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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Tools that manage md devices can be found at
2 http://www.<country>.kernel.org/pub/linux/utils/raid/....
3
4
5Boot time assembly of RAID arrays
6---------------------------------
7
8You can boot with your md device with the following kernel command
9lines:
10
11for old raid arrays without persistent superblocks:
12 md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
13
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
18
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
25
26raid level = -1 linear mode
27 0 striped mode
28 other modes are only supported with persistent super blocks
29
30chunk size factor = (raid-0 and raid-1 only)
31 Set the chunk size as 4k << n.
32
33fault level = totally ignored
34
35dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
36
37A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this:
38
39e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
40
41
42Boot time autodetection of RAID arrays
43--------------------------------------
44
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.
50
51The kernel parameter "raid=partitionable" (or "raid=part") means
52that all auto-detected arrays are assembled as partitionable.
53
NeilBrown6ff8d8ec2006-01-06 00:20:15 -080054Boot time assembly of degraded/dirty arrays
55-------------------------------------------
56
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).
62
63For this reason, md will normally refuse to start such an array. This
64requires the sysadmin to take action to explicitly start the array
Matt LaPlantefff92892006-10-03 22:47:42 +020065despite possible corruption. This is normally done with
NeilBrown6ff8d8ec2006-01-06 00:20:15 -080066 mdadm --assemble --force ....
67
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.
73
74So, to boot with a root filesystem of a dirty degraded raid[56], use
75
76 md-mod.start_dirty_degraded=1
77
Linus Torvalds1da177e2005-04-16 15:20:36 -070078
79Superblock formats
80------------------
81
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.
85
86The kernel will autodetect which format superblock is being used.
87
88Superblock format '0' is treated differently to others for legacy
89reasons - it is the original superblock format.
90
91
92General Rules - apply for all superblock formats
93------------------------------------------------
94
95An array is 'created' by writing appropriate superblocks to all
96devices.
97
98It is 'assembled' by associating each of these devices with an
99particular md virtual device. Once it is completely assembled, it can
100be accessed.
101
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).
107
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
113superblock.
114
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.
118
119The array is started with the RUN_ARRAY ioctl.
120
121Once started, new devices can be added. They should have an
122appropriate superblock written to them, and then passed be in with
123ADD_NEW_DISK.
124
125Devices that have failed or are not yet active can be detached from an
126array using HOT_REMOVE_DISK.
127
128
129Specific Rules that apply to format-0 super block arrays, and
130 arrays with no superblock (non-persistent).
131-------------------------------------------------------------
132
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.
136
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.
140
141Once started with RUN_ARRAY, uninitialized spares can be added with
142HOT_ADD_DISK.
NeilBrownbb636542005-11-08 21:39:45 -0800143
144
145
146MD devices in sysfs
147-------------------
148md devices appear in sysfs (/sys) as regular block devices,
149e.g.
150 /sys/block/md0
151
152Each 'md' device will contain a subdirectory called 'md' which
153contains further md-specific information about the device.
154
155All md devices contain:
156 level
NeilBrownd33a56d2006-10-03 01:15:58 -0700157 a text file indicating the 'raid level'. e.g. raid0, raid1,
158 raid5, linear, multipath, faulty.
NeilBrownbb636542005-11-08 21:39:45 -0800159 If no raid level has been set yet (array is still being
NeilBrownd33a56d2006-10-03 01:15:58 -0700160 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.
NeilBrownbb636542005-11-08 21:39:45 -0800163
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.
NeilBrownda943b992006-01-06 00:20:54 -0800169 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.
NeilBrownbb636542005-11-08 21:39:45 -0800172
NeilBrown3b343802006-01-06 00:20:47 -0800173 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.
Matt LaPlante3f6dee92006-10-03 22:45:33 +0200178 The size should be at least PAGE_SIZE (4k) and should be a power
NeilBrown3b343802006-01-06 00:20:47 -0800179 of 2. This can only be set while assembling an array
180
NeilBrowna35b0d62006-01-06 00:20:49 -0800181 component_size
182 For arrays with data redundancy (i.e. not raid0, linear, faulty,
183 multipath), all components must be the same size - or at least
184 there must a size that they all provide space for. This is a key
185 part or the geometry of the array. It is measured in sectors
186 and can be read from here. Writing to this value may resize
187 the array if the personality supports it (raid1, raid5, raid6),
188 and if the component drives are large enough.
189
NeilBrown8bb93aa2006-01-06 00:20:50 -0800190 metadata_version
191 This indicates the format that is being used to record metadata
192 about the array. It can be 0.90 (traditional format), 1.0, 1.1,
193 1.2 (newer format in varying locations) or "none" indicating that
194 the kernel isn't managing metadata at all.
195
NeilBrownd4dbd022006-06-26 00:27:59 -0700196 layout
197 The "layout" for the array for the particular level. This is
198 simply a number that is interpretted differently by different
199 levels. It can be written while assembling an array.
