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
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 ...
26 raid level = -1 linear 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 desipite 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
74 So, to boot with a root filesystem of a dirty degraded raid[56], use
76 md-mod.start_dirty_degraded=1
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
98 It is 'assembled' by associating each of these devices with an
99 particular md virtual device. Once it is completely assembled, it can
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
115 Then each device is added using the ADD_NEW_DISK ioctl. This
116 provides, in particular, a major and minor number identifying the
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
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
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
148 md devices appear in sysfs (/sys) as regular block devices,
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:
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.
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.
170 This is the size if bytes for 'chunks' and is only relevant to
171 raid levels that involve striping (1,4,5,6,10). The address space
172 of the array is conceptually divided into chunks and consecutive
173 chunks are striped onto neighbouring devices.
174 The size should be atleast PAGE_SIZE (4k) and should be a power
175 of 2. This can only be set while assembling an array
177 As component devices are added to an md array, they appear in the 'md'
178 directory as new directories named
180 where XXX is a name that the kernel knows for the device, e.g. hdb1.
181 Each directory contains:
184 a symlink to the block device in /sys/block, e.g.
185 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
188 A file containing an image of the superblock read from, or
189 written to, that device.
192 A file recording the current state of the device in the array
193 which can be a comma separated list of
194 faulty - device has been kicked from active use due to
196 in_sync - device is a fully in-sync member of the array
197 spare - device is working, but not a full member.
198 This includes spares that are in the process
199 of being recoverred to
200 This list make grow in future.
203 An active md device will also contain and entry for each active device
204 in the array. These are named
208 where 'NN' is the possition in the array, starting from 0.
209 So for a 3 drive array there will be rd0, rd1, rd2.
210 These are symbolic links to the appropriate 'dev-XXX' entry.
212 cat /sys/block/md*/md/rd*/state
213 will show 'in_sync' on every line.
217 Active md devices for levels that support data redundancy (1,4,5,6)
221 a text file that can be used to monitor and control the rebuild
222 process. It contains one word which can be one of:
223 resync - redundancy is being recalculated after unclean
225 recover - a hot spare is being built to replace a
226 failed/missing device
227 idle - nothing is happening
228 check - A full check of redundancy was requested and is
229 happening. This reads all block and checks
230 them. A repair may also happen for some raid
232 repair - A full check and repair is happening. This is
233 similar to 'resync', but was requested by the
234 user, and the write-intent bitmap is NOT used to
235 optimise the process.
237 This file is writable, and each of the strings that could be
238 read are meaningful for writing.
240 'idle' will stop an active resync/recovery etc. There is no
241 guarantee that another resync/recovery may not be automatically
242 started again, though some event will be needed to trigger
244 'resync' or 'recovery' can be used to restart the
245 corresponding operation if it was stopped with 'idle'.
246 'check' and 'repair' will start the appropriate process
247 providing the current state is 'idle'.
250 When performing 'check' and 'repair', and possibly when
251 performing 'resync', md will count the number of errors that are
252 found. The count in 'mismatch_cnt' is the number of sectors
253 that were re-written, or (for 'check') would have been
254 re-written. As most raid levels work in units of pages rather
255 than sectors, this my be larger than the number of actual errors
256 by a factor of the number of sectors in a page.
258 Each active md device may also have attributes specific to the
259 personality module that manages it.
260 These are specific to the implementation of the module and could
261 change substantially if the implementation changes.
263 These currently include
265 stripe_cache_size (currently raid5 only)
266 number of entries in the stripe cache. This is writable, but
267 there are upper and lower limits (32768, 16). Default is 128.
268 strip_cache_active (currently raid5 only)
269 number of active entries in the stripe cache