl.  This
 provides, in particular, a major and minor number identifying the
 device to add.
 
 The array is started with the RUN_ARRAY ioctl.
 
 Once started, new devices can be added.  They should have an
 appropriate superblock written to them, and then be passed in with
 ADD_NEW_DISK.
 
 Devices that have failed or are not yet active can be detached from an
 array using HOT_REMOVE_DISK.
 
 
 Specific Rules that apply to format-0 super block arrays, and arrays with no superblock (non-persistent)
 --------------------------------------------------------------------------------------------------------
 
 An array can be ``created`` by describing the array (level, chunksize
 etc) in a SET_ARRAY_INFO ioctl.  This must have ``major_version==0`` and
 ``raid_disks != 0``.
 
 Then uninitialized devices can be added with ADD_NEW_DISK.  The
 structure passed to ADD_NEW_DISK must specify the state of the device
 and its role in the array.
 
 Once started with RUN_ARRAY, uninitialized spares can be added with
 HOT_ADD_DISK.
 
 
 MD devices in sysfs
 -------------------
 
 md devices appear in sysfs (``/sys``) as regular block devices,
 e.g.::
 
    /sys/block/md0
 
 Each ``md`` device will contain a subdirectory called ``md`` which
 contains further md-specific information about the device.
 
 All md devices contain:
 
   level
      a text file indicating the ``raid level``. e.g. raid0, raid1,
      raid5, linear, multipath, faulty.
      If no raid level has been set yet (array is still being
      assembled), the value will reflect whatever has been written
      to it, which may be a name like the above, or may be a number
      such as ``0``, ``5``, etc.
 
   raid_disks
      a text file with a simple number indicating the number of devices
      in a fully functional array.  If this is not yet known, the file
      will be empty.  If an array is being resized this will contain
      the new number of devices.
      Some raid levels allow this value to be set while the array is
      active.  This will reconfigure the array.   Otherwise it can only
      be set while assembling an array.
      A change to this attribute will not be permitted if it would
      reduce the size of the array.  To reduce the number of drives
      in an e.g. raid5, the array size must first be reduced by
      setting the ``array_size`` attribute.
 
   chunk_size
      This is the size in bytes for ``chunks`` and is only relevant to
      raid levels that involve striping (0,4,5,6,10). The address space
      of the array is conceptually divided into chunks and consecutive
      chunks are striped onto neighbouring devices.
      The size should be at least PAGE_SIZE (4k) and should be a power
      of 2.  This can only be set while assembling an array
 
   layout
      The ``layout`` for the array for the particular level.  This is
      simply a number that is interpreted differently by different
      levels.  It can be written while assembling an array.
 
   array_size
      This can be used to artificially constrain the available space in
      the array to be less than is actually available on the combined
      devices.  Writing a number (in Kilobytes) which is less than
      the available size will set the size.  Any reconfiguration of the
      array (e.g. adding devices) will not cause the size to change.
      Writing the word ``default`` will cause the effective size of the
      array to be whatever size is actually available based on
      ``level``, ``chunk_size`` and ``component_size``.
 
      This can be used to reduce the size of the array before reducing
      the number of devices in a raid4/5/6, or to support external
      metadata formats which mandate such clipping.
 
   reshape_position
      This is either ``none`` or a sector number within the devices of
      the array where ``reshape`` is up to.  If this is set, the three
      attributes mentioned above (raid_disks, chunk_size, layout) can
      potentially have 2 values, an old and a new value.  If these
      values differ, reading the attribute returns::
 
         new (old)
 
      and writing will effect the ``new`` value, leaving the ``old``
      unchanged.
 
   component_size
      For arrays with data redundancy (i.e. not raid0, linear, faulty,
      multipath), all components must be the same size - or at least
      there must a size that they all provide space for.  This is a key
      part or the geometry of the array.  It is measured in sectors
      and can be read from here.  Writing to this value may resize
      the array if the personality supports it (raid1, raid5, raid6),
      and if the component drives are large enough.
 
