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+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
+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
+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
+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
+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 its role in the array.
+Once started with RUN_ARRAY, uninitialized spares can be added with
+MD devices in sysfs
+md devices appear in sysfs (/sys) as regular block devices,
+ /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 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, 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 is 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 and 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)
+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'.
+ 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 with 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 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.
+ 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.
+ A 'select' on this attribute will return when resync completes,
+ when it reaches the current sync_max (below) and possibly at
+ other times.
+ sync_max
+ This is a number of sectors at which point a resync/recovery
+ process will pause. When a resync is active, the value can
+ only ever be increased, never decreased. The value of 'max'
+ effectively disables the limit.
+ 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".
+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
+ 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.