Tools that manage md devices can be found at

     http://www.kernel.org/pub/linux/utils/raid/ 

  
  
  Boot time assembly of RAID arrays
  ---------------------------------
  
  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 ... 
                0 means md0, 
  	      1 md1,
  	      2 md2,
  	      3 md3,
  	      4 md4
  
  raid 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-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[56], 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 raid1, parity
  calculation in raid4/5).
  
  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 interpretted 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.

  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.

  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.