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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
NeilBrown6ff8d8e2006-01-06 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
65desipite possible corruption. This is normally done with
66 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
157 a text file indicating the 'raid level'. This may be a standard
158 numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
159 other name such as "linear" or "multipath".
160 If no raid level has been set yet (array is still being
161 assembled), this file will be empty.
162
163 raid_disks
164 a text file with a simple number indicating the number of devices
165 in a fully functional array. If this is not yet known, the file
166 will be empty. If an array is being resized (not currently
167 possible) this will contain the larger of the old and new sizes.
NeilBrownda943b92006-01-06 00:20:54 -0800168 Some raid level (RAID1) allow this value to be set while the
169 array is active. This will reconfigure the array. Otherwise
170 it can only be set while assembling an array.
NeilBrownbb636542005-11-08 21:39:45 -0800171
NeilBrown3b343802006-01-06 00:20:47 -0800172 chunk_size
173 This is the size if bytes for 'chunks' and is only relevant to
174 raid levels that involve striping (1,4,5,6,10). The address space
175 of the array is conceptually divided into chunks and consecutive
176 chunks are striped onto neighbouring devices.
177 The size should be atleast PAGE_SIZE (4k) and should be a power
178 of 2. This can only be set while assembling an array
179
NeilBrowna35b0d62006-01-06 00:20:49 -0800180 component_size
181 For arrays with data redundancy (i.e. not raid0, linear, faulty,
182 multipath), all components must be the same size - or at least
183 there must a size that they all provide space for. This is a key
184 part or the geometry of the array. It is measured in sectors
185 and can be read from here. Writing to this value may resize
186 the array if the personality supports it (raid1, raid5, raid6),
187 and if the component drives are large enough.
188
NeilBrown8bb93aa2006-01-06 00:20:50 -0800189 metadata_version
190 This indicates the format that is being used to record metadata
191 about the array. It can be 0.90 (traditional format), 1.0, 1.1,
192 1.2 (newer format in varying locations) or "none" indicating that
193 the kernel isn't managing metadata at all.
194
NeilBrownd9d166c2006-01-06 00:20:51 -0800195 level
196 The raid 'level' for this array. The name will often (but not
197 always) be the same as the name of the module that implements the
198 level. To be auto-loaded the module must have an alias
199 md-$LEVEL e.g. md-raid5
200 This can be written only while the array is being assembled, not
201 after it is started.
202
NeilBrown6d7ff732006-01-06 00:21:16 -0800203 new_dev
204 This file can be written but not read. The value written should
205 be a block device number as major:minor. e.g. 8:0
206 This will cause that device to be attached to the array, if it is
207 available. It will then appear at md/dev-XXX (depending on the
208 name of the device) and further configuration is then possible.
209
NeilBrown88202a02006-01-06 00:21:36 -0800210 sync_speed_min
211 sync_speed_max
212 This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
213 however they only apply to the particular array.
214 If no value has been written to these, of if the word 'system'
215 is written, then the system-wide value is used. If a value,
216 in kibibytes-per-second is written, then it is used.
217 When the files are read, they show the currently active value
218 followed by "(local)" or "(system)" depending on whether it is
219 a locally set or system-wide value.
220
221 sync_completed
222 This shows the number of sectors that have been completed of
223 whatever the current sync_action is, followed by the number of
224 sectors in total that could need to be processed. The two
225 numbers are separated by a '/' thus effectively showing one
226 value, a fraction of the process that is complete.
227
228 sync_speed
229 This shows the current actual speed, in K/sec, of the current
230 sync_action. It is averaged over the last 30 seconds.
231
NeilBrown6d7ff732006-01-06 00:21:16 -0800232
NeilBrownbb636542005-11-08 21:39:45 -0800233As component devices are added to an md array, they appear in the 'md'
234directory as new directories named
235 dev-XXX
236where XXX is a name that the kernel knows for the device, e.g. hdb1.
237Each directory contains:
238
239 block
240 a symlink to the block device in /sys/block, e.g.
241 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
242
243 super
244 A file containing an image of the superblock read from, or
245 written to, that device.
