Documentation/md.txt - kernel/msm-5.4 - Gitiles

 Tools that manage md devices can be found at
    http://www.<country>.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 passed be 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 has 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 it's 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 (not currently
      possible) this will contain the larger of the old and new sizes.
      Some raid level (RAID1) 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.

   chunk_size
      This is the size if bytes for 'chunks' and is only relevant to
      raid levels that involve striping (1,4,5,6,10). 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

   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.

   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.

   resync_start
      The point at which resync should start.  If no resync is needed,
      this will be a very large number.  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.

      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).


    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, of 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.

    sync_speed
      This shows the current actual speed, in K/sec, of the current
      sync_action.  It is averaged over the last 30 seconds.


 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
 	      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.
 	      spare    - device is working, but not a full member.
 			 This includes spares that are in the process
 			 of being recoverred to
 	This list make 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.

       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 possition
 	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 us 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, component_size will be reduced to this value.


 An active md device will also contain and entry for each active device
 in the array.  These are named

     rdNN

 where 'NN' is the possition 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)
 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 block 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'.

    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 my 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.

 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, 16).  Default is 128.
   strip_cache_active (currently raid5 only)
       number of active entries in the stripe cache
	Tools that manage md devices can be found at
	http://www.<country>.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 passed be 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 has 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 it's 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 (not currently
	possible) this will contain the larger of the old and new sizes.
	Some raid level (RAID1) 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.

	chunk_size
	This is the size if bytes for 'chunks' and is only relevant to
	raid levels that involve striping (1,4,5,6,10). 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

	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.

	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.

	resync_start
	The point at which resync should start. If no resync is needed,
	this will be a very large number. 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.

	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).


	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, of 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.

	sync_speed
	This shows the current actual speed, in K/sec, of the current
	sync_action. It is averaged over the last 30 seconds.


	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
	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.
	spare - device is working, but not a full member.
	This includes spares that are in the process
	of being recoverred to
	This list make 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.

	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 possition
	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 us 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, component_size will be reduced to this value.


	An active md device will also contain and entry for each active device
	in the array. These are named

	rdNN

	where 'NN' is the possition 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)
	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 block 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'.

	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 my 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.

	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, 16). Default is 128.
	strip_cache_active (currently raid5 only)
	number of active entries in the stripe cache