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1da177e4 LT |
1 | Tools that manage md devices can be found at |
2 | http://www.<country>.kernel.org/pub/linux/utils/raid/.... | |
3 | ||
4 | ||
5 | Boot time assembly of RAID arrays | |
6 | --------------------------------- | |
7 | ||
8 | You can boot with your md device with the following kernel command | |
9 | lines: | |
10 | ||
11 | for old raid arrays without persistent superblocks: | |
12 | md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn | |
13 | ||
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 | |
18 | ||
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 | |
25 | ||
26 | raid level = -1 linear mode | |
27 | 0 striped mode | |
28 | other modes are only supported with persistent super blocks | |
29 | ||
30 | chunk size factor = (raid-0 and raid-1 only) | |
31 | Set the chunk size as 4k << n. | |
32 | ||
33 | fault level = totally ignored | |
34 | ||
35 | dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1 | |
36 | ||
37 | A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>) looks like this: | |
38 | ||
39 | e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro | |
40 | ||
41 | ||
42 | Boot time autodetection of RAID arrays | |
43 | -------------------------------------- | |
44 | ||
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. | |
50 | ||
51 | The kernel parameter "raid=partitionable" (or "raid=part") means | |
52 | that all auto-detected arrays are assembled as partitionable. | |
53 | ||
54 | ||
55 | Superblock formats | |
56 | ------------------ | |
57 | ||
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. | |
61 | ||
62 | The kernel will autodetect which format superblock is being used. | |
63 | ||
64 | Superblock format '0' is treated differently to others for legacy | |
65 | reasons - it is the original superblock format. | |
66 | ||
67 | ||
68 | General Rules - apply for all superblock formats | |
69 | ------------------------------------------------ | |
70 | ||
71 | An array is 'created' by writing appropriate superblocks to all | |
72 | devices. | |
73 | ||
74 | It is 'assembled' by associating each of these devices with an | |
75 | particular md virtual device. Once it is completely assembled, it can | |
76 | be accessed. | |
77 | ||
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). | |
83 | ||
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 | |
89 | superblock. | |
90 | ||
91 | Then each device is added using the ADD_NEW_DISK ioctl. This | |
92 | provides, in particular, a major and minor number identifying the | |
93 | device to add. | |
94 | ||
95 | The array is started with the RUN_ARRAY ioctl. | |
96 | ||
97 | Once started, new devices can be added. They should have an | |
98 | appropriate superblock written to them, and then passed be in with | |
99 | ADD_NEW_DISK. | |
100 | ||
101 | Devices that have failed or are not yet active can be detached from an | |
102 | array using HOT_REMOVE_DISK. | |
103 | ||
104 | ||
105 | Specific Rules that apply to format-0 super block arrays, and | |
106 | arrays with no superblock (non-persistent). | |
107 | ------------------------------------------------------------- | |
108 | ||
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 | |
111 | raid_disks != 0. | |
112 | ||
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. | |
116 | ||
117 | Once started with RUN_ARRAY, uninitialized spares can be added with | |
118 | HOT_ADD_DISK. | |
bb636547 N |
119 | |
120 | ||
121 | ||
122 | MD devices in sysfs | |
123 | ------------------- | |
124 | md devices appear in sysfs (/sys) as regular block devices, | |
125 | e.g. | |
126 | /sys/block/md0 | |
127 | ||
128 | Each 'md' device will contain a subdirectory called 'md' which | |
129 | contains further md-specific information about the device. | |
130 | ||
131 | All md devices contain: | |
132 | level | |
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. | |
138 | ||
139 | raid_disks | |
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. | |
144 | ||
145 | As component devices are added to an md array, they appear in the 'md' | |
146 | directory as new directories named | |
147 | dev-XXX | |
148 | where XXX is a name that the kernel knows for the device, e.g. hdb1. | |
149 | Each directory contains: | |
150 | ||
151 | block | |
152 | a symlink to the block device in /sys/block, e.g. | |
153 | /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1 | |
154 | ||
155 | super | |
156 | A file containing an image of the superblock read from, or | |
157 | written to, that device. | |
158 | ||
159 | state | |
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 | |
163 | a detected fault | |
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. | |
169 | ||
170 | ||
171 | An active md device will also contain and entry for each active device | |
172 | in the array. These are named | |
173 | ||
174 | rdNN | |
175 | ||
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. | |
179 | Thus, for example, | |
180 | cat /sys/block/md*/md/rd*/state | |
181 | will show 'in_sync' on every line. | |
182 | ||
183 | ||
184 | ||
185 | Active md devices for levels that support data redundancy (1,4,5,6) | |
186 | also have | |
187 | ||
188 | sync_action | |
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 | |
192 | shutdown or creation | |
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 | |
199 | levels. | |
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. | |
204 | ||
205 | This file is writable, and each of the strings that could be | |
206 | read are meaningful for writing. | |
207 | ||
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 | |
211 | this. | |
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'. | |
216 | ||
217 | mismatch_count | |
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. | |
225 | ||
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. | |
230 | ||
231 | These currently include | |
232 | ||
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 |