Documentation/md.txt

   1 Tools that manage md devices can be found at
   2    http://www.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 be passed 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 have 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 its 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 this will contain
 168      the new number of devices.
 169      Some raid levels allow this value to be set while the array is
 170      active.  This will reconfigure the array.   Otherwise it can only
 171      be set while assembling an array.
 172      A change to this attribute will not be permitted if it would
 173      reduce the size of the array.  To reduce the number of drives
 174      in an e.g. raid5, the array size must first be reduced by
 175      setting the 'array_size' attribute.
 176
 177   chunk_size
 178      This is the size in bytes for 'chunks' and is only relevant to
 179      raid levels that involve striping (0,4,5,6,10). The address space
 180      of the array is conceptually divided into chunks and consecutive
 181      chunks are striped onto neighbouring devices.
 182      The size should be at least PAGE_SIZE (4k) and should be a power
 183      of 2.  This can only be set while assembling an array
 184
 185   layout
 186      The "layout" for the array for the particular level.  This is
 187      simply a number that is interpretted differently by different
 188      levels.  It can be written while assembling an array.
 189
 190   array_size
 191      This can be used to artificially constrain the available space in
 192      the array to be less than is actually available on the combined
 193      devices.  Writing a number (in Kilobytes) which is less than
 194      the available size will set the size.  Any reconfiguration of the
 195      array (e.g. adding devices) will not cause the size to change.
 196      Writing the word 'default' will cause the effective size of the
 197      array to be whatever size is actually available based on
 198      'level', 'chunk_size' and 'component_size'.
 199
 200      This can be used to reduce the size of the array before reducing
 201      the number of devices in a raid4/5/6, or to support external
 202      metadata formats which mandate such clipping.
 203
 204   reshape_position
 205      This is either "none" or a sector number within the devices of
 206      the array where "reshape" is up to.  If this is set, the three
 207      attributes mentioned above (raid_disks, chunk_size, layout) can
 208      potentially have 2 values, an old and a new value.  If these
 209      values differ, reading the attribute returns
 210         new (old)
 211      and writing will effect the 'new' value, leaving the 'old'
 212      unchanged.
 213
 214   component_size
 215      For arrays with data redundancy (i.e. not raid0, linear, faulty,
 216      multipath), all components must be the same size - or at least
 217      there must a size that they all provide space for.  This is a key
 218      part or the geometry of the array.  It is measured in sectors
 219      and can be read from here.  Writing to this value may resize
 220      the array if the personality supports it (raid1, raid5, raid6),
 221      and if the component drives are large enough.
 222
 223   metadata_version
 224      This indicates the format that is being used to record metadata
 225      about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
 226      1.2 (newer format in varying locations) or "none" indicating that
 227      the kernel isn't managing metadata at all.
 228      Alternately it can be "external:" followed by a string which
 229      is set by user-space.  This indicates that metadata is managed
 230      by a user-space program.  Any device failure or other event that
 231      requires a metadata update will cause array activity to be
 232      suspended until the event is acknowledged.
 233
 234   resync_start
 235      The point at which resync should start.  If no resync is needed,
 236      this will be a very large number (or 'none' since 2.6.30-rc1).  At
 237      array creation it will default to 0, though starting the array as
 238      'clean' will set it much larger.
 239
 240    new_dev
 241      This file can be written but not read.  The value written should
 242      be a block device number as major:minor.  e.g. 8:0
 243      This will cause that device to be attached to the array, if it is
 244      available.  It will then appear at md/dev-XXX (depending on the
 245      name of the device) and further configuration is then possible.
 246
 247    safe_mode_delay
 248      When an md array has seen no write requests for a certain period
 249      of time, it will be marked as 'clean'.  When another write
 250      request arrives, the array is marked as 'dirty' before the write
 251      commences.  This is known as 'safe_mode'.
 252      The 'certain period' is controlled by this file which stores the
 253      period as a number of seconds.  The default is 200msec (0.200).
 254      Writing a value of 0 disables safemode.
 255
 256    array_state
 257      This file contains a single word which describes the current
 258      state of the array.  In many cases, the state can be set by
 259      writing the word for the desired state, however some states
 260      cannot be explicitly set, and some transitions are not allowed.
 261
 262      Select/poll works on this file.  All changes except between
 263         active_idle and active (which can be frequent and are not
 264         very interesting) are notified.  active->active_idle is
 265         reported if the metadata is externally managed.
