1Tools that manage md devices can be found at
   2   http://www.<country>
   5Boot time assembly of RAID arrays
   8You can boot with your md device with the following kernel command
  11for old raid arrays without persistent superblocks:
  12  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
  14for raid arrays with persistent superblocks
  15  md=<md device no.>,dev0,dev1,...,devn
  16or, to assemble a partitionable array:
  17  md=d<md device no.>,dev0,dev1,...,devn
  19md device no. = the number of the md device ... 
  20              0 means md0, 
  21              1 md1,
  22              2 md2,
  23              3 md3,
  24              4 md4
  26raid level = -1 linear mode
  27              0 striped mode
  28              other modes are only supported with persistent super blocks
  30chunk size factor = (raid-0 and raid-1 only)
  31              Set  the chunk size as 4k << n.
  33fault level = totally ignored
  35dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
  37A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this:
  39e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
  42Boot time autodetection of RAID arrays
  45When md is compiled into the kernel (not as module), partitions of
  46type 0xfd are scanned and automatically assembled into RAID arrays.
  47This autodetection may be suppressed with the kernel parameter
  48"raid=noautodetect".  As of kernel 2.6.9, only drives with a type 0
  49superblock can be autodetected and run at boot time.
  51The kernel parameter "raid=partitionable" (or "raid=part") means
  52that all auto-detected arrays are assembled as partitionable.
  54Boot time assembly of degraded/dirty arrays
  57If a raid5 or raid6 array is both dirty and degraded, it could have
  58undetectable data corruption.  This is because the fact that it is
  59'dirty' means that the parity cannot be trusted, and the fact that it
  60is degraded means that some datablocks are missing and cannot reliably
  61be reconstructed (due to no parity).
  63For this reason, md will normally refuse to start such an array.  This
  64requires the sysadmin to take action to explicitly start the array
  65despite possible corruption.  This is normally done with
  66   mdadm --assemble --force ....
  68This option is not really available if the array has the root
  69filesystem on it.  In order to support this booting from such an
  70array, md supports a module parameter "start_dirty_degraded" which,
  71when set to 1, bypassed the checks and will allows dirty degraded
  72arrays to be started.
  74So, to boot with a root filesystem of a dirty degraded raid[56], use
  76   md-mod.start_dirty_degraded=1
  79Superblock formats
  82The md driver can support a variety of different superblock formats.
  83Currently, it supports superblock formats "0.90.0" and the "md-1" format
  84introduced in the 2.5 development series.
  86The kernel will autodetect which format superblock is being used.
  88Superblock format '0' is treated differently to others for legacy
  89reasons - it is the original superblock format.
  92General Rules - apply for all superblock formats
  95An array is 'created' by writing appropriate superblocks to all
  98It is 'assembled' by associating each of these devices with an
  99particular md virtual device.  Once it is completely assembled, it can
 100be accessed.
 102An array should be created by a user-space tool.  This will write
 103superblocks to all devices.  It will usually mark the array as
 104'unclean', or with some devices missing so that the kernel md driver
 105can create appropriate redundancy (copying in raid1, parity
 106calculation in raid4/5).
 108When an array is assembled, it is first initialized with the
 109SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
 110version number.  The major version number selects which superblock
 111format is to be used.  The minor number might be used to tune handling
 112of the format, such as suggesting where on each device to look for the
 115Then each device is added using the ADD_NEW_DISK ioctl.  This
 116provides, in particular, a major and minor number identifying the
 117device to add.
 119The array is started with the RUN_ARRAY ioctl.
 121Once started, new devices can be added.  They should have an
 122appropriate superblock written to them, and then passed be in with
 125Devices that have failed or are not yet active can be detached from an
 126array using HOT_REMOVE_DISK.
 129Specific Rules that apply to format-0 super block arrays, and
 130       arrays with no superblock (non-persistent).
 133An array can be 'created' by describing the array (level, chunksize
 134etc) in a SET_ARRAY_INFO ioctl.  This must has major_version==0 and
 135raid_disks != 0.
 137Then uninitialized devices can be added with ADD_NEW_DISK.  The
 138structure passed to ADD_NEW_DISK must specify the state of the device
 139and it's role in the array.
