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 despite 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'. e.g. raid0, raid1,
 158      raid5, linear, multipath, faulty.
 159      If no raid level has been set yet (array is still being
 160      assembled), the value will reflect whatever has been written
 161      to it, which may be a name like the above, or may be a number
 162      such as '0', '5', etc.
 163
 164   raid_disks
 165      a text file with a simple number indicating the number of devices
 166      in a fully functional array.  If this is not yet known, the file
 167      will be empty.  If an array is being resized (not currently
 168      possible) this will contain the larger of the old and new sizes.
 169      Some raid level (RAID1) allow this value to be set while the
 170      array is active.  This will reconfigure the array.   Otherwise
 171      it can only be set while assembling an array.
 172
 173   chunk_size
 174      This is the size if bytes for 'chunks' and is only relevant to
 175      raid levels that involve striping (1,4,5,6,10). The address space
 176      of the array is conceptually divided into chunks and consecutive
 177      chunks are striped onto neighbouring devices.
 178      The size should be at least PAGE_SIZE (4k) and should be a power
 179      of 2.  This can only be set while assembling an array
 180
 181   layout
 182      The "layout" for the array for the particular level.  This is
 183      simply a number that is interpretted differently by different
 184      levels.  It can be written while assembling an array.
 185
 186   reshape_position
 187      This is either "none" or a sector number within the devices of
 188      the array where "reshape" is up to.  If this is set, the three
 189      attributes mentioned above (raid_disks, chunk_size, layout) can
 190      potentially have 2 values, an old and a new value.  If these
 191      values differ, reading the attribute returns
 192         new (old)
 193      and writing will effect the 'new' value, leaving the 'old'
 194      unchanged.
 195
 196   component_size
 197      For arrays with data redundancy (i.e. not raid0, linear, faulty,
 198      multipath), all components must be the same size - or at least
 199      there must a size that they all provide space for.  This is a key
 200      part or the geometry of the array.  It is measured in sectors
 201      and can be read from here.  Writing to this value may resize
 202      the array if the personality supports it (raid1, raid5, raid6),
 203      and if the component drives are large enough.
 204
 205   metadata_version
 206      This indicates the format that is being used to record metadata
 207      about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
 208      1.2 (newer format in varying locations) or "none" indicating that
 209      the kernel isn't managing metadata at all.
 210
 211   resync_start
 212      The point at which resync should start.  If no resync is needed,
 213      this will be a very large number.  At array creation it will
 214      default to 0, though starting the array as 'clean' will
 215      set it much larger.
 216
 217    new_dev
 218      This file can be written but not read.  The value written should
 219      be a block device number as major:minor.  e.g. 8:0
 220      This will cause that device to be attached to the array, if it is
 221      available.  It will then appear at md/dev-XXX (depending on the
 222      name of the device) and further configuration is then possible.
 223
 224    safe_mode_delay
 225      When an md array has seen no write requests for a certain period
 226      of time, it will be marked as 'clean'.  When another write
 227      request arrives, the array is marked as 'dirty' before the write
 228      commences.  This is known as 'safe_mode'.
 229      The 'certain period' is controlled by this file which stores the
 230      period as a number of seconds.  The default is 200msec (0.200).
 231      Writing a value of 0 disables safemode.
 232
 233    array_state
 234      This file contains a single word which describes the current
 235      state of the array.  In many cases, the state can be set by
 236      writing the word for the desired state, however some states
 237      cannot be explicitly set, and some transitions are not allowed.
 238
 239      Select/poll works on this file.  All changes except between
 240         active_idle and active (which can be frequent and are not
 241         very interesting) are notified.  active->active_idle is
 242         reported if the metadata is externally managed.
 243
 244      clear
 245          No devices, no size, no level
 246          Writing is equivalent to STOP_ARRAY ioctl
 247      inactive
 248          May have some settings, but array is not active
 249             all IO results in error
 250          When written, doesn't tear down array, but just stops it
 251      suspended (not supported yet)
 252          All IO requests will block. The array can be reconfigured.
 253          Writing this, if accepted, will block until array is quiessent
 254      readonly
 255          no resync can happen.  no superblocks get written.