 266
 267      clear
 268          No devices, no size, no level
 269          Writing is equivalent to STOP_ARRAY ioctl
 270      inactive
 271          May have some settings, but array is not active
 272             all IO results in error
 273          When written, doesn't tear down array, but just stops it
 274      suspended (not supported yet)
 275          All IO requests will block. The array can be reconfigured.
 276          Writing this, if accepted, will block until array is quiessent
 277      readonly
 278          no resync can happen.  no superblocks get written.
 279          write requests fail
 280      read-auto
 281          like readonly, but behaves like 'clean' on a write request.
 282
 283      clean - no pending writes, but otherwise active.
 284          When written to inactive array, starts without resync
 285          If a write request arrives then
 286            if metadata is known, mark 'dirty' and switch to 'active'.
 287            if not known, block and switch to write-pending
 288          If written to an active array that has pending writes, then fails.
 289      active
 290          fully active: IO and resync can be happening.
 291          When written to inactive array, starts with resync
 292
 293      write-pending
 294          clean, but writes are blocked waiting for 'active' to be written.
 295
 296      active-idle
 297          like active, but no writes have been seen for a while (safe_mode_delay).
 298
 299   bitmap/location
 300      This indicates where the write-intent bitmap for the array is
 301      stored.
 302      It can be one of "none", "file" or "[+-]N".
 303      "file" may later be extended to "file:/file/name"
 304      "[+-]N" means that many sectors from the start of the metadata.
 305        This is replicated on all devices.  For arrays with externally
 306        managed metadata, the offset is from the beginning of the
 307        device.
 308   bitmap/chunksize
 309      The size, in bytes, of the chunk which will be represented by a
 310      single bit.  For RAID456, it is a portion of an individual
 311      device. For RAID10, it is a portion of the array.  For RAID1, it
 312      is both (they come to the same thing).
 313   bitmap/time_base
 314      The time, in seconds, between looking for bits in the bitmap to
 315      be cleared. In the current implementation, a bit will be cleared
 316      between 2 and 3 times "time_base" after all the covered blocks
 317      are known to be in-sync.
 318   bitmap/backlog
 319      When write-mostly devices are active in a RAID1, write requests
 320      to those devices proceed in the background - the filesystem (or
 321      other user of the device) does not have to wait for them.
 322      'backlog' sets a limit on the number of concurrent background
 323      writes.  If there are more than this, new writes will by
 324      synchronous.
 325   bitmap/metadata
 326      This can be either 'internal' or 'external'.
 327      'internal' is the default and means the metadata for the bitmap
 328      is stored in the first 256 bytes of the allocated space and is
 329      managed by the md module.
 330      'external' means that bitmap metadata is managed externally to
 331      the kernel (i.e. by some userspace program)
 332   bitmap/can_clear
 333      This is either 'true' or 'false'.  If 'true', then bits in the
 334      bitmap will be cleared when the corresponding blocks are thought
 335      to be in-sync.  If 'false', bits will never be cleared.
 336      This is automatically set to 'false' if a write happens on a
 337      degraded array, or if the array becomes degraded during a write.
 338      When metadata is managed externally, it should be set to true
 339      once the array becomes non-degraded, and this fact has been
 340      recorded in the metadata.
 341
 342
 343
 344
 345 As component devices are added to an md array, they appear in the 'md'
 346 directory as new directories named
 347       dev-XXX
 348 where XXX is a name that the kernel knows for the device, e.g. hdb1.
 349 Each directory contains:
 350
 351       block
 352         a symlink to the block device in /sys/block, e.g.
 353              /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
 354
 355       super
 356         A file containing an image of the superblock read from, or
 357         written to, that device.
 358
 359       state
 360         A file recording the current state of the device in the array
 361         which can be a comma separated list of
 362               faulty   - device has been kicked from active use due to
 363                          a detected fault, or it has unacknowledged bad
 364                          blocks
 365               in_sync  - device is a fully in-sync member of the array
 366               writemostly - device will only be subject to read
 367                          requests if there are no other options.
 368                          This applies only to raid1 arrays.
 369               blocked  - device has failed, and the failure hasn't been
 370                          acknowledged yet by the metadata handler.
 371                          Writes that would write to this device if
 372                          it were not faulty are blocked.
 373               spare    - device is working, but not a full member.
 374                          This includes spares that are in the process
 375                          of being recovered to
 376               write_error - device has ever seen a write error.
 377               want_replacement - device is (mostly) working but probably
 378                          should be replaced, either due to errors or
 379                          due to user request.
 380               replacement - device is a replacement for another active
 381                          device with same raid_disk.
 382
 383
 384         This list may grow in future.
 385         This can be written to.
 386         Writing "faulty"  simulates a failure on the device.
 387         Writing "remove" removes the device from the array.
 388         Writing "writemostly" sets the writemostly flag.
 389         Writing "-writemostly" clears the writemostly flag.
 390         Writing "blocked" sets the "blocked" flag.
 391         Writing "-blocked" clears the "blocked" flags and allows writes
 392                 to complete and possibly simulates an error.