   metadata_version
      This indicates the format that is being used to record metadata
      about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
      1.2 (newer format in varying locations) or ``none`` indicating that
      the kernel isn't managing metadata at all.
      Alternately it can be ``external:`` followed by a string which
      is set by user-space.  This indicates that metadata is managed
      by a user-space program.  Any device failure or other event that
      requires a metadata update will cause array activity to be
      suspended until the event is acknowledged.
 
   resync_start
      The point at which resync should start.  If no resync is needed,
      this will be a very large number (or ``none`` since 2.6.30-rc1).  At
      array creation it will default to 0, though starting the array as
      ``clean`` will set it much larger.
 
   new_dev
      This file can be written but not read.  The value written should
      be a block device number as major:minor.  e.g. 8:0
      This will cause that device to be attached to the array, if it is
      available.  It will then appear at md/dev-XXX (depending on the
      name of the device) and further configuration is then possible.
 
   safe_mode_delay
      When an md array has seen no write requests for a certain period
      of time, it will be marked as ``clean``.  When another write
      request arrives, the array is marked as ``dirty`` before the write
      commences.  This is known as ``safe_mode``.
      The ``certain period`` is controlled by this file which stores the
      period as a number of seconds.  The default is 200msec (0.200).
      Writing a value of 0 disables safemode.
 
   array_state
      This file contains a single word which describes the current
      state of the array.  In many cases, the state can be set by
      writing the word for the desired state, however some states
      cannot be explicitly set, and some transitions are not allowed.
 
      Select/poll works on this file.  All changes except between
      Active_idle and active (which can be frequent and are not
      very interesting) are notified.  active->active_idle is
      reported if the metadata is externally managed.
 
      clear
          No devices, no size, no level
 
          Writing is equivalent to STOP_ARRAY ioctl
 
      inactive
          May have some settings, but array is not active
          all IO results in error
 
          When written, doesn't tear down array, but just stops it
 
      suspended (not supported yet)
          All IO requests will block. The array can be reconfigured.
 
          Writing this, if accepted, will block until array is quiessent
 
      readonly
          no resync can happen.  no superblocks get written.
 
          Write requests fail
 
      read-auto
          like readonly, but behaves like ``clean`` on a write request.
 
      clean
          no pending writes, but otherwise active.
 
          When written to inactive array, starts without resync
 
          If a write request arrives then
          if metadata is known, mark ``dirty`` and switch to ``active``.
          if not known, block and switch to write-pending
 
          If written to an active array that has pending writes, then fails.
      active
          fully active: IO and resync can be happening.
          When written to inactive array, starts with resync
 
      write-pending
          clean, but writes are blocked waiting for ``active`` to be written.
 
      active-idle
          like active, but no writes have been seen for a while (safe_mode_delay).
 
   bitmap/location
      This indicates where the write-intent bitmap for the array is
      stored.
 
      It can be one of ``none``, ``file`` or ``[+-]N``.
      ``file`` may later be extended to ``file:/file/name``
      ``[+-]N`` means that many sectors from the start of the metadata.
 
      This is replicated on all devices.  For arrays with externally
      managed metadata, the offset is from the beginning of the
      device.
 
   bitmap/chunksize
      The size, in bytes, of the chunk which will be represented by a
      single bit.  For RAID456, it is a portion of an individual
      device. For RAID10, it is a portion of the array.  For RAID1, it
      is both (they come to the same thing).
 
   bitmap/time_base
      The time, in seconds, between looking for bits in the bitmap to
      be cleared. In the current implementation, a bit will be cleared
      between 2 and 3 times ``time_base`` after all the covered blocks
      are known to be in-sync.
 
   bitmap/backlog
      When write-mostly devices are active in a RAID1, write requests
      to those devices proceed in the background - the filesystem (or
      other user of the device) does not have to wait for them.
      ``backlog`` sets a limit on the number of concurrent background
      writes.  If there are more than this, new writes will by
      synchronous.
 
   bitmap/metadata
      This can be either ``internal`` or ``external``.
 
