1Tools that manage md devices can be found at
2   http://www.<country>
5Boot time assembly of RAID arrays
8You can boot with your md device with the following kernel command
11for old raid arrays without persistent superblocks:
12  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
14for raid arrays with persistent superblocks
15  md=<md device no.>,dev0,dev1,...,devn
16or, to assemble a partitionable array:
17  md=d<md device no.>,dev0,dev1,...,devn
19md device no. = the number of the md device ...
20              0 means md0,
21          1 md1,
22          2 md2,
23          3 md3,
24          4 md4
26raid level = -1 linear mode
27              0 striped mode
28          other modes are only supported with persistent super blocks
30chunk size factor = (raid-0 and raid-1 only)
31              Set the chunk size as 4k << n.
33fault level = totally ignored
35dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
37A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:
39e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
42Boot time autodetection of RAID arrays
45When md is compiled into the kernel (not as module), partitions of
46type 0xfd are scanned and automatically assembled into RAID arrays.
47This autodetection may be suppressed with the kernel parameter
48"raid=noautodetect". As of kernel 2.6.9, only drives with a type 0
49superblock can be autodetected and run at boot time.
51The kernel parameter "raid=partitionable" (or "raid=part") means
52that all auto-detected arrays are assembled as partitionable.
54Boot time assembly of degraded/dirty arrays
57If a raid5 or raid6 array is both dirty and degraded, it could have
58undetectable data corruption. This is because the fact that it is
59'dirty' means that the parity cannot be trusted, and the fact that it
60is degraded means that some datablocks are missing and cannot reliably
61be reconstructed (due to no parity).
63For this reason, md will normally refuse to start such an array. This
64requires the sysadmin to take action to explicitly start the array
65despite possible corruption. This is normally done with
66   mdadm --assemble --force ....
68This option is not really available if the array has the root
69filesystem on it. In order to support this booting from such an
70array, md supports a module parameter "start_dirty_degraded" which,
71when set to 1, bypassed the checks and will allows dirty degraded
72arrays to be started.
74So, to boot with a root filesystem of a dirty degraded raid[56], use
76   md-mod.start_dirty_degraded=1
79Superblock formats
82The md driver can support a variety of different superblock formats.
83Currently, it supports superblock formats "0.90.0" and the "md-1" format
84introduced in the 2.5 development series.
86The kernel will autodetect which format superblock is being used.
88Superblock format '0' is treated differently to others for legacy
89reasons - it is the original superblock format.
92General Rules - apply for all superblock formats
95An array is 'created' by writing appropriate superblocks to all
98It is 'assembled' by associating each of these devices with an
99particular md virtual device. Once it is completely assembled, it can
100be accessed.
102An array should be created by a user-space tool. This will write
103superblocks to all devices. It will usually mark the array as
104'unclean', or with some devices missing so that the kernel md driver
105can create appropriate redundancy (copying in raid1, parity
106calculation in raid4/5).
108When an array is assembled, it is first initialized with the
109SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor
110version number. The major version number selects which superblock
111format is to be used. The minor number might be used to tune handling
112of the format, such as suggesting where on each device to look for the
115Then each device is added using the ADD_NEW_DISK ioctl. This
116provides, in particular, a major and minor number identifying the
117device to add.
119The array is started with the RUN_ARRAY ioctl.
121Once started, new devices can be added. They should have an
122appropriate superblock written to them, and then passed be in with
125Devices that have failed or are not yet active can be detached from an
126array using HOT_REMOVE_DISK.
129Specific Rules that apply to format-0 super block arrays, and
130       arrays with no superblock (non-persistent).
133An array can be 'created' by describing the array (level, chunksize
134etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and
135raid_disks != 0.
137Then uninitialized devices can be added with ADD_NEW_DISK. The
138structure passed to ADD_NEW_DISK must specify the state of the device
139and it's role in the array.