 393         Writing "in_sync" sets the in_sync flag.
 394         Writing "write_error" sets writeerrorseen flag.
 395         Writing "-write_error" clears writeerrorseen flag.
 396         Writing "want_replacement" is allowed at any time except to a
 397                 replacement device or a spare.  It sets the flag.
 398         Writing "-want_replacement" is allowed at any time.  It clears
 399                 the flag.
 400         Writing "replacement" or "-replacement" is only allowed before
 401                 starting the array.  It sets or clears the flag.
 402
 403
 404         This file responds to select/poll. Any change to 'faulty'
 405         or 'blocked' causes an event.
 406
 407       errors
 408         An approximate count of read errors that have been detected on
 409         this device but have not caused the device to be evicted from
 410         the array (either because they were corrected or because they
 411         happened while the array was read-only).  When using version-1
 412         metadata, this value persists across restarts of the array.
 413
 414         This value can be written while assembling an array thus
 415         providing an ongoing count for arrays with metadata managed by
 416         userspace.
 417
 418       slot
 419         This gives the role that the device has in the array.  It will
 420         either be 'none' if the device is not active in the array
 421         (i.e. is a spare or has failed) or an integer less than the
 422         'raid_disks' number for the array indicating which position
 423         it currently fills.  This can only be set while assembling an
 424         array.  A device for which this is set is assumed to be working.
 425
 426       offset
 427         This gives the location in the device (in sectors from the
 428         start) where data from the array will be stored.  Any part of
 429         the device before this offset is not touched, unless it is
 430         used for storing metadata (Formats 1.1 and 1.2).
 431
 432       size
 433         The amount of the device, after the offset, that can be used
 434         for storage of data.  This will normally be the same as the
 435         component_size.  This can be written while assembling an
 436         array.  If a value less than the current component_size is
 437         written, it will be rejected.
 438
 439       recovery_start
 440         When the device is not 'in_sync', this records the number of
 441         sectors from the start of the device which are known to be
 442         correct.  This is normally zero, but during a recovery
 443         operation it will steadily increase, and if the recovery is
 444         interrupted, restoring this value can cause recovery to
 445         avoid repeating the earlier blocks.  With v1.x metadata, this
 446         value is saved and restored automatically.
 447
 448         This can be set whenever the device is not an active member of
 449         the array, either before the array is activated, or before
 450         the 'slot' is set.
 451
 452         Setting this to 'none' is equivalent to setting 'in_sync'.
 453         Setting to any other value also clears the 'in_sync' flag.
 454
 455       bad_blocks
 456         This gives the list of all known bad blocks in the form of
 457         start address and length (in sectors respectively). If output
 458         is too big to fit in a page, it will be truncated. Writing
 459         "sector length" to this file adds new acknowledged (i.e.
 460         recorded to disk safely) bad blocks.
 461
 462       unacknowledged_bad_blocks
 463         This gives the list of known-but-not-yet-saved-to-disk bad
 464         blocks in the same form of 'bad_blocks'. If output is too big
 465         to fit in a page, it will be truncated. Writing to this file
 466         adds bad blocks without acknowledging them. This is largely
 467         for testing.
 468
 469
 470
 471 An active md device will also contain an entry for each active device
 472 in the array.  These are named
 473
 474     rdNN
 475
 476 where 'NN' is the position in the array, starting from 0.
 477 So for a 3 drive array there will be rd0, rd1, rd2.
 478 These are symbolic links to the appropriate 'dev-XXX' entry.
 479 Thus, for example,
 480        cat /sys/block/md*/md/rd*/state
 481 will show 'in_sync' on every line.
 482
 483
 484
 485 Active md devices for levels that support data redundancy (1,4,5,6,10)
 486 also have
 487
 488    sync_action
 489      a text file that can be used to monitor and control the rebuild
 490      process.  It contains one word which can be one of:
 491        resync        - redundancy is being recalculated after unclean
 492                        shutdown or creation
 493        recover       - a hot spare is being built to replace a
 494                        failed/missing device
 495        idle          - nothing is happening
 496        check         - A full check of redundancy was requested and is
 497                        happening.  This reads all blocks and checks
 498                        them. A repair may also happen for some raid
 499                        levels.
 500        repair        - A full check and repair is happening.  This is
 501                        similar to 'resync', but was requested by the
 502                        user, and the write-intent bitmap is NOT used to
 503                        optimise the process.