 256          write requests fail
 257      read-auto
 258          like readonly, but behaves like 'clean' on a write request.
 259
 260      clean - no pending writes, but otherwise active.
 261          When written to inactive array, starts without resync
 262          If a write request arrives then
 263            if metadata is known, mark 'dirty' and switch to 'active'.
 264            if not known, block and switch to write-pending
 265          If written to an active array that has pending writes, then fails.
 266      active
 267          fully active: IO and resync can be happening.
 268          When written to inactive array, starts with resync
 269
 270      write-pending
 271          clean, but writes are blocked waiting for 'active' to be written.
 272
 273      active-idle
 274          like active, but no writes have been seen for a while (safe_mode_delay).
 275
 276
 277 As component devices are added to an md array, they appear in the 'md'
 278 directory as new directories named
 279       dev-XXX
 280 where XXX is a name that the kernel knows for the device, e.g. hdb1.
 281 Each directory contains:
 282
 283       block
 284         a symlink to the block device in /sys/block, e.g.
 285              /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
 286
 287       super
 288         A file containing an image of the superblock read from, or
 289         written to, that device.
 290
 291       state
 292         A file recording the current state of the device in the array
 293         which can be a comma separated list of
 294               faulty   - device has been kicked from active use due to
 295                          a detected fault
 296               in_sync  - device is a fully in-sync member of the array
 297               writemostly - device will only be subject to read
 298                          requests if there are no other options.
 299                          This applies only to raid1 arrays.
 300               spare    - device is working, but not a full member.
 301                          This includes spares that are in the process
 302                          of being recovered to
 303         This list may grow in future.
 304         This can be written to.
 305         Writing "faulty"  simulates a failure on the device.
 306         Writing "remove" removes the device from the array.
 307         Writing "writemostly" sets the writemostly flag.
 308         Writing "-writemostly" clears the writemostly flag.
 309
 310       errors
 311         An approximate count of read errors that have been detected on
 312         this device but have not caused the device to be evicted from
 313         the array (either because they were corrected or because they
 314         happened while the array was read-only).  When using version-1
 315         metadata, this value persists across restarts of the array.
 316
 317         This value can be written while assembling an array thus
 318         providing an ongoing count for arrays with metadata managed by
 319         userspace.
 320
 321       slot
 322         This gives the role that the device has in the array.  It will
 323         either be 'none' if the device is not active in the array
 324         (i.e. is a spare or has failed) or an integer less than the
 325         'raid_disks' number for the array indicating which position
 326         it currently fills.  This can only be set while assembling an
 327         array.  A device for which this is set is assumed to be working.
 328
 329       offset
 330         This gives the location in the device (in sectors from the
 331         start) where data from the array will be stored.  Any part of
 332         the device before this offset us not touched, unless it is
 333         used for storing metadata (Formats 1.1 and 1.2).
 334
 335       size
 336         The amount of the device, after the offset, that can be used
 337         for storage of data.  This will normally be the same as the
 338         component_size.  This can be written while assembling an
 339         array.  If a value less than the current component_size is
 340         written, component_size will be reduced to this value.
 341
 342
 343 An active md device will also contain and entry for each active device
 344 in the array.  These are named
 345
 346     rdNN
 347
 348 where 'NN' is the position in the array, starting from 0.
 349 So for a 3 drive array there will be rd0, rd1, rd2.
 350 These are symbolic links to the appropriate 'dev-XXX' entry.
 351 Thus, for example,
 352        cat /sys/block/md*/md/rd*/state
 353 will show 'in_sync' on every line.
 354
 355
 356
 357 Active md devices for levels that support data redundancy (1,4,5,6)
 358 also have
 359
 360    sync_action
 361      a text file that can be used to monitor and control the rebuild
 362      process.  It contains one word which can be one of:
 363        resync        - redundancy is being recalculated after unclean
 364                        shutdown or creation
 365        recover       - a hot spare is being built to replace a
 366                        failed/missing device
 367        idle          - nothing is happening
 368        check         - A full check of redundancy was requested and is
 369                        happening.  This reads all block and checks
 370                        them. A repair may also happen for some raid
 371                        levels.
