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.
58 The md driver can support a variety of different superblock formats.
59 Currently, it supports superblock formats "0.90.0" and the "md-1" format
60 introduced in the 2.5 development series.
62 The kernel will autodetect which format superblock is being used.
64 Superblock format '0' is treated differently to others for legacy
65 reasons - it is the original superblock format.
68 General Rules - apply for all superblock formats
69 ------------------------------------------------
71 An array is 'created' by writing appropriate superblocks to all
74 It is 'assembled' by associating each of these devices with an
75 particular md virtual device. Once it is completely assembled, it can
78 An array should be created by a user-space tool. This will write
79 superblocks to all devices. It will usually mark the array as
80 'unclean', or with some devices missing so that the kernel md driver
81 can create appropriate redundancy (copying in raid1, parity
82 calculation in raid4/5).
84 When an array is assembled, it is first initialized with the
85 SET_ARRAY_INFO ioctl. This contains, in particular, a major and minor
86 version number. The major version number selects which superblock
87 format is to be used. The minor number might be used to tune handling
88 of the format, such as suggesting where on each device to look for the
91 Then each device is added using the ADD_NEW_DISK ioctl. This
92 provides, in particular, a major and minor number identifying the
95 The array is started with the RUN_ARRAY ioctl.
97 Once started, new devices can be added. They should have an
98 appropriate superblock written to them, and then passed be in with
101 Devices that have failed or are not yet active can be detached from an
102 array using HOT_REMOVE_DISK.
105 Specific Rules that apply to format-0 super block arrays, and
106 arrays with no superblock (non-persistent).
107 -------------------------------------------------------------
109 An array can be 'created' by describing the array (level, chunksize
110 etc) in a SET_ARRAY_INFO ioctl. This must has major_version==0 and
113 Then uninitialized devices can be added with ADD_NEW_DISK. The
114 structure passed to ADD_NEW_DISK must specify the state of the device
115 and it's role in the array.
117 Once started with RUN_ARRAY, uninitialized spares can be added with
124 md devices appear in sysfs (/sys) as regular block devices,
128 Each 'md' device will contain a subdirectory called 'md' which
129 contains further md-specific information about the device.
131 All md devices contain:
133 a text file indicating the 'raid level'. This may be a standard
134 numerical level prefixed by "RAID-" - e.g. "RAID-5", or some
135 other name such as "linear" or "multipath".
136 If no raid level has been set yet (array is still being
137 assembled), this file will be empty.
140 a text file with a simple number indicating the number of devices
141 in a fully functional array. If this is not yet known, the file
142 will be empty. If an array is being resized (not currently
143 possible) this will contain the larger of the old and new sizes.
145 As component devices are added to an md array, they appear in the 'md'
146 directory as new directories named
148 where XXX is a name that the kernel knows for the device, e.g. hdb1.
149 Each directory contains:
152 a symlink to the block device in /sys/block, e.g.
153 /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
156 A file containing an image of the superblock read from, or
157 written to, that device.
160 A file recording the current state of the device in the array
161 which can be a comma separated list of
162 faulty - device has been kicked from active use due to
164 in_sync - device is a fully in-sync member of the array
165 spare - device is working, but not a full member.
166 This includes spares that are in the process
167 of being recoverred to
168 This list make grow in future.
171 An active md device will also contain and entry for each active device
172 in the array. These are named
176 where 'NN' is the possition in the array, starting from 0.
177 So for a 3 drive array there will be rd0, rd1, rd2.
178 These are symbolic links to the appropriate 'dev-XXX' entry.
180 cat /sys/block/md*/md/rd*/state
181 will show 'in_sync' on every line.
185 Active md devices for levels that support data redundancy (1,4,5,6)
189 a text file that can be used to monitor and control the rebuild
190 process. It contains one word which can be one of:
191 resync - redundancy is being recalculated after unclean
193 recover - a hot spare is being built to replace a
194 failed/missing device
195 idle - nothing is happening
196 check - A full check of redundancy was requested and is
197 happening. This reads all block and checks
198 them. A repair may also happen for some raid
200 repair - A full check and repair is happening. This is
201 similar to 'resync', but was requested by the
202 user, and the write-intent bitmap is NOT used to
203 optimise the process.
205 This file is writable, and each of the strings that could be
206 read are meaningful for writing.
208 'idle' will stop an active resync/recovery etc. There is no
209 guarantee that another resync/recovery may not be automatically
210 started again, though some event will be needed to trigger
212 'resync' or 'recovery' can be used to restart the
213 corresponding operation if it was stopped with 'idle'.
214 'check' and 'repair' will start the appropriate process
215 providing the current state is 'idle'.
218 When performing 'check' and 'repair', and possibly when
219 performing 'resync', md will count the number of errors that are
220 found. The count in 'mismatch_cnt' is the number of sectors
221 that were re-written, or (for 'check') would have been
222 re-written. As most raid levels work in units of pages rather
223 than sectors, this my be larger than the number of actual errors
224 by a factor of the number of sectors in a page.
226 Each active md device may also have attributes specific to the
227 personality module that manages it.
228 These are specific to the implementation of the module and could
229 change substantially if the implementation changes.
231 These currently include
233 stripe_cache_size (currently raid5 only)
234 number of entries in the stripe cache. This is writable, but
235 there are upper and lower limits (32768, 16). Default is 128.
236 strip_cache_active (currently raid5 only)
237 number of active entries in the stripe cache