 504
 505       This file is writable, and each of the strings that could be
 506       read are meaningful for writing.
 507
 508        'idle' will stop an active resync/recovery etc.  There is no
 509            guarantee that another resync/recovery may not be automatically
 510            started again, though some event will be needed to trigger
 511            this.
 512         'resync' or 'recovery' can be used to restart the
 513            corresponding operation if it was stopped with 'idle'.
 514         'check' and 'repair' will start the appropriate process
 515            providing the current state is 'idle'.
 516
 517       This file responds to select/poll.  Any important change in the value
 518       triggers a poll event.  Sometimes the value will briefly be
 519       "recover" if a recovery seems to be needed, but cannot be
 520       achieved. In that case, the transition to "recover" isn't
 521       notified, but the transition away is.
 522
 523    degraded
 524       This contains a count of the number of devices by which the
 525       arrays is degraded.  So an optimal array will show '0'.  A
 526       single failed/missing drive will show '1', etc.
 527       This file responds to select/poll, any increase or decrease
 528       in the count of missing devices will trigger an event.
 529
 530    mismatch_count
 531       When performing 'check' and 'repair', and possibly when
 532       performing 'resync', md will count the number of errors that are
 533       found.  The count in 'mismatch_cnt' is the number of sectors
 534       that were re-written, or (for 'check') would have been
 535       re-written.  As most raid levels work in units of pages rather
 536       than sectors, this may be larger than the number of actual errors
 537       by a factor of the number of sectors in a page.
 538
 539    bitmap_set_bits
 540       If the array has a write-intent bitmap, then writing to this
 541       attribute can set bits in the bitmap, indicating that a resync
 542       would need to check the corresponding blocks. Either individual
 543       numbers or start-end pairs can be written.  Multiple numbers
 544       can be separated by a space.
 545       Note that the numbers are 'bit' numbers, not 'block' numbers.
 546       They should be scaled by the bitmap_chunksize.
 547
 548    sync_speed_min
 549    sync_speed_max
 550      This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
 551      however they only apply to the particular array.
 552      If no value has been written to these, of if the word 'system'
 553      is written, then the system-wide value is used.  If a value,
 554      in kibibytes-per-second is written, then it is used.
 555      When the files are read, they show the currently active value
 556      followed by "(local)" or "(system)" depending on whether it is
 557      a locally set or system-wide value.
 558
 559    sync_completed
 560      This shows the number of sectors that have been completed of
 561      whatever the current sync_action is, followed by the number of
 562      sectors in total that could need to be processed.  The two
 563      numbers are separated by a '/'  thus effectively showing one
 564      value, a fraction of the process that is complete.
 565      A 'select' on this attribute will return when resync completes,
 566      when it reaches the current sync_max (below) and possibly at
 567      other times.
 568
 569    sync_speed
 570      This shows the current actual speed, in K/sec, of the current
 571      sync_action.  It is averaged over the last 30 seconds.
 572
 573    suspend_lo
 574    suspend_hi
 575      The two values, given as numbers of sectors, indicate a range
 576      within the array where IO will be blocked.  This is currently
 577      only supported for raid4/5/6.
 578
 579    sync_min
 580    sync_max
 581      The two values, given as numbers of sectors, indicate a range
 582      within the array where 'check'/'repair' will operate. Must be
 583      a multiple of chunk_size. When it reaches "sync_max" it will
 584      pause, rather than complete.
 585      You can use 'select' or 'poll' on "sync_completed" to wait for
 586      that number to reach sync_max.  Then you can either increase
 587      "sync_max", or can write 'idle' to "sync_action".
 588
 589      The value of 'max' for "sync_max" effectively disables the limit.
 590      When a resync is active, the value can only ever be increased,
 591      never decreased.
 592      The value of '0' is the minimum for "sync_min".
 593
 594
 595
 596 Each active md device may also have attributes specific to the
 597 personality module that manages it.
 598 These are specific to the implementation of the module and could
 599 change substantially if the implementation changes.
 600
 601 These currently include
 602
 603   stripe_cache_size  (currently raid5 only)
 604       number of entries in the stripe cache.  This is writable, but
 605       there are upper and lower limits (32768, 16).  Default is 128.
 606   strip_cache_active (currently raid5 only)
 607       number of active entries in the stripe cache
 608   preread_bypass_threshold (currently raid5 only)
 609       number of times a stripe requiring preread will be bypassed by
 610       a stripe that does not require preread.  For fairness defaults
 611       to 1.  Setting this to 0 disables bypass accounting and
 612       requires preread stripes to wait until all full-width stripe-
 613       writes are complete.  Valid values are 0 to stripe_cache_size.