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1 Tools that manage md devices can be found at
2 http://www.<country>.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 it's 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
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.
440 An active md device will also contain and entry for each active device
441 in the array. These are named
443 rdNN
445 where 'NN' is the position in the array, starting from 0.
446 So for a 3 drive array there will be rd0, rd1, rd2.
447 These are symbolic links to the appropriate 'dev-XXX' entry.
448 Thus, for example,
449 cat /sys/block/md*/md/rd*/state
450 will show 'in_sync' on every line.
454 Active md devices for levels that support data redundancy (1,4,5,6)
455 also 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.
556 Each active md device may also have attributes specific to the
557 personality module that manages it.
558 These are specific to the implementation of the module and could
559 change substantially if the implementation changes.
561 These 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.