      ``internal``
        is the default and means the metadata for the bitmap
        is stored in the first 256 bytes of the allocated space and is
        managed by the md module.
 
      ``external``
        means that bitmap metadata is managed externally to
        the kernel (i.e. by some userspace program)
 
   bitmap/can_clear
      This is either ``true`` or ``false``.  If ``true``, then bits in the
      bitmap will be cleared when the corresponding blocks are thought
      to be in-sync.  If ``false``, bits will never be cleared.
      This is automatically set to ``false`` if a write happens on a
      degraded array, or if the array becomes degraded during a write.
      When metadata is managed externally, it should be set to true
      once the array becomes non-degraded, and this fact has been
      recorded in the metadata.
 
   consistency_policy
      This indicates how the array maintains consistency in case of unexpected
      shutdown. It can be:
 
      none
        Array has no redundancy information, e.g. raid0, linear.
 
      resync
        Full resync is performed and all redundancy is regenerated when the
        array is started after unclean shutdown.
 
      bitmap
        Resync assisted by a write-intent bitmap.
 
      journal
        For raid4/5/6, journal device is used to log transactions and replay
        after unclean shutdown.
 
      ppl
        For raid5 only, Partial Parity Log is used to close the write hole and
        eliminate resync.
 
      The accepted values when writing to this file are ``ppl`` and ``resync``,
      used to enable and disable PPL.
 
   uuid
      This indicates the UUID of the array in the following format:
      xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
 
 
 As component devices are added to an md array, they appear in the ``md``
 directory as new directories named::
 
       dev-XXX
 
 where ``XXX`` is a name that the kernel knows for the device, e.g. hdb1.
 Each directory contains:
 
       block
         a symlink to the block device in /sys/block, e.g.::
 
              /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
 
       super
         A file containing an image of the superblock read from, or
         written to, that device.
 
       state
         A file recording the current state of the device in the array
         which can be a comma separated list of:
 
               faulty
                         device has been kicked from active use due to
                         a detected fault, or it has unacknowledged bad
                         blocks
 
               in_sync
                         device is a fully in-sync member of the array
 
               writemostly
                         device will only be subject to read
                         requests if there are no other options.
 
                         This applies only to raid1 arrays.
 
               blocked
                         device has failed, and the failure hasn't been
                         acknowledged yet by the metadata handler.
 
                         Writes that would write to this device if
                         it were not faulty are blocked.
 
               spare
                         device is working, but not a full member.
 
                         This includes spares that are in the process
                         of being recovered to
 
               write_error
                         device has ever seen a write error.
 
               want_replacement
                         device is (mostly) working but probably
                         should be replaced, either due to errors or
                         due to user request.
 
               replacement
                         device is a replacement for another active
                         device with same raid_disk.
 
 
         This list may grow in future.
 
         This can be written to.
 
         Writing ``faulty``  simulates a failure on the device.
 
         Writing ``remove`` removes the device from the array.
 
         Writing ``writemostly`` sets the writemostly flag.
 
         Writing ``-writemostly`` clears the writemostly flag.
 
         Writing ``blocked`` sets the ``blocked`` flag.
 
         Writing ``-blocked`` clears the ``blocked`` flags and allows writes
         to complete and possibly simulates an error.
 
         Writing ``in_sync`` sets the in_sync flag.
 
         Writing ``write_error`` sets writeerrorseen flag.
 
         Writing ``-write_error`` clears writeerrorseen flag.
 
         Writing ``want_replacement`` is allowed at any time except to a
         replacement device or a spare.  It sets the flag.
 
         Writing ``-want_replacement`` is allowed at any time.  It clears
         the flag.
 
         Writing ``replacement`` or ``-replacement`` is only allowed before
         starting the array.  It sets or clears the flag.
 
 
         This file responds to select/poll. Any change to ``faulty``
         or ``blocked`` causes an event.
 
       errors
         An approximate count of read errors that have been detected on
         this device but have not caused the device to be evicted from
         the array (either because they were corrected or because they
         happened while the array was read-only).  When using version-1
         metadata, this value persists across restarts of the array.
 