141Once started with RUN_ARRAY, uninitialized spares can be added with
146MD devices in sysfs
148md devices appear in sysfs (/sys) as regular block devices,
150   /sys/block/md0
152Each 'md' device will contain a subdirectory called 'md' which
153contains further md-specific information about the device.
155All md devices contain:
156  level
157     a text file indicating the 'raid level'. e.g. raid0, raid1,
158     raid5, linear, multipath, faulty.
159     If no raid level has been set yet (array is still being
160     assembled), the value will reflect whatever has been written
161     to it, which may be a name like the above, or may be a number
162     such as '0', '5', etc.
164  raid_disks
165     a text file with a simple number indicating the number of devices
166     in a fully functional array. If this is not yet known, the file
167     will be empty. If an array is being resized this will contain
168     the new number of devices.
169     Some raid levels allow this value to be set while the array is
170     active. This will reconfigure the array. Otherwise it can only
171     be set while assembling an array.
172     A change to this attribute will not be permitted if it would
173     reduce the size of the array. To reduce the number of drives
174     in an e.g. raid5, the array size must first be reduced by
175     setting the 'array_size' attribute.
177  chunk_size
178     This is the size in bytes for 'chunks' and is only relevant to
179     raid levels that involve striping (0,4,5,6,10). The address space
180     of the array is conceptually divided into chunks and consecutive
181     chunks are striped onto neighbouring devices.
182     The size should be at least PAGE_SIZE (4k) and should be a power
183     of 2. This can only be set while assembling an array
185  layout
186     The "layout" for the array for the particular level. This is
187     simply a number that is interpretted differently by different
188     levels. It can be written while assembling an array.
190  array_size
191     This can be used to artificially constrain the available space in
192     the array to be less than is actually available on the combined
193     devices. Writing a number (in Kilobytes) which is less than
194     the available size will set the size. Any reconfiguration of the
195     array (e.g. adding devices) will not cause the size to change.
196     Writing the word 'default' will cause the effective size of the
197     array to be whatever size is actually available based on
198     'level', 'chunk_size' and 'component_size'.
200     This can be used to reduce the size of the array before reducing
201     the number of devices in a raid4/5/6, or to support external
202     metadata formats which mandate such clipping.
204  reshape_position
205     This is either "none" or a sector number within the devices of
206     the array where "reshape" is up to. If this is set, the three
207     attributes mentioned above (raid_disks, chunk_size, layout) can
208     potentially have 2 values, an old and a new value. If these
209     values differ, reading the attribute returns
210        new (old)
211     and writing will effect the 'new' value, leaving the 'old'
212     unchanged.
214  component_size
215     For arrays with data redundancy (i.e. not raid0, linear, faulty,
216     multipath), all components must be the same size - or at least
217     there must a size that they all provide space for. This is a key
218     part or the geometry of the array. It is measured in sectors
219     and can be read from here. Writing to this value may resize
220     the array if the personality supports it (raid1, raid5, raid6),
221     and if the component drives are large enough.
223  metadata_version
224     This indicates the format that is being used to record metadata
225     about the array. It can be 0.90 (traditional format), 1.0, 1.1,
226     1.2 (newer format in varying locations) or "none" indicating that
227     the kernel isn't managing metadata at all.
228     Alternately it can be "external:" followed by a string which
229     is set by user-space. This indicates that metadata is managed
230     by a user-space program. Any device failure or other event that
231     requires a metadata update will cause array activity to be
232     suspended until the event is acknowledged.
234  resync_start
235     The point at which resync should start. If no resync is needed,
236     this will be a very large number (or 'none' since 2.6.30-rc1). At
237     array creation it will default to 0, though starting the array as
238     'clean' will set it much larger.
240   new_dev
241     This file can be written but not read. The value written should
242     be a block device number as major:minor. e.g. 8:0
243     This will cause that device to be attached to the array, if it is
244     available. It will then appear at md/dev-XXX (depending on the
245     name of the device) and further configuration is then possible.
