Documentation/md.txt

   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.
 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