Documentation/md.txt - LeafOS-Devices/android_kernel_samsung_gta4xl - 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
 desipite 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'.  This may be a standard
      numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
      other name such as "linear" or "multipath".
      If no raid level has been set yet (array is still being
      assembled), this file will be empty.

   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.

   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 atleast 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.

 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
 	      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.


 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.

 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
	desipite 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'. This may be a standard
	numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
	other name such as "linear" or "multipath".
	If no raid level has been set yet (array is still being
	assembled), this file will be empty.

	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.

	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 atleast 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.

	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
	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.


	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.

	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