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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 (not currently
168 possible) this will contain the larger of the old and new sizes.
169 Some raid level (RAID1) allow this value to be set while the
170 array is active. This will reconfigure the array. Otherwise
171 it can only be set while assembling an array.
173 chunk_size
174 This is the size if bytes for 'chunks' and is only relevant to
175 raid levels that involve striping (1,4,5,6,10). The address space
176 of the array is conceptually divided into chunks and consecutive
177 chunks are striped onto neighbouring devices.
178 The size should be at least PAGE_SIZE (4k) and should be a power
179 of 2. This can only be set while assembling an array
181 layout
182 The "layout" for the array for the particular level. This is
183 simply a number that is interpretted differently by different
184 levels. It can be written while assembling an array.
186 reshape_position
187 This is either "none" or a sector number within the devices of
188 the array where "reshape" is up to. If this is set, the three
189 attributes mentioned above (raid_disks, chunk_size, layout) can
190 potentially have 2 values, an old and a new value. If these
191 values differ, reading the attribute returns
192 new (old)
193 and writing will effect the 'new' value, leaving the 'old'
194 unchanged.
196 component_size
197 For arrays with data redundancy (i.e. not raid0, linear, faulty,
198 multipath), all components must be the same size - or at least
199 there must a size that they all provide space for. This is a key
200 part or the geometry of the array. It is measured in sectors
201 and can be read from here. Writing to this value may resize
202 the array if the personality supports it (raid1, raid5, raid6),
203 and if the component drives are large enough.
205 metadata_version
206 This indicates the format that is being used to record metadata
207 about the array. It can be 0.90 (traditional format), 1.0, 1.1,
208 1.2 (newer format in varying locations) or "none" indicating that
209 the kernel isn't managing metadata at all.
211 resync_start
212 The point at which resync should start. If no resync is needed,
213 this will be a very large number. At array creation it will
214 default to 0, though starting the array as 'clean' will
215 set it much larger.
217 new_dev
218 This file can be written but not read. The value written should
219 be a block device number as major:minor. e.g. 8:0
220 This will cause that device to be attached to the array, if it is
221 available. It will then appear at md/dev-XXX (depending on the
222 name of the device) and further configuration is then possible.
224 safe_mode_delay
225 When an md array has seen no write requests for a certain period
226 of time, it will be marked as 'clean'. When another write
227 request arrives, the array is marked as 'dirty' before the write
228 commences. This is known as 'safe_mode'.
229 The 'certain period' is controlled by this file which stores the
230 period as a number of seconds. The default is 200msec (0.200).
231 Writing a value of 0 disables safemode.
233 array_state
234 This file contains a single word which describes the current
235 state of the array. In many cases, the state can be set by
236 writing the word for the desired state, however some states
237 cannot be explicitly set, and some transitions are not allowed.
239 Select/poll works on this file. All changes except between
240 active_idle and active (which can be frequent and are not
241 very interesting) are notified. active->active_idle is
242 reported if the metadata is externally managed.
244 clear
245 No devices, no size, no level
246 Writing is equivalent to STOP_ARRAY ioctl
247 inactive
248 May have some settings, but array is not active
249 all IO results in error
250 When written, doesn't tear down array, but just stops it
251 suspended (not supported yet)
252 All IO requests will block. The array can be reconfigured.
253 Writing this, if accepted, will block until array is quiessent
254 readonly
255 no resync can happen. no superblocks get written.
256 write requests fail
257 read-auto
258 like readonly, but behaves like 'clean' on a write request.
260 clean - no pending writes, but otherwise active.
261 When written to inactive array, starts without resync
262 If a write request arrives then
263 if metadata is known, mark 'dirty' and switch to 'active'.
264 if not known, block and switch to write-pending
265 If written to an active array that has pending writes, then fails.
266 active
267 fully active: IO and resync can be happening.
268 When written to inactive array, starts with resync
270 write-pending
271 clean, but writes are blocked waiting for 'active' to be written.
273 active-idle
274 like active, but no writes have been seen for a while (safe_mode_delay).
277 As component devices are added to an md array, they appear in the 'md'
278 directory as new directories named
279 dev-XXX
280 where XXX is a name that the kernel knows for the device, e.g. hdb1.
281 Each directory contains:
283 block
284 a symlink to the block device in /sys/block, e.g.
285 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
287 super
288 A file containing an image of the superblock read from, or
289 written to, that device.
291 state
292 A file recording the current state of the device in the array
293 which can be a comma separated list of
294 faulty - device has been kicked from active use due to
295 a detected fault
296 in_sync - device is a fully in-sync member of the array
297 writemostly - device will only be subject to read
298 requests if there are no other options.
299 This applies only to raid1 arrays.
300 blocked - device has failed, metadata is "external",
301 and the failure hasn't been acknowledged yet.
302 Writes that would write to this device if
303 it were not faulty are blocked.
304 spare - device is working, but not a full member.
305 This includes spares that are in the process
306 of being recovered to
307 This list may grow in future.
308 This can be written to.
309 Writing "faulty" simulates a failure on the device.
310 Writing "remove" removes the device from the array.
311 Writing "writemostly" sets the writemostly flag.
312 Writing "-writemostly" clears the writemostly flag.
313 Writing "blocked" sets the "blocked" flag.
314 Writing "-blocked" clear the "blocked" flag and allows writes
315 to complete.
317 This file responds to select/poll. Any change to 'faulty'
318 or 'blocked' causes an event.
320 errors
321 An approximate count of read errors that have been detected on
322 this device but have not caused the device to be evicted from
323 the array (either because they were corrected or because they
324 happened while the array was read-only). When using version-1
325 metadata, this value persists across restarts of the array.