 141Once started with RUN_ARRAY, uninitialized spares can be added with
 146MD devices in sysfs
 148md devices appear in sysfs (/sys) as regular block devices,
 150   /sys/block/md0
 152Each 'md' device will contain a subdirectory called 'md' which
 153contains further md-specific information about the device.
 155All 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.
 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.
 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
 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.
 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'.
 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.
 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.
 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.
 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.
 234  resync_start
 235     The point at which resync should start.  If no resync is needed,
 236     this will be a very large number.  At array creation it will
 237     default to 0, though starting the array as 'clean' will
 238     set it much larger.
 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.
 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.
 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.
 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.
 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.
 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
 293     write-pending
 294         clean, but writes are blocked waiting for 'active' to be written.
 296     active-idle
 297         like active, but no writes have been seen for a while (safe_mode_delay).
 300As component devices are added to an md array, they appear in the 'md'
 301directory as new directories named
 302      dev-XXX
 303where XXX is a name that the kernel knows for the device, e.g. hdb1.
 304Each directory contains:
 306      block
 307        a symlink to the block device in /sys/block, e.g.
 308             /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
 310      super
 311        A file containing an image of the superblock read from, or
 312        written to, that device.
 314      state
 315        A file recording the current state of the device in the array
 316        which can be a comma separated list of
 317              faulty   - device has been kicked from active use due to
 318                         a detected fault
 319              in_sync  - device is a fully in-sync member of the array
 320              writemostly - device will only be subject to read
 321                         requests if there are no other options.
 322                         This applies only to raid1 arrays.
 323              blocked  - device has failed, metadata is "external",
 324                         and the failure hasn't been acknowledged yet.
 325                         Writes that would write to this device if
 326                         it were not faulty are blocked.
 327              spare    - device is working, but not a full member.
 328                         This includes spares that are in the process
 329                         of being recovered to
 330        This list may grow in future.
 331        This can be written to.
 332        Writing "faulty"  simulates a failure on the device.
 333        Writing "remove" removes the device from the array.
 334        Writing "writemostly" sets the writemostly flag.
 335        Writing "-writemostly" clears the writemostly flag.
 336        Writing "blocked" sets the "blocked" flag.
 337        Writing "-blocked" clear the "blocked" flag and allows writes
 338                to complete.
 340        This file responds to select/poll. Any change to 'faulty'
 341        or 'blocked' causes an event.
 343      errors
 344        An approximate count of read errors that have been detected on
 345        this device but have not caused the device to be evicted from
 346        the array (either because they were corrected or because they
 347        happened while the array was read-only).  When using version-1
 348        metadata, this value persists across restarts of the array.
 350        This value can be written while assembling an array thus
 351        providing an ongoing count for arrays with metadata managed by
 352        userspace.
 354      slot
 355        This gives the role that the device has in the array.  It will
 356        either be 'none' if the device is not active in the array
 357        (i.e. is a spare or has failed) or an integer less than the
 358        'raid_disks' number for the array indicating which position
 359        it currently fills.  This can only be set while assembling an
 360        array.  A device for which this is set is assumed to be working.
 362      offset
 363        This gives the location in the device (in sectors from the
 364        start) where data from the array will be stored.  Any part of
 365        the device before this offset us not touched, unless it is
 366        used for storing metadata (Formats 1.1 and 1.2).
 368      size
 369        The amount of the device, after the offset, that can be used
 370        for storage of data.  This will normally be the same as the
 371        component_size.  This can be written while assembling an
 372        array.  If a value less than the current component_size is
 373        written, it will be rejected.
 376An active md device will also contain and entry for each active device
 377in the array.  These are named
 379    rdNN
 381where 'NN' is the position in the array, starting from 0.
 382So for a 3 drive array there will be rd0, rd1, rd2.
 383These are symbolic links to the appropriate 'dev-XXX' entry.
 384Thus, for example,
 385       cat /sys/block/md*/md/rd*/state
 386will show 'in_sync' on every line.
 390Active md devices for levels that support data redundancy (1,4,5,6)
 391also have
 393   sync_action
 394     a text file that can be used to monitor and control the rebuild
 395     process.  It contains one word which can be one of:
 396       resync        - redundancy is being recalculated after unclean
 397                       shutdown or creation
 398       recover       - a hot spare is being built to replace a
 399                       failed/missing device
 400       idle          - nothing is happening
 401       check         - A full check of redundancy was requested and is
 402                       happening.  This reads all block and checks
 403                       them. A repair may also happen for some raid
 404                       levels.