 372        repair        - A full check and repair is happening.  This is
 373                        similar to 'resync', but was requested by the
 374                        user, and the write-intent bitmap is NOT used to
 375                        optimise the process.
 376
 377       This file is writable, and each of the strings that could be
 378       read are meaningful for writing.
 379
 380        'idle' will stop an active resync/recovery etc.  There is no
 381            guarantee that another resync/recovery may not be automatically
 382            started again, though some event will be needed to trigger
 383            this.
 384         'resync' or 'recovery' can be used to restart the
 385            corresponding operation if it was stopped with 'idle'.
 386         'check' and 'repair' will start the appropriate process
 387            providing the current state is 'idle'.
 388
 389       This file responds to select/poll.  Any important change in the value
 390       triggers a poll event.  Sometimes the value will briefly be
 391       "recover" if a recovery seems to be needed, but cannot be
 392       achieved. In that case, the transition to "recover" isn't
 393       notified, but the transition away is.
 394
 395    mismatch_count
 396       When performing 'check' and 'repair', and possibly when
 397       performing 'resync', md will count the number of errors that are
 398       found.  The count in 'mismatch_cnt' is the number of sectors
 399       that were re-written, or (for 'check') would have been
 400       re-written.  As most raid levels work in units of pages rather
 401       than sectors, this my be larger than the number of actual errors
 402       by a factor of the number of sectors in a page.
 403
 404    bitmap_set_bits
 405       If the array has a write-intent bitmap, then writing to this
 406       attribute can set bits in the bitmap, indicating that a resync
 407       would need to check the corresponding blocks. Either individual
 408       numbers or start-end pairs can be written.  Multiple numbers
 409       can be separated by a space.
 410       Note that the numbers are 'bit' numbers, not 'block' numbers.
 411       They should be scaled by the bitmap_chunksize.
 412
 413    sync_speed_min
 414    sync_speed_max
 415      This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
 416      however they only apply to the particular array.
 417      If no value has been written to these, of if the word 'system'
 418      is written, then the system-wide value is used.  If a value,
 419      in kibibytes-per-second is written, then it is used.
 420      When the files are read, they show the currently active value
 421      followed by "(local)" or "(system)" depending on whether it is
 422      a locally set or system-wide value.
 423
 424    sync_completed
 425      This shows the number of sectors that have been completed of
 426      whatever the current sync_action is, followed by the number of
 427      sectors in total that could need to be processed.  The two
 428      numbers are separated by a '/'  thus effectively showing one
 429      value, a fraction of the process that is complete.
 430      A 'select' on this attribute will return when resync completes,
 431      when it reaches the current sync_max (below) and possibly at
 432      other times.
 433
 434    sync_max
 435      This is a number of sectors at which point a resync/recovery
 436      process will pause.  When a resync is active, the value can
 437      only ever be increased, never decreased.  The value of 'max'
 438      effectively disables the limit.
 439
 440
 441    sync_speed
 442      This shows the current actual speed, in K/sec, of the current
 443      sync_action.  It is averaged over the last 30 seconds.
 444
 445    suspend_lo
 446    suspend_hi
 447      The two values, given as numbers of sectors, indicate a range
 448      within the array where IO will be blocked.  This is currently
 449      only supported for raid4/5/6.
 450
 451
 452 Each active md device may also have attributes specific to the
 453 personality module that manages it.
 454 These are specific to the implementation of the module and could
 455 change substantially if the implementation changes.
 456
 457 These currently include
 458
 459   stripe_cache_size  (currently raid5 only)
 460       number of entries in the stripe cache.  This is writable, but
 461       there are upper and lower limits (32768, 16).  Default is 128.
 462   strip_cache_active (currently raid5 only)
 463       number of active entries in the stripe cache
 464   preread_bypass_threshold (currently raid5 only)
 465       number of times a stripe requiring preread will be bypassed by
 466       a stripe that does not require preread.  For fairness defaults
 467       to 1.  Setting this to 0 disables bypass accounting and
 468       requires preread stripes to wait until all full-width stripe-
 469       writes are complete.  Valid values are 0 to stripe_cache_size.