         This value can be written while assembling an array thus
         providing an ongoing count for arrays with metadata managed by
         userspace.
 
       slot
         This gives the role that the device has in the array.  It will
         either be ``none`` if the device is not active in the array
         (i.e. is a spare or has failed) or an integer less than the
         ``raid_disks`` number for the array indicating which position
         it currently fills.  This can only be set while assembling an
         array.  A device for which this is set is assumed to be working.
 
       offset
         This gives the location in the device (in sectors from the
         start) where data from the array will be stored.  Any part of
         the device before this offset is not touched, unless it is
         used for storing metadata (Formats 1.1 and 1.2).
 
       size
         The amount of the device, after the offset, that can be used
         for storage of data.  This will normally be the same as the
         component_size.  This can be written while assembling an
         array.  If a value less than the current component_size is
         written, it will be rejected.
 
       recovery_start
         When the device is not ``in_sync``, this records the number of
         sectors from the start of the device which are known to be
         correct.  This is normally zero, but during a recovery
         operation it will steadily increase, and if the recovery is
         interrupted, restoring this value can cause recovery to
         avoid repeating the earlier blocks.  With v1.x metadata, this
         value is saved and restored automatically.
 
         This can be set whenever the device is not an active member of
         the array, either before the array is activated, or before
         the ``slot`` is set.
 
         Setting this to ``none`` is equivalent to setting ``in_sync``.
         Setting to any other value also clears the ``in_sync`` flag.
 
       bad_blocks
         This gives the list of all known bad blocks in the form of
         start address and length (in sectors respectively). If output
         is too big to fit in a page, it will be truncated. Writing
         ``sector length`` to this file adds new acknowledged (i.e.
         recorded to disk safely) bad blocks.
 
       unacknowledged_bad_blocks
         This gives the list of known-but-not-yet-saved-to-disk bad
         blocks in the same form of ``bad_blocks``. If output is too big
         to fit in a page, it will be truncated. Writing to this file
         adds bad blocks without acknowledging them. This is largely
         for testing.
 
       ppl_sector, ppl_size
         Location and size (in sectors) of the space used for Partial Parity Log
         on this device.
 
 
 An active md device will also contain an entry for each active device
 in the array.  These are named::
 
     rdNN
 
 where ``NN`` is the position in the array, starting from 0.
 So for a 3 drive array there will be rd0, rd1, rd2.
 These are symbolic links to the appropriate ``dev-XXX`` entry.
 Thus, for example::
 
        cat /sys/block/md*/md/rd*/state
 
 will show ``in_sync`` on every line.
 
 
 
 Active md devices for levels that support data redundancy (1,4,5,6,10)
 also have
 
    sync_action
      a text file that can be used to monitor and control the rebuild
      process.  It contains one word which can be one of:
 
        resync
                 redundancy is being recalculated after unclean
                 shutdown or creation
 
        recover
                 a hot spare is being built to replace a
                 failed/missing device
 
        idle
                 nothing is happening
        check
                 A full check of redundancy was requested and is
                 happening.  This reads all blocks and checks
                 them. A repair may also happen for some raid
                 levels.
 
        repair
                 A full check and repair is happening.  This is
                 similar to ``resync``, but was requested by the
                 user, and the write-intent bitmap is NOT used to
                 optimise the process.
 
       This file is writable, and each of the strings that could be
       read are meaningful for writing.
 
         ``idle`` will stop an active resync/recovery etc.  There is no
         guarantee that another resync/recovery may not be automatically
         started again, though some event will be needed to trigger
         this.
 
         ``resync`` or ``recovery`` can be used to restart the
         corresponding operation if it was stopped with ``idle``.
 
         ``check`` and ``repair`` will start the appropriate process
         providing the current state is ``idle``.
 