 405       repair        - A full check and repair is happening.  This is
 406                       similar to 'resync', but was requested by the
 407                       user, and the write-intent bitmap is NOT used to
 408                       optimise the process.
 410      This file is writable, and each of the strings that could be
 411      read are meaningful for writing.
 413       'idle' will stop an active resync/recovery etc.  There is no
 414           guarantee that another resync/recovery may not be automatically
 415           started again, though some event will be needed to trigger
 416           this.
 417        'resync' or 'recovery' can be used to restart the
 418           corresponding operation if it was stopped with 'idle'.
 419        'check' and 'repair' will start the appropriate process
 420           providing the current state is 'idle'.
 422      This file responds to select/poll.  Any important change in the value
 423      triggers a poll event.  Sometimes the value will briefly be
 424      "recover" if a recovery seems to be needed, but cannot be
 425      achieved. In that case, the transition to "recover" isn't
 426      notified, but the transition away is.
 428   degraded
 429      This contains a count of the number of devices by which the
 430      arrays is degraded.  So an optimal array with show '0'.  A
 431      single failed/missing drive will show '1', etc.
 432      This file responds to select/poll, any increase or decrease
 433      in the count of missing devices will trigger an event.
 435   mismatch_count
 436      When performing 'check' and 'repair', and possibly when
 437      performing 'resync', md will count the number of errors that are
 438      found.  The count in 'mismatch_cnt' is the number of sectors
 439      that were re-written, or (for 'check') would have been
 440      re-written.  As most raid levels work in units of pages rather
 441      than sectors, this my be larger than the number of actual errors
 442      by a factor of the number of sectors in a page.
 444   bitmap_set_bits
 445      If the array has a write-intent bitmap, then writing to this
 446      attribute can set bits in the bitmap, indicating that a resync
 447      would need to check the corresponding blocks. Either individual
 448      numbers or start-end pairs can be written.  Multiple numbers
 449      can be separated by a space.
 450      Note that the numbers are 'bit' numbers, not 'block' numbers.
 451      They should be scaled by the bitmap_chunksize.
 453   sync_speed_min
 454   sync_speed_max
 455     This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
 456     however they only apply to the particular array.
 457     If no value has been written to these, of if the word 'system'
 458     is written, then the system-wide value is used.  If a value,
 459     in kibibytes-per-second is written, then it is used.
 460     When the files are read, they show the currently active value
 461     followed by "(local)" or "(system)" depending on whether it is
 462     a locally set or system-wide value.
 464   sync_completed
 465     This shows the number of sectors that have been completed of
 466     whatever the current sync_action is, followed by the number of
 467     sectors in total that could need to be processed.  The two
 468     numbers are separated by a '/'  thus effectively showing one
 469     value, a fraction of the process that is complete.
 470     A 'select' on this attribute will return when resync completes,
 471     when it reaches the current sync_max (below) and possibly at
 472     other times.
 474   sync_max
 475     This is a number of sectors at which point a resync/recovery
 476     process will pause.  When a resync is active, the value can
 477     only ever be increased, never decreased.  The value of 'max'
 478     effectively disables the limit.
 481   sync_speed
 482     This shows the current actual speed, in K/sec, of the current
 483     sync_action.  It is averaged over the last 30 seconds.
 485   suspend_lo
 486   suspend_hi
 487     The two values, given as numbers of sectors, indicate a range
 488     within the array where IO will be blocked.  This is currently
 489     only supported for raid4/5/6.
 492Each active md device may also have attributes specific to the
 493personality module that manages it.
 494These are specific to the implementation of the module and could
 495change substantially if the implementation changes.
 497These currently include
 499  stripe_cache_size  (currently raid5 only)
 500      number of entries in the stripe cache.  This is writable, but
 501      there are upper and lower limits (32768, 16).  Default is 128.
 502  strip_cache_active (currently raid5 only)
 503      number of active entries in the stripe cache
 504  preread_bypass_threshold (currently raid5 only)
 505      number of times a stripe requiring preread will be bypassed by
 506      a stripe that does not require preread.  For fairness defaults
 507      to 1.  Setting this to 0 disables bypass accounting and
 508      requires preread stripes to wait until all full-width stripe-
 509      writes are complete.  Valid values are 0 to stripe_cache_size.