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1 Tools that manage md devices can be found at
2 http://www.kernel.org/pub/linux/utils/raid/
5 Boot time assembly of RAID arrays
6 ---------------------------------
8 You can boot with your md device with the following kernel command
9 lines:
11 for old raid arrays without persistent superblocks:
12 md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
14 for raid arrays with persistent superblocks
15 md=<md device no.>,dev0,dev1,...,devn
16 or, to assemble a partitionable array:
17 md=d<md device no.>,dev0,dev1,...,devn
19 md device no. = the number of the md device ...
20 0 means md0,
21 1 md1,
22 2 md2,
23 3 md3,
24 4 md4
26 raid level = -1 linear mode
27 0 striped mode
28 other modes are only supported with persistent super blocks
30 chunk size factor = (raid-0 and raid-1 only)
31 Set the chunk size as 4k << n.
33 fault level = totally ignored
35 dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
37 A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:
39 e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
42 Boot time autodetection of RAID arrays
43 --------------------------------------
45 When md is compiled into the kernel (not as module), partitions of
46 type 0xfd are scanned and automatically assembled into RAID arrays.
47 This autodetection may be suppressed with the kernel parameter
48 "raid=noautodetect". As of kernel 2.6.9, only drives with a type 0
49 superblock can be autodetected and run at boot time.
51 The kernel parameter "raid=partitionable" (or "raid=part") means
52 that all auto-detected arrays are assembled as partitionable.
54 Boot time assembly of degraded/dirty arrays
55 -------------------------------------------
57 If a raid5 or raid6 array is both dirty and degraded, it could have
58 undetectable data corruption. This is because the fact that it is
59 'dirty' means that the parity cannot be trusted, and the fact that it
60 is degraded means that some datablocks are missing and cannot reliably
61 be reconstructed (due to no parity).
63 For this reason, md will normally refuse to start such an array. This
64 requires the sysadmin to take action to explicitly start the array
65 despite possible corruption. This is normally done with
66 mdadm --assemble --force ....
68 This option is not really available if the array has the root
69 filesystem on it. In order to support this booting from such an
70 array, md supports a module parameter "start_dirty_degraded" which,
71 when set to 1, bypassed the checks and will allows dirty degraded
72 arrays to be started.
74 So, to boot with a root filesystem of a dirty degraded raid[56], use
76 md-mod.start_dirty_degraded=1
79 Superblock formats
80 ------------------
82 The md driver can support a variety of different superblock formats.
83 Currently, it supports superblock formats "0.90.0" and the "md-1" format
84 introduced in the 2.5 development series.
86 The kernel will autodetect which format superblock is being used.
88 Superblock format '0' is treated differently to others for legacy
89 reasons - it is the original superblock format.
92 General Rules - apply for all superblock formats
93 ------------------------------------------------
95 An array is 'created' by writing appropriate superblocks to all
96 devices.
98 It is 'assembled' by associating each of these devices with an
99 particular md virtual device. Once it is completely assembled, it can
100 be accessed.
102 An array should be created by a user-space tool. This will write
103 superblocks to all devices. It will usually mark the array as
104 'unclean', or with some devices missing so that the kernel md driver
105 can create appropriate redundancy (copying in raid1, parity
106 calculation in raid4/5).
108 When an array is assembled, it is first initialized with the
109 SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor
110 version number. The major version number selects which superblock
111 format is to be used. The minor number might be used to tune handling
112 of the format, such as suggesting where on each device to look for the
113 superblock.
115 Then each device is added using the ADD_NEW_DISK ioctl. This
116 provides, in particular, a major and minor number identifying the
117 device to add.
119 The array is started with the RUN_ARRAY ioctl.
121 Once started, new devices can be added. They should have an
122 appropriate superblock written to them, and then passed be in with
123 ADD_NEW_DISK.
125 Devices that have failed or are not yet active can be detached from an
126 array using HOT_REMOVE_DISK.
129 Specific Rules that apply to format-0 super block arrays, and
130 arrays with no superblock (non-persistent).
131 -------------------------------------------------------------
133 An array can be 'created' by describing the array (level, chunksize
134 etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and
135 raid_disks != 0.
137 Then uninitialized devices can be added with ADD_NEW_DISK. The
138 structure passed to ADD_NEW_DISK must specify the state of the device
139 and its role in the array.
141 Once started with RUN_ARRAY, uninitialized spares can be added with
142 HOT_ADD_DISK.
146 MD devices in sysfs
147 -------------------
148 md devices appear in sysfs (/sys) as regular block devices,
149 e.g.
150 /sys/block/md0
152 Each 'md' device will contain a subdirectory called 'md' which
153 contains further md-specific information about the device.