327 This value can be written while assembling an array thus
328 providing an ongoing count for arrays with metadata managed by
329 userspace.
331 slot
332 This gives the role that the device has in the array. It will
333 either be 'none' if the device is not active in the array
334 (i.e. is a spare or has failed) or an integer less than the
335 'raid_disks' number for the array indicating which position
336 it currently fills. This can only be set while assembling an
337 array. A device for which this is set is assumed to be working.
339 offset
340 This gives the location in the device (in sectors from the
341 start) where data from the array will be stored. Any part of
342 the device before this offset us not touched, unless it is
343 used for storing metadata (Formats 1.1 and 1.2).
345 size
346 The amount of the device, after the offset, that can be used
347 for storage of data. This will normally be the same as the
348 component_size. This can be written while assembling an
349 array. If a value less than the current component_size is
350 written, it will be rejected.
353 An active md device will also contain and entry for each active device
354 in the array. These are named
356 rdNN
358 where 'NN' is the position in the array, starting from 0.
359 So for a 3 drive array there will be rd0, rd1, rd2.
360 These are symbolic links to the appropriate 'dev-XXX' entry.
361 Thus, for example,
362 cat /sys/block/md*/md/rd*/state
363 will show 'in_sync' on every line.
367 Active md devices for levels that support data redundancy (1,4,5,6)
368 also have
370 sync_action
371 a text file that can be used to monitor and control the rebuild
372 process. It contains one word which can be one of:
373 resync - redundancy is being recalculated after unclean
374 shutdown or creation
375 recover - a hot spare is being built to replace a
376 failed/missing device
377 idle - nothing is happening
378 check - A full check of redundancy was requested and is
379 happening. This reads all block and checks
380 them. A repair may also happen for some raid
381 levels.
382 repair - A full check and repair is happening. This is
383 similar to 'resync', but was requested by the
384 user, and the write-intent bitmap is NOT used to
385 optimise the process.
387 This file is writable, and each of the strings that could be
388 read are meaningful for writing.
390 'idle' will stop an active resync/recovery etc. There is no
391 guarantee that another resync/recovery may not be automatically
392 started again, though some event will be needed to trigger
393 this.
394 'resync' or 'recovery' can be used to restart the
395 corresponding operation if it was stopped with 'idle'.
396 'check' and 'repair' will start the appropriate process
397 providing the current state is 'idle'.
399 This file responds to select/poll. Any important change in the value
400 triggers a poll event. Sometimes the value will briefly be
401 "recover" if a recovery seems to be needed, but cannot be
402 achieved. In that case, the transition to "recover" isn't
403 notified, but the transition away is.
405 degraded
406 This contains a count of the number of devices by which the
407 arrays is degraded. So an optimal array with show '0'. A
408 single failed/missing drive will show '1', etc.
409 This file responds to select/poll, any increase or decrease
410 in the count of missing devices will trigger an event.
412 mismatch_count
413 When performing 'check' and 'repair', and possibly when
414 performing 'resync', md will count the number of errors that are
415 found. The count in 'mismatch_cnt' is the number of sectors
416 that were re-written, or (for 'check') would have been
417 re-written. As most raid levels work in units of pages rather
418 than sectors, this my be larger than the number of actual errors
419 by a factor of the number of sectors in a page.
421 bitmap_set_bits
422 If the array has a write-intent bitmap, then writing to this
423 attribute can set bits in the bitmap, indicating that a resync
424 would need to check the corresponding blocks. Either individual
425 numbers or start-end pairs can be written. Multiple numbers
426 can be separated by a space.
427 Note that the numbers are 'bit' numbers, not 'block' numbers.
428 They should be scaled by the bitmap_chunksize.
430 sync_speed_min
431 sync_speed_max
432 This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
433 however they only apply to the particular array.
434 If no value has been written to these, of if the word 'system'
435 is written, then the system-wide value is used. If a value,
436 in kibibytes-per-second is written, then it is used.
437 When the files are read, they show the currently active value
438 followed by "(local)" or "(system)" depending on whether it is
439 a locally set or system-wide value.
441 sync_completed
442 This shows the number of sectors that have been completed of
443 whatever the current sync_action is, followed by the number of
444 sectors in total that could need to be processed. The two
445 numbers are separated by a '/' thus effectively showing one
446 value, a fraction of the process that is complete.
447 A 'select' on this attribute will return when resync completes,
448 when it reaches the current sync_max (below) and possibly at
449 other times.
451 sync_max
452 This is a number of sectors at which point a resync/recovery
453 process will pause. When a resync is active, the value can
454 only ever be increased, never decreased. The value of 'max'
455 effectively disables the limit.
458 sync_speed
459 This shows the current actual speed, in K/sec, of the current
460 sync_action. It is averaged over the last 30 seconds.
462 suspend_lo
463 suspend_hi
464 The two values, given as numbers of sectors, indicate a range
465 within the array where IO will be blocked. This is currently
466 only supported for raid4/5/6.
469 Each active md device may also have attributes specific to the
470 personality module that manages it.
471 These are specific to the implementation of the module and could
472 change substantially if the implementation changes.
474 These currently include
476 stripe_cache_size (currently raid5 only)
477 number of entries in the stripe cache. This is writable, but
478 there are upper and lower limits (32768, 16). Default is 128.
479 strip_cache_active (currently raid5 only)
480 number of active entries in the stripe cache
481 preread_bypass_threshold (currently raid5 only)
482 number of times a stripe requiring preread will be bypassed by
483 a stripe that does not require preread. For fairness defaults
484 to 1. Setting this to 0 disables bypass accounting and
485 requires preread stripes to wait until all full-width stripe-
486 writes are complete. Valid values are 0 to stripe_cache_size.