200
NeilBrowna94213b2006-06-26 00:28:00 -0700201 resync_start
202 The point at which resync should start. If no resync is needed,
203 this will be a very large number. At array creation it will
204 default to 0, though starting the array as 'clean' will
205 set it much larger.
206
NeilBrown6d7ff732006-01-06 00:21:16 -0800207 new_dev
208 This file can be written but not read. The value written should
209 be a block device number as major:minor. e.g. 8:0
210 This will cause that device to be attached to the array, if it is
211 available. It will then appear at md/dev-XXX (depending on the
212 name of the device) and further configuration is then possible.
213
NeilBrown16f17b32006-06-26 00:27:37 -0700214 safe_mode_delay
215 When an md array has seen no write requests for a certain period
216 of time, it will be marked as 'clean'. When another write
Matt LaPlantefff92892006-10-03 22:47:42 +0200217 request arrives, the array is marked as 'dirty' before the write
218 commences. This is known as 'safe_mode'.
NeilBrown16f17b32006-06-26 00:27:37 -0700219 The 'certain period' is controlled by this file which stores the
220 period as a number of seconds. The default is 200msec (0.200).
221 Writing a value of 0 disables safemode.
222
NeilBrown9e653b62006-06-26 00:27:58 -0700223 array_state
224 This file contains a single word which describes the current
225 state of the array. In many cases, the state can be set by
226 writing the word for the desired state, however some states
227 cannot be explicitly set, and some transitions are not allowed.
228
229 clear
230 No devices, no size, no level
231 Writing is equivalent to STOP_ARRAY ioctl
232 inactive
233 May have some settings, but array is not active
234 all IO results in error
235 When written, doesn't tear down array, but just stops it
236 suspended (not supported yet)
237 All IO requests will block. The array can be reconfigured.
238 Writing this, if accepted, will block until array is quiessent
239 readonly
240 no resync can happen. no superblocks get written.
241 write requests fail
242 read-auto
243 like readonly, but behaves like 'clean' on a write request.
244
245 clean - no pending writes, but otherwise active.
246 When written to inactive array, starts without resync
247 If a write request arrives then
248 if metadata is known, mark 'dirty' and switch to 'active'.
249 if not known, block and switch to write-pending
250 If written to an active array that has pending writes, then fails.
251 active
252 fully active: IO and resync can be happening.
253 When written to inactive array, starts with resync
254
255 write-pending
256 clean, but writes are blocked waiting for 'active' to be written.
257
258 active-idle
259 like active, but no writes have been seen for a while (safe_mode_delay).
260
261
NeilBrown88202a02006-01-06 00:21:36 -0800262 sync_speed_min
263 sync_speed_max
264 This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
265 however they only apply to the particular array.
266 If no value has been written to these, of if the word 'system'
267 is written, then the system-wide value is used. If a value,
268 in kibibytes-per-second is written, then it is used.
269 When the files are read, they show the currently active value
270 followed by "(local)" or "(system)" depending on whether it is
271 a locally set or system-wide value.
272
273 sync_completed
274 This shows the number of sectors that have been completed of
275 whatever the current sync_action is, followed by the number of
276 sectors in total that could need to be processed. The two
277 numbers are separated by a '/' thus effectively showing one
278 value, a fraction of the process that is complete.
279
280 sync_speed
281 This shows the current actual speed, in K/sec, of the current
282 sync_action. It is averaged over the last 30 seconds.
283
NeilBrown6d7ff732006-01-06 00:21:16 -0800284
NeilBrownbb636542005-11-08 21:39:45 -0800285As component devices are added to an md array, they appear in the 'md'
286directory as new directories named
287 dev-XXX
288where XXX is a name that the kernel knows for the device, e.g. hdb1.
289Each directory contains:
290
291 block
292 a symlink to the block device in /sys/block, e.g.
293 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
294
295 super
296 A file containing an image of the superblock read from, or
297 written to, that device.
298
299 state
300 A file recording the current state of the device in the array
301 which can be a comma separated list of
302 faulty - device has been kicked from active use due to
303 a detected fault
304 in_sync - device is a fully in-sync member of the array
NeilBrownf6556752006-06-26 00:28:01 -0700305 writemostly - device will only be subject to read
306 requests if there are no other options.
307 This applies only to raid1 arrays.
NeilBrownbb636542005-11-08 21:39:45 -0800308 spare - device is working, but not a full member.
309 This includes spares that are in the process
Matt LaPlanted6bc8ac2006-10-03 22:54:15 +0200310 of being recovered to
311 This list may grow in future.
NeilBrown45dc2de2006-06-26 00:27:58 -0700312 This can be written to.
313 Writing "faulty" simulates a failure on the device.
314 Writing "remove" removes the device from the array.
NeilBrownf6556752006-06-26 00:28:01 -0700315 Writing "writemostly" sets the writemostly flag.