155 All 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.
345 As component devices are added to an md array, they appear in the 'md'
346 directory as new directories named
347 dev-XXX
348 where XXX is a name that the kernel knows for the device, e.g. hdb1.
349 Each 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, or it has unacknowledged bad
364 blocks
365 in_sync - device is a fully in-sync member of the array
366 writemostly - device will only be subject to read
367 requests if there are no other options.
368 This applies only to raid1 arrays.
369 blocked - device has failed, and the failure hasn't been
370 acknowledged yet by the metadata handler.
371 Writes that would write to this device if
372 it were not faulty are blocked.
373 spare - device is working, but not a full member.
374 This includes spares that are in the process
375 of being recovered to
376 write_error - device has ever seen a write error.
377 want_replacement - device is (mostly) working but probably
378 should be replaced, either due to errors or
379 due to user request.
380 replacement - device is a replacement for another active
381 device with same raid_disk.
384 This list may grow in future.
385 This can be written to.
386 Writing "faulty" simulates a failure on the device.
387 Writing "remove" removes the device from the array.
388 Writing "writemostly" sets the writemostly flag.
389 Writing "-writemostly" clears the writemostly flag.
390 Writing "blocked" sets the "blocked" flag.
391 Writing "-blocked" clears the "blocked" flags and allows writes
392 to complete and possibly simulates an error.
393 Writing "in_sync" sets the in_sync flag.
394 Writing "write_error" sets writeerrorseen flag.
395 Writing "-write_error" clears writeerrorseen flag.
396 Writing "want_replacement" is allowed at any time except to a
397 replacement device or a spare. It sets the flag.
398 Writing "-want_replacement" is allowed at any time. It clears
399 the flag.
400 Writing "replacement" or "-replacement" is only allowed before
401 starting the array. It sets or clears the flag.
404 This file responds to select/poll. Any change to 'faulty'
405 or 'blocked' causes an event.
407 errors
408 An approximate count of read errors that have been detected on
409 this device but have not caused the device to be evicted from
410 the array (either because they were corrected or because they
411 happened while the array was read-only). When using version-1
412 metadata, this value persists across restarts of the array.
414 This value can be written while assembling an array thus
415 providing an ongoing count for arrays with metadata managed by
416 userspace.
418 slot
419 This gives the role that the device has in the array. It will
420 either be 'none' if the device is not active in the array
421 (i.e. is a spare or has failed) or an integer less than the
422 'raid_disks' number for the array indicating which position
423 it currently fills. This can only be set while assembling an
424 array. A device for which this is set is assumed to be working.
426 offset
427 This gives the location in the device (in sectors from the
428 start) where data from the array will be stored. Any part of
429 the device before this offset us not touched, unless it is
430 used for storing metadata (Formats 1.1 and 1.2).
432 size
433 The amount of the device, after the offset, that can be used
434 for storage of data. This will normally be the same as the
435 component_size. This can be written while assembling an
436 array. If a value less than the current component_size is
437 written, it will be rejected.
439 recovery_start
440 When the device is not 'in_sync', this records the number of
441 sectors from the start of the device which are known to be
442 correct. This is normally zero, but during a recovery
443 operation is will steadily increase, and if the recovery is
444 interrupted, restoring this value can cause recovery to
445 avoid repeating the earlier blocks. With v1.x metadata, this
446 value is saved and restored automatically.
448 This can be set whenever the device is not an active member of
449 the array, either before the array is activated, or before
450 the 'slot' is set.
452 Setting this to 'none' is equivalent to setting 'in_sync'.
453 Setting to any other value also clears the 'in_sync' flag.
455 bad_blocks
456 This gives the list of all known bad blocks in the form of
457 start address and length (in sectors respectively). If output
458 is too big to fit in a page, it will be truncated. Writing
459 "sector length" to this file adds new acknowledged (i.e.
460 recorded to disk safely) bad blocks.
462 unacknowledged_bad_blocks
463 This gives the list of known-but-not-yet-saved-to-disk bad
464 blocks in the same form of 'bad_blocks'. If output is too big
465 to fit in a page, it will be truncated. Writing to this file
466 adds bad blocks without acknowledging them. This is largely
467 for testing.
471 An active md device will also contain and entry for each active device
472 in the array. These are named
474 rdNN
476 where 'NN' is the position in the array, starting from 0.
477 So for a 3 drive array there will be rd0, rd1, rd2.
478 These are symbolic links to the appropriate 'dev-XXX' entry.
479 Thus, for example,
480 cat /sys/block/md*/md/rd*/state
481 will show 'in_sync' on every line.