247   safe_mode_delay
248     When an md array has seen no write requests for a certain period
249     of time, it will be marked as 'clean'. When another write
250     request arrives, the array is marked as 'dirty' before the write
251     commences. This is known as 'safe_mode'.
252     The 'certain period' is controlled by this file which stores the
253     period as a number of seconds. The default is 200msec (0.200).
254     Writing a value of 0 disables safemode.
256   array_state
257     This file contains a single word which describes the current
258     state of the array. In many cases, the state can be set by
259     writing the word for the desired state, however some states
260     cannot be explicitly set, and some transitions are not allowed.
262     Select/poll works on this file. All changes except between
263         active_idle and active (which can be frequent and are not
264    very interesting) are notified. active->active_idle is
265    reported if the metadata is externally managed.
267     clear
268         No devices, no size, no level
269         Writing is equivalent to STOP_ARRAY ioctl
270     inactive
271         May have some settings, but array is not active
272            all IO results in error
273         When written, doesn't tear down array, but just stops it
274     suspended (not supported yet)
275         All IO requests will block. The array can be reconfigured.
276         Writing this, if accepted, will block until array is quiessent
277     readonly
278         no resync can happen. no superblocks get written.
279         write requests fail
280     read-auto
281         like readonly, but behaves like 'clean' on a write request.
283     clean - no pending writes, but otherwise active.
284         When written to inactive array, starts without resync
285         If a write request arrives then
286           if metadata is known, mark 'dirty' and switch to 'active'.
287           if not known, block and switch to write-pending
288         If written to an active array that has pending writes, then fails.
289     active
290         fully active: IO and resync can be happening.
291         When written to inactive array, starts with resync
293     write-pending
294         clean, but writes are blocked waiting for 'active' to be written.
296     active-idle
297         like active, but no writes have been seen for a while (safe_mode_delay).
299  bitmap/location
300     This indicates where the write-intent bitmap for the array is
301     stored.
302     It can be one of "none", "file" or "[+-]N".
303     "file" may later be extended to "file:/file/name"
304     "[+-]N" means that many sectors from the start of the metadata.
305       This is replicated on all devices. For arrays with externally
306       managed metadata, the offset is from the beginning of the
307       device.
308  bitmap/chunksize
309     The size, in bytes, of the chunk which will be represented by a
310     single bit. For RAID456, it is a portion of an individual
311     device. For RAID10, it is a portion of the array. For RAID1, it
312     is both (they come to the same thing).
313  bitmap/time_base
314     The time, in seconds, between looking for bits in the bitmap to
315     be cleared. In the current implementation, a bit will be cleared
316     between 2 and 3 times "time_base" after all the covered blocks
317     are known to be in-sync.
318  bitmap/backlog
319     When write-mostly devices are active in a RAID1, write requests
320     to those devices proceed in the background - the filesystem (or
321     other user of the device) does not have to wait for them.
322     'backlog' sets a limit on the number of concurrent background
323     writes. If there are more than this, new writes will by
324     synchronous.
325  bitmap/metadata
326     This can be either 'internal' or 'external'.
327     'internal' is the default and means the metadata for the bitmap
328     is stored in the first 256 bytes of the allocated space and is
329     managed by the md module.
330     'external' means that bitmap metadata is managed externally to
331     the kernel (i.e. by some userspace program)
332  bitmap/can_clear
333     This is either 'true' or 'false'. If 'true', then bits in the
334     bitmap will be cleared when the corresponding blocks are thought
335     to be in-sync. If 'false', bits will never be cleared.
336     This is automatically set to 'false' if a write happens on a
337     degraded array, or if the array becomes degraded during a write.
338     When metadata is managed externally, it should be set to true
339     once the array becomes non-degraded, and this fact has been
340     recorded in the metadata.
345As component devices are added to an md array, they appear in the 'md'
346directory as new directories named
347      dev-XXX
348where XXX is a name that the kernel knows for the device, e.g. hdb1.