316 Writing "-writemostly" clears the writemostly flag.
NeilBrownbb636542005-11-08 21:39:45 -0800317
NeilBrown4dbcdc72006-01-06 00:20:52 -0800318 errors
319 An approximate count of read errors that have been detected on
320 this device but have not caused the device to be evicted from
321 the array (either because they were corrected or because they
322 happened while the array was read-only). When using version-1
323 metadata, this value persists across restarts of the array.
324
325 This value can be written while assembling an array thus
326 providing an ongoing count for arrays with metadata managed by
327 userspace.
328
NeilBrown014236d2006-01-06 00:20:55 -0800329 slot
330 This gives the role that the device has in the array. It will
331 either be 'none' if the device is not active in the array
332 (i.e. is a spare or has failed) or an integer less than the
Matt LaPlante992caac2006-10-03 22:52:05 +0200333 'raid_disks' number for the array indicating which position
NeilBrown014236d2006-01-06 00:20:55 -0800334 it currently fills. This can only be set while assembling an
335 array. A device for which this is set is assumed to be working.
336
NeilBrown93c8cad2006-01-06 00:20:56 -0800337 offset
338 This gives the location in the device (in sectors from the
339 start) where data from the array will be stored. Any part of
340 the device before this offset us not touched, unless it is
341 used for storing metadata (Formats 1.1 and 1.2).
342
NeilBrown83303b62006-01-06 00:21:06 -0800343 size
344 The amount of the device, after the offset, that can be used
345 for storage of data. This will normally be the same as the
346 component_size. This can be written while assembling an
347 array. If a value less than the current component_size is
348 written, component_size will be reduced to this value.
349
NeilBrownbb636542005-11-08 21:39:45 -0800350
351An active md device will also contain and entry for each active device
352in the array. These are named
353
354 rdNN
355
Matt LaPlante992caac2006-10-03 22:52:05 +0200356where 'NN' is the position in the array, starting from 0.
NeilBrownbb636542005-11-08 21:39:45 -0800357So for a 3 drive array there will be rd0, rd1, rd2.
358These are symbolic links to the appropriate 'dev-XXX' entry.
359Thus, for example,
360 cat /sys/block/md*/md/rd*/state
361will show 'in_sync' on every line.
362
363
364
365Active md devices for levels that support data redundancy (1,4,5,6)
366also have
367
368 sync_action
369 a text file that can be used to monitor and control the rebuild
370 process. It contains one word which can be one of:
371 resync - redundancy is being recalculated after unclean
372 shutdown or creation
373 recover - a hot spare is being built to replace a
374 failed/missing device
375 idle - nothing is happening
376 check - A full check of redundancy was requested and is
377 happening. This reads all block and checks
378 them. A repair may also happen for some raid
379 levels.
380 repair - A full check and repair is happening. This is
381 similar to 'resync', but was requested by the
382 user, and the write-intent bitmap is NOT used to
383 optimise the process.
384
385 This file is writable, and each of the strings that could be
386 read are meaningful for writing.
387
388 'idle' will stop an active resync/recovery etc. There is no
389 guarantee that another resync/recovery may not be automatically
390 started again, though some event will be needed to trigger
391 this.
392 'resync' or 'recovery' can be used to restart the
393 corresponding operation if it was stopped with 'idle'.
394 'check' and 'repair' will start the appropriate process
395 providing the current state is 'idle'.
396
397 mismatch_count
398 When performing 'check' and 'repair', and possibly when
399 performing 'resync', md will count the number of errors that are
400 found. The count in 'mismatch_cnt' is the number of sectors
401 that were re-written, or (for 'check') would have been
402 re-written. As most raid levels work in units of pages rather
403 than sectors, this my be larger than the number of actual errors
404 by a factor of the number of sectors in a page.
405
Paul Clements9b1d1da2006-10-03 01:15:49 -0700406 bitmap_set_bits
407 If the array has a write-intent bitmap, then writing to this
408 attribute can set bits in the bitmap, indicating that a resync
409 would need to check the corresponding blocks. Either individual
410 numbers or start-end pairs can be written. Multiple numbers
411 can be separated by a space.
412 Note that the numbers are 'bit' numbers, not 'block' numbers.
413 They should be scaled by the bitmap_chunksize.
414
NeilBrownbb636542005-11-08 21:39:45 -0800415Each active md device may also have attributes specific to the
416personality module that manages it.
417These are specific to the implementation of the module and could
418change substantially if the implementation changes.
419
420These currently include
421
422 stripe_cache_size (currently raid5 only)
423 number of entries in the stripe cache. This is writable, but
424 there are upper and lower limits (32768, 16). Default is 128.
425 strip_cache_active (currently raid5 only)
426 number of active entries in the stripe cache