       This file responds to select/poll.  Any important change in the value
       triggers a poll event.  Sometimes the value will briefly be
       ``recover`` if a recovery seems to be needed, but cannot be
       achieved. In that case, the transition to ``recover`` isn't
       notified, but the transition away is.
 
    degraded
       This contains a count of the number of devices by which the
       arrays is degraded.  So an optimal array will show ``0``.  A
       single failed/missing drive will show ``1``, etc.
 
       This file responds to select/poll, any increase or decrease
       in the count of missing devices will trigger an event.
 
    mismatch_count
       When performing ``check`` and ``repair``, and possibly when
       performing ``resync``, md will count the number of errors that are
       found.  The count in ``mismatch_cnt`` is the number of sectors
       that were re-written, or (for ``check``) would have been
       re-written.  As most raid levels work in units of pages rather
       than sectors, this may be larger than the number of actual errors
       by a factor of the number of sectors in a page.
 
    bitmap_set_bits
       If the array has a write-intent bitmap, then writing to this
       attribute can set bits in the bitmap, indicating that a resync
       would need to check the corresponding blocks. Either individual
       numbers or start-end pairs can be written.  Multiple numbers
       can be separated by a space.
 
       Note that the numbers are ``bit`` numbers, not ``block`` numbers.
       They should be scaled by the bitmap_chunksize.
 
    sync_speed_min, sync_speed_max
      This are similar to ``/proc/sys/dev/raid/speed_limit_{min,max}``
      however they only apply to the particular array.
 
      If no value has been written to these, or if the word ``system``
      is written, then the system-wide value is used.  If a value,
      in kibibytes-per-second is written, then it is used.
 
      When the files are read, they show the currently active value
      followed by ``(local)`` or ``(system)`` depending on whether it is
      a locally set or system-wide value.
 
    sync_completed
      This shows the number of sectors that have been completed of
      whatever the current sync_action is, followed by the number of
      sectors in total that could need to be processed.  The two
      numbers are separated by a ``/``  thus effectively showing one
      value, a fraction of the process that is complete.
 
      A ``select`` on this attribute will return when resync completes,
      when it reaches the current sync_max (below) and possibly at
      other times.
 
    sync_speed
      This shows the current actual speed, in K/sec, of the current
      sync_action.  It is averaged over the last 30 seconds.
 
    suspend_lo, suspend_hi
      The two values, given as numbers of sectors, indicate a range
      within the array where IO will be blocked.  This is currently
      only supported for raid4/5/6.
 
    sync_min, sync_max
      The two values, given as numbers of sectors, indicate a range
      within the array where ``check``/``repair`` will operate. Must be
      a multiple of chunk_size. When it reaches ``sync_max`` it will
      pause, rather than complete.
      You can use ``select`` or ``poll`` on ``sync_completed`` to wait for
      that number to reach sync_max.  Then you can either increase
      ``sync_max``, or can write ``idle`` to ``sync_action``.
 
      The value of ``max`` for ``sync_max`` effectively disables the limit.
      When a resync is active, the value can only ever be increased,
      never decreased.
      The value of ``0`` is the minimum for ``sync_min``.
 
 
 
 Each active md device may also have attributes specific to the
 personality module that manages it.
 These are specific to the implementation of the module and could
 change substantially if the implementation changes.
 
 These currently include:
 
   stripe_cache_size  (currently raid5 only)
       number of entries in the stripe cache.  This is writable, but
       there are upper and lower limits (32768, 17).  Default is 256.
 
   strip_cache_active (currently raid5 only)
       number of active entries in the stripe cache
 
   preread_bypass_threshold (currently raid5 only)
       number of times a stripe requiring preread will be bypassed by
       a stripe that does not require preread.  For fairness defaults
       to 1.  Setting this to 0 disables bypass accounting and
       requires preread stripes to wait until all full-width stripe-
       writes are complete.  Valid values are 0 to stripe_cache_size.
 
   journal_mode (currently raid5 only)
       The cache mode for raid5. raid5 could include an extra disk for
       caching. The mode can be "write-throuth" and "write-back". The
       default is "write-through".
 
   ppl_write_hint
       NVMe stream ID to be set for each PPL write request.

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