349Each directory contains:
351      block
352        a symlink to the block device in /sys/block, e.g.
353         /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
355      super
356        A file containing an image of the superblock read from, or
357        written to, that device.
359      state
360        A file recording the current state of the device in the array
361    which can be a comma separated list of
362          faulty - device has been kicked from active use due to
363                         a detected fault
364          in_sync - device is a fully in-sync member of the array
365          writemostly - device will only be subject to read
366                 requests if there are no other options.
367             This applies only to raid1 arrays.
368          blocked - device has failed, metadata is "external",
369                     and the failure hasn't been acknowledged yet.
370             Writes that would write to this device if
371             it were not faulty are blocked.
372          spare - device is working, but not a full member.
373             This includes spares that are in the process
374             of being recovered to
375    This list may grow in future.
376    This can be written to.
377    Writing "faulty" simulates a failure on the device.
378    Writing "remove" removes the device from the array.
379    Writing "writemostly" sets the writemostly flag.
380    Writing "-writemostly" clears the writemostly flag.
381    Writing "blocked" sets the "blocked" flag.
382    Writing "-blocked" clears the "blocked" flag and allows writes
383        to complete.
384    Writing "in_sync" sets the in_sync flag.
386    This file responds to select/poll. Any change to 'faulty'
387    or 'blocked' causes an event.
389      errors
390    An approximate count of read errors that have been detected on
391    this device but have not caused the device to be evicted from
392    the array (either because they were corrected or because they
393    happened while the array was read-only). When using version-1
394    metadata, this value persists across restarts of the array.
396    This value can be written while assembling an array thus
397    providing an ongoing count for arrays with metadata managed by
398    userspace.
400      slot
401        This gives the role that the device has in the array. It will
402    either be 'none' if the device is not active in the array
403        (i.e. is a spare or has failed) or an integer less than the
404    'raid_disks' number for the array indicating which position
405    it currently fills. This can only be set while assembling an
406    array. A device for which this is set is assumed to be working.
408      offset
409        This gives the location in the device (in sectors from the
410        start) where data from the array will be stored. Any part of
411        the device before this offset us not touched, unless it is
412        used for storing metadata (Formats 1.1 and 1.2).
414      size
415        The amount of the device, after the offset, that can be used
416        for storage of data. This will normally be the same as the
417    component_size. This can be written while assembling an
418        array. If a value less than the current component_size is
419        written, it will be rejected.
421      recovery_start
423        When the device is not 'in_sync', this records the number of
424    sectors from the start of the device which are known to be
425    correct. This is normally zero, but during a recovery
426    operation is will steadily increase, and if the recovery is
427    interrupted, restoring this value can cause recovery to
428    avoid repeating the earlier blocks. With v1.x metadata, this
429    value is saved and restored automatically.
431    This can be set whenever the device is not an active member of
432    the array, either before the array is activated, or before
433    the 'slot' is set.
435    Setting this to 'none' is equivalent to setting 'in_sync'.
436    Setting to any other value also clears the 'in_sync' flag.
440An active md device will also contain and entry for each active device
441in the array. These are named
443    rdNN
445where 'NN' is the position in the array, starting from 0.
446So for a 3 drive array there will be rd0, rd1, rd2.
447These are symbolic links to the appropriate 'dev-XXX' entry.
448Thus, for example,
449       cat /sys/block/md*/md/rd*/state
450will show 'in_sync' on every line.
454Active md devices for levels that support data redundancy (1,4,5,6)
455also have
457   sync_action
458     a text file that can be used to monitor and control the rebuild
459     process. It contains one word which can be one of:
460       resync - redundancy is being recalculated after unclean
461                       shutdown or creation
462       recover - a hot spare is being built to replace a
463                       failed/missing device
464       idle - nothing is happening
465       check - A full check of redundancy was requested and is
466                       happening. This reads all block and checks
467                       them. A repair may also happen for some raid
468                       levels.