485 Active md devices for levels that support data redundancy (1,4,5,6)
486 also have
488 sync_action
489 a text file that can be used to monitor and control the rebuild
490 process. It contains one word which can be one of:
491 resync - redundancy is being recalculated after unclean
492 shutdown or creation
493 recover - a hot spare is being built to replace a
494 failed/missing device
495 idle - nothing is happening
496 check - A full check of redundancy was requested and is
497 happening. This reads all block and checks
498 them. A repair may also happen for some raid
499 levels.
500 repair - A full check and repair is happening. This is
501 similar to 'resync', but was requested by the
502 user, and the write-intent bitmap is NOT used to
503 optimise the process.
505 This file is writable, and each of the strings that could be
506 read are meaningful for writing.
508 'idle' will stop an active resync/recovery etc. There is no
509 guarantee that another resync/recovery may not be automatically
510 started again, though some event will be needed to trigger
511 this.
512 'resync' or 'recovery' can be used to restart the
513 corresponding operation if it was stopped with 'idle'.
514 'check' and 'repair' will start the appropriate process
515 providing the current state is 'idle'.
517 This file responds to select/poll. Any important change in the value
518 triggers a poll event. Sometimes the value will briefly be
519 "recover" if a recovery seems to be needed, but cannot be
520 achieved. In that case, the transition to "recover" isn't
521 notified, but the transition away is.
523 degraded
524 This contains a count of the number of devices by which the
525 arrays is degraded. So an optimal array with show '0'. A
526 single failed/missing drive will show '1', etc.
527 This file responds to select/poll, any increase or decrease
528 in the count of missing devices will trigger an event.
530 mismatch_count
531 When performing 'check' and 'repair', and possibly when
532 performing 'resync', md will count the number of errors that are
533 found. The count in 'mismatch_cnt' is the number of sectors
534 that were re-written, or (for 'check') would have been
535 re-written. As most raid levels work in units of pages rather
536 than sectors, this my be larger than the number of actual errors
537 by a factor of the number of sectors in a page.
539 bitmap_set_bits
540 If the array has a write-intent bitmap, then writing to this
541 attribute can set bits in the bitmap, indicating that a resync
542 would need to check the corresponding blocks. Either individual
543 numbers or start-end pairs can be written. Multiple numbers
544 can be separated by a space.
545 Note that the numbers are 'bit' numbers, not 'block' numbers.
546 They should be scaled by the bitmap_chunksize.
548 sync_speed_min
549 sync_speed_max
550 This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
551 however they only apply to the particular array.
552 If no value has been written to these, of if the word 'system'
553 is written, then the system-wide value is used. If a value,
554 in kibibytes-per-second is written, then it is used.
555 When the files are read, they show the currently active value
556 followed by "(local)" or "(system)" depending on whether it is
557 a locally set or system-wide value.
559 sync_completed
560 This shows the number of sectors that have been completed of
561 whatever the current sync_action is, followed by the number of
562 sectors in total that could need to be processed. The two
563 numbers are separated by a '/' thus effectively showing one
564 value, a fraction of the process that is complete.
565 A 'select' on this attribute will return when resync completes,
566 when it reaches the current sync_max (below) and possibly at
567 other times.
569 sync_max
570 This is a number of sectors at which point a resync/recovery
571 process will pause. When a resync is active, the value can
572 only ever be increased, never decreased. The value of 'max'
573 effectively disables the limit.
576 sync_speed
577 This shows the current actual speed, in K/sec, of the current
578 sync_action. It is averaged over the last 30 seconds.
580 suspend_lo
581 suspend_hi
582 The two values, given as numbers of sectors, indicate a range
583 within the array where IO will be blocked. This is currently
584 only supported for raid4/5/6.
586 sync_min
587 sync_max
588 The two values, given as numbers of sectors, indicate a range
589 within the array where 'check'/'repair' will operate. Must be
590 a multiple of chunk_size. When it reaches "sync_max" it will
591 pause, rather than complete.
592 You can use 'select' or 'poll' on "sync_completed" to wait for
593 that number to reach sync_max. Then you can either increase
594 "sync_max", or can write 'idle' to "sync_action".
597 Each active md device may also have attributes specific to the
598 personality module that manages it.
599 These are specific to the implementation of the module and could
600 change substantially if the implementation changes.
602 These currently include
604 stripe_cache_size (currently raid5 only)
605 number of entries in the stripe cache. This is writable, but
606 there are upper and lower limits (32768, 16). Default is 128.
607 strip_cache_active (currently raid5 only)
608 number of active entries in the stripe cache
609 preread_bypass_threshold (currently raid5 only)
610 number of times a stripe requiring preread will be bypassed by
611 a stripe that does not require preread. For fairness defaults
612 to 1. Setting this to 0 disables bypass accounting and
613 requires preread stripes to wait until all full-width stripe-
614 writes are complete. Valid values are 0 to stripe_cache_size.