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".
+"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.
-(It doesn't yet, but it can)
+Currently, it supports superblock formats "0.90.0" and the "md-1" format
+introduced in the 2.5 development series.
-The kernel does *NOT* autodetect which format superblock is being
-used. It must be told.
+The kernel will autodetect which format superblock is being used.
Superblock format '0' is treated differently to others for legacy
-reasons.
+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 approrpriate redundancy (copying in raid1, parity
+can create appropriate redundancy (copying in raid1, parity
calculation in raid4/5).
-When an array is assembled, it is first initialised with the
+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
Specific Rules that apply to format-0 super block arrays, and
- arrays with no superblock (non-persistant).
+ 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 uninitialised devices can be added with ADD_NEW_DISK. The
+
+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.
-One started with RUN_ARRAY, uninitialised spares can be added with
+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 recovered to
+ 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.
+
+ 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, 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 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'.
+
+ 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
git://git.onelab.eu / linux-2.6.git / blobdiff