469       repair - A full check and repair is happening. This is
470                       similar to 'resync', but was requested by the
471                       user, and the write-intent bitmap is NOT used to
472               optimise the process.
474      This file is writable, and each of the strings that could be
475      read are meaningful for writing.
477       'idle' will stop an active resync/recovery etc. There is no
478           guarantee that another resync/recovery may not be automatically
479       started again, though some event will be needed to trigger
480           this.
481    'resync' or 'recovery' can be used to restart the
482           corresponding operation if it was stopped with 'idle'.
483    'check' and 'repair' will start the appropriate process
484           providing the current state is 'idle'.
486      This file responds to select/poll. Any important change in the value
487      triggers a poll event. Sometimes the value will briefly be
488      "recover" if a recovery seems to be needed, but cannot be
489      achieved. In that case, the transition to "recover" isn't
490      notified, but the transition away is.
492   degraded
493      This contains a count of the number of devices by which the
494      arrays is degraded. So an optimal array with show '0'. A
495      single failed/missing drive will show '1', etc.
496      This file responds to select/poll, any increase or decrease
497      in the count of missing devices will trigger an event.
499   mismatch_count
500      When performing 'check' and 'repair', and possibly when
501      performing 'resync', md will count the number of errors that are
502      found. The count in 'mismatch_cnt' is the number of sectors
503      that were re-written, or (for 'check') would have been
504      re-written. As most raid levels work in units of pages rather
505      than sectors, this my be larger than the number of actual errors
506      by a factor of the number of sectors in a page.
508   bitmap_set_bits
509      If the array has a write-intent bitmap, then writing to this
510      attribute can set bits in the bitmap, indicating that a resync
511      would need to check the corresponding blocks. Either individual
512      numbers or start-end pairs can be written. Multiple numbers
513      can be separated by a space.
514      Note that the numbers are 'bit' numbers, not 'block' numbers.
515      They should be scaled by the bitmap_chunksize.
517   sync_speed_min
518   sync_speed_max
519     This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
520     however they only apply to the particular array.
521     If no value has been written to these, of if the word 'system'
522     is written, then the system-wide value is used. If a value,
523     in kibibytes-per-second is written, then it is used.
524     When the files are read, they show the currently active value
525     followed by "(local)" or "(system)" depending on whether it is
526     a locally set or system-wide value.
528   sync_completed
529     This shows the number of sectors that have been completed of
530     whatever the current sync_action is, followed by the number of
531     sectors in total that could need to be processed. The two
532     numbers are separated by a '/' thus effectively showing one
533     value, a fraction of the process that is complete.
534     A 'select' on this attribute will return when resync completes,
535     when it reaches the current sync_max (below) and possibly at
536     other times.
538   sync_max
539     This is a number of sectors at which point a resync/recovery
540     process will pause. When a resync is active, the value can
541     only ever be increased, never decreased. The value of 'max'
542     effectively disables the limit.
545   sync_speed
546     This shows the current actual speed, in K/sec, of the current
547     sync_action. It is averaged over the last 30 seconds.
549   suspend_lo
550   suspend_hi
551     The two values, given as numbers of sectors, indicate a range
552     within the array where IO will be blocked. This is currently
553     only supported for raid4/5/6.
556Each active md device may also have attributes specific to the
557personality module that manages it.
558These are specific to the implementation of the module and could
559change substantially if the implementation changes.
561These currently include
563  stripe_cache_size (currently raid5 only)
564      number of entries in the stripe cache. This is writable, but
565      there are upper and lower limits (32768, 16). Default is 128.
566  strip_cache_active (currently raid5 only)
567      number of active entries in the stripe cache
568  preread_bypass_threshold (currently raid5 only)
569      number of times a stripe requiring preread will be bypassed by
570      a stripe that does not require preread. For fairness defaults
571      to 1. Setting this to 0 disables bypass accounting and
572      requires preread stripes to wait until all full-width stripe-
573      writes are complete. Valid values are 0 to stripe_cache_size.

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