246
247 state
248 A file recording the current state of the device in the array
249 which can be a comma separated list of
250 faulty - device has been kicked from active use due to
251 a detected fault
252 in_sync - device is a fully in-sync member of the array
253 spare - device is working, but not a full member.
254 This includes spares that are in the process
255 of being recoverred to
256 This list make grow in future.
257
NeilBrown4dbcdc72006-01-06 00:20:52 -0800258 errors
259 An approximate count of read errors that have been detected on
260 this device but have not caused the device to be evicted from
261 the array (either because they were corrected or because they
262 happened while the array was read-only). When using version-1
263 metadata, this value persists across restarts of the array.
264
265 This value can be written while assembling an array thus
266 providing an ongoing count for arrays with metadata managed by
267 userspace.
268
NeilBrown014236d2006-01-06 00:20:55 -0800269 slot
270 This gives the role that the device has in the array. It will
271 either be 'none' if the device is not active in the array
272 (i.e. is a spare or has failed) or an integer less than the
273 'raid_disks' number for the array indicating which possition
274 it currently fills. This can only be set while assembling an
275 array. A device for which this is set is assumed to be working.
276
NeilBrown93c8cad2006-01-06 00:20:56 -0800277 offset
278 This gives the location in the device (in sectors from the
279 start) where data from the array will be stored. Any part of
280 the device before this offset us not touched, unless it is
281 used for storing metadata (Formats 1.1 and 1.2).
282
NeilBrown83303b62006-01-06 00:21:06 -0800283 size
284 The amount of the device, after the offset, that can be used
285 for storage of data. This will normally be the same as the
286 component_size. This can be written while assembling an
287 array. If a value less than the current component_size is
288 written, component_size will be reduced to this value.
289
NeilBrownbb636542005-11-08 21:39:45 -0800290
291An active md device will also contain and entry for each active device
292in the array. These are named
293
294 rdNN
295
296where 'NN' is the possition in the array, starting from 0.
297So for a 3 drive array there will be rd0, rd1, rd2.
298These are symbolic links to the appropriate 'dev-XXX' entry.
299Thus, for example,
300 cat /sys/block/md*/md/rd*/state
301will show 'in_sync' on every line.
302
303
304
305Active md devices for levels that support data redundancy (1,4,5,6)
306also have
307
308 sync_action
309 a text file that can be used to monitor and control the rebuild
310 process. It contains one word which can be one of:
311 resync - redundancy is being recalculated after unclean
312 shutdown or creation
313 recover - a hot spare is being built to replace a
314 failed/missing device
315 idle - nothing is happening
316 check - A full check of redundancy was requested and is
317 happening. This reads all block and checks
318 them. A repair may also happen for some raid
319 levels.
320 repair - A full check and repair is happening. This is
321 similar to 'resync', but was requested by the
322 user, and the write-intent bitmap is NOT used to
323 optimise the process.
324
325 This file is writable, and each of the strings that could be
326 read are meaningful for writing.
327
328 'idle' will stop an active resync/recovery etc. There is no
329 guarantee that another resync/recovery may not be automatically
330 started again, though some event will be needed to trigger
331 this.
332 'resync' or 'recovery' can be used to restart the
333 corresponding operation if it was stopped with 'idle'.
334 'check' and 'repair' will start the appropriate process
335 providing the current state is 'idle'.
336
337 mismatch_count
338 When performing 'check' and 'repair', and possibly when
339 performing 'resync', md will count the number of errors that are
340 found. The count in 'mismatch_cnt' is the number of sectors
341 that were re-written, or (for 'check') would have been
342 re-written. As most raid levels work in units of pages rather
343 than sectors, this my be larger than the number of actual errors
344 by a factor of the number of sectors in a page.
345
346Each active md device may also have attributes specific to the
347personality module that manages it.
348These are specific to the implementation of the module and could
349change substantially if the implementation changes.
350
351These currently include
352
353 stripe_cache_size (currently raid5 only)
354 number of entries in the stripe cache. This is writable, but
355 there are upper and lower limits (32768, 16). Default is 128.
356 strip_cache_active (currently raid5 only)
357 number of active entries in the stripe cache