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 Boot time assembly of degraded/dirty arrays
  55 -------------------------------------------
  56
  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).
  62
  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 ....
  67
  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
  72 arrays to be started.
  73
  74 So, to boot with a root filesystem of a dirty degraded raid[56], use
  75
  76    md-mod.start_dirty_degraded=1
  77
  78
  79 Superblock formats
  80 ------------------
  81
  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.
  85
  86 The kernel will autodetect which format superblock is being used.
  87
  88 Superblock format '0' is treated differently to others for legacy
  89 reasons - it is the original superblock format.
  90
  91
  92 General Rules - apply for all superblock formats
  93 ------------------------------------------------
  94
  95 An array is 'created' by writing appropriate superblocks to all
  96 devices.
  97
  98 It is 'assembled' by associating each of these devices with an
  99 particular md virtual device.  Once it is completely assembled, it can
 100 be accessed.
 101
 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).
 107
 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
 113 superblock.
 114
 115 Then each device is added using the ADD_NEW_DISK ioctl.  This
 116 provides, in particular, a major and minor number identifying the
 117 device to add.
 118
 119 The array is started with the RUN_ARRAY ioctl.
 120
 121 Once started, new devices can be added.  They should have an
 122 appropriate superblock written to them, and then passed be in with
 123 ADD_NEW_DISK.
 124
 125 Devices that have failed or are not yet active can be detached from an
 126 array using HOT_REMOVE_DISK.
 127
 128
 129 Specific Rules that apply to format-0 super block arrays, and
 130        arrays with no superblock (non-persistent).
 131 -------------------------------------------------------------
 132
 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
 135 raid_disks != 0.
 136
 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.
 140
 141 Once started with RUN_ARRAY, uninitialized spares can be added with
 142 HOT_ADD_DISK.
 143
 144
 145
 146 MD devices in sysfs
 147 -------------------
 148 md devices appear in sysfs (/sys) as regular block devices,
 149 e.g.
 150    /sys/block/md0
 151
 152 Each 'md' device will contain a subdirectory called 'md' which
 153 contains further md-specific information about the device.
 154
 155 All 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.
 168
 169   chunk_size
 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
 176
 177   component_size
 178      For arrays with data redundancy (i.e. not raid0, linear, faulty,
 179      multipath), all components must be the same size - or at least
 180      there must a size that they all provide space for.  This is a key
 181      part or the geometry of the array.  It is measured in sectors
 182      and can be read from here.  Writing to this value may resize
 183      the array if the personality supports it (raid1, raid5, raid6),
 184      and if the component drives are large enough.
 185
 186   metadata_version
 187      This indicates the format that is being used to record metadata
 188      about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
 189      1.2 (newer format in varying locations) or "none" indicating that
 190      the kernel isn't managing metadata at all.
 191
 192   level
 193      The raid 'level' for this array.  The name will often (but not
 194      always) be the same as the name of the module that implements the
 195      level.  To be auto-loaded the module must have an alias
 196         md-$LEVEL  e.g. md-raid5
 197      This can be written only while the array is being assembled, not
 198      after it is started.
 199
 200 As component devices are added to an md array, they appear in the 'md'
 201 directory as new directories named
 202       dev-XXX
 203 where XXX is a name that the kernel knows for the device, e.g. hdb1.
 204 Each directory contains:
 205
 206       block
 207         a symlink to the block device in /sys/block, e.g.
 208              /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
 209
 210       super
 211         A file containing an image of the superblock read from, or
 212         written to, that device.
 213
 214       state
 215         A file recording the current state of the device in the array
 216         which can be a comma separated list of
 217               faulty   - device has been kicked from active use due to
 218                          a detected fault
 219               in_sync  - device is a fully in-sync member of the array
 220               spare    - device is working, but not a full member.
 221                          This includes spares that are in the process
 222                          of being recoverred to
 223         This list make grow in future.
 224
 225       errors
 226         An approximate count of read errors that have been detected on
 227         this device but have not caused the device to be evicted from
 228         the array (either because they were corrected or because they
 229         happened while the array was read-only).  When using version-1
 230         metadata, this value persists across restarts of the array.
 231
 232         This value can be written while assembling an array thus
 233         providing an ongoing count for arrays with metadata managed by
 234         userspace.
 235
 236
 237 An active md device will also contain and entry for each active device
 238 in the array.  These are named
 239
 240     rdNN
 241
 242 where 'NN' is the possition in the array, starting from 0.
 243 So for a 3 drive array there will be rd0, rd1, rd2.
 244 These are symbolic links to the appropriate 'dev-XXX' entry.
 245 Thus, for example,
 246        cat /sys/block/md*/md/rd*/state
 247 will show 'in_sync' on every line.
 248
 249
 250
 251 Active md devices for levels that support data redundancy (1,4,5,6)
 252 also have
 253
 254    sync_action
 255      a text file that can be used to monitor and control the rebuild
 256      process.  It contains one word which can be one of:
 257        resync        - redundancy is being recalculated after unclean
 258                        shutdown or creation
 259        recover       - a hot spare is being built to replace a
 260                        failed/missing device
 261        idle          - nothing is happening
 262        check         - A full check of redundancy was requested and is
 263                        happening.  This reads all block and checks
 264                        them. A repair may also happen for some raid
 265                        levels.
 266        repair        - A full check and repair is happening.  This is
 267                        similar to 'resync', but was requested by the
 268                        user, and the write-intent bitmap is NOT used to
 269                        optimise the process.
 270
 271       This file is writable, and each of the strings that could be
 272       read are meaningful for writing.
 273
 274        'idle' will stop an active resync/recovery etc.  There is no
 275            guarantee that another resync/recovery may not be automatically
 276            started again, though some event will be needed to trigger
 277            this.
 278         'resync' or 'recovery' can be used to restart the
 279            corresponding operation if it was stopped with 'idle'.
 280         'check' and 'repair' will start the appropriate process
 281            providing the current state is 'idle'.
 282
 283    mismatch_count
 284       When performing 'check' and 'repair', and possibly when
 285       performing 'resync', md will count the number of errors that are
 286       found.  The count in 'mismatch_cnt' is the number of sectors
 287       that were re-written, or (for 'check') would have been
 288       re-written.  As most raid levels work in units of pages rather
 289       than sectors, this my be larger than the number of actual errors
 290       by a factor of the number of sectors in a page.
 291
 292 Each active md device may also have attributes specific to the
 293 personality module that manages it.
 294 These are specific to the implementation of the module and could
 295 change substantially if the implementation changes.
 296
 297 These currently include
 298
 299   stripe_cache_size  (currently raid5 only)
 300       number of entries in the stripe cache.  This is writable, but
 301       there are upper and lower limits (32768, 16).  Default is 128.
 302   strip_cache_active (currently raid5 only)
 303       number of active entries in the stripe cache