linux/Documentation/md.txt
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   1Tools that manage md devices can be found at
   2   http://www.<country>.kernel.org/pub/linux/utils/raid/....
   3
   4
   5Boot time assembly of RAID arrays
   6---------------------------------
   7
   8You can boot with your md device with the following kernel command
   9lines:
  10
  11for old raid arrays without persistent superblocks:
  12  md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
  13
  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
  18  
  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
  25
  26raid level = -1 linear mode
  27              0 striped mode
  28              other modes are only supported with persistent super blocks
  29
  30chunk size factor = (raid-0 and raid-1 only)
  31              Set  the chunk size as 4k << n.
  32              
  33fault level = totally ignored
  34                            
  35dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
  36                            
  37A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this:
  38
  39e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
  40
  41
  42Boot time autodetection of RAID arrays
  43--------------------------------------
  44
  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.
  50
  51The kernel parameter "raid=partitionable" (or "raid=part") means
  52that all auto-detected arrays are assembled as partitionable.
  53
  54Boot time assembly of degraded/dirty arrays
  55-------------------------------------------
  56
  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).
  62
  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 ....
  67
  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.
  73
  74So, to boot with a root filesystem of a dirty degraded raid[56], use
  75
  76   md-mod.start_dirty_degraded=1
  77
  78
  79Superblock formats
  80------------------
  81
  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.
  85
  86The kernel will autodetect which format superblock is being used.
  87
  88Superblock format '0' is treated differently to others for legacy
  89reasons - it is the original superblock format.
  90
  91
  92General Rules - apply for all superblock formats
  93------------------------------------------------
  94
  95An array is 'created' by writing appropriate superblocks to all
  96devices.
  97
  98It is 'assembled' by associating each of these devices with an
  99particular md virtual device.  Once it is completely assembled, it can
 100be accessed.
 101
 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).
 107
 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
 113superblock.
 114
 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.
 118
 119The array is started with the RUN_ARRAY ioctl.
 120
 121Once started, new devices can be added.  They should have an
 122appropriate superblock written to them, and then passed be in with
 123ADD_NEW_DISK.
 124
 125Devices that have failed or are not yet active can be detached from an
 126array using HOT_REMOVE_DISK.
 127
 128
 129Specific Rules that apply to format-0 super block arrays, and
 130       arrays with no superblock (non-persistent).
 131-------------------------------------------------------------
 132
 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.
 136
 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.
 140
 141Once started with RUN_ARRAY, uninitialized spares can be added with
 142HOT_ADD_DISK.
 143
 144
 145
 146MD devices in sysfs
 147-------------------
 148md devices appear in sysfs (/sys) as regular block devices,
 149e.g.
 150   /sys/block/md0
 151
 152Each 'md' device will contain a subdirectory called 'md' which
 153contains further md-specific information about the device.
 154
 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.
 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
 277As component devices are added to an md array, they appear in the 'md'
 278directory as new directories named
 279      dev-XXX
 280where XXX is a name that the kernel knows for the device, e.g. hdb1.
 281Each 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              blocked  - device has failed, metadata is "external",
 301                         and the failure hasn't been acknowledged yet.
 302                         Writes that would write to this device if
 303                         it were not faulty are blocked.
 304              spare    - device is working, but not a full member.
 305                         This includes spares that are in the process
 306                         of being recovered to
 307        This list may grow in future.
 308        This can be written to.
 309        Writing "faulty"  simulates a failure on the device.
 310        Writing "remove" removes the device from the array.
 311        Writing "writemostly" sets the writemostly flag.
 312        Writing "-writemostly" clears the writemostly flag.
 313        Writing "blocked" sets the "blocked" flag.
 314        Writing "-blocked" clear the "blocked" flag and allows writes
 315                to complete.
 316
 317        This file responds to select/poll. Any change to 'faulty'
 318        or 'blocked' causes an event.
 319
 320      errors
 321        An approximate count of read errors that have been detected on
 322        this device but have not caused the device to be evicted from
 323        the array (either because they were corrected or because they
 324        happened while the array was read-only).  When using version-1
 325        metadata, this value persists across restarts of the array.
 326
 327        This value can be written while assembling an array thus
 328        providing an ongoing count for arrays with metadata managed by
 329        userspace.
 330
 331      slot
 332        This gives the role that the device has in the array.  It will
 333        either be 'none' if the device is not active in the array
 334        (i.e. is a spare or has failed) or an integer less than the
 335        'raid_disks' number for the array indicating which position
 336        it currently fills.  This can only be set while assembling an
 337        array.  A device for which this is set is assumed to be working.
 338
 339      offset
 340        This gives the location in the device (in sectors from the
 341        start) where data from the array will be stored.  Any part of
 342        the device before this offset us not touched, unless it is
 343        used for storing metadata (Formats 1.1 and 1.2).
 344
 345      size
 346        The amount of the device, after the offset, that can be used
 347        for storage of data.  This will normally be the same as the
 348        component_size.  This can be written while assembling an
 349        array.  If a value less than the current component_size is
 350        written, it will be rejected.
 351
 352
 353An active md device will also contain and entry for each active device
 354in the array.  These are named
 355
 356    rdNN
 357
 358where 'NN' is the position in the array, starting from 0.
 359So for a 3 drive array there will be rd0, rd1, rd2.
 360These are symbolic links to the appropriate 'dev-XXX' entry.
 361Thus, for example,
 362       cat /sys/block/md*/md/rd*/state
 363will show 'in_sync' on every line.
 364
 365
 366
 367Active md devices for levels that support data redundancy (1,4,5,6)
 368also have
 369
 370   sync_action
 371     a text file that can be used to monitor and control the rebuild
 372     process.  It contains one word which can be one of:
 373       resync        - redundancy is being recalculated after unclean
 374                       shutdown or creation
 375       recover       - a hot spare is being built to replace a
 376                       failed/missing device
 377       idle          - nothing is happening
 378       check         - A full check of redundancy was requested and is
 379                       happening.  This reads all block and checks
 380                       them. A repair may also happen for some raid
 381                       levels.
 382       repair        - A full check and repair is happening.  This is
 383                       similar to 'resync', but was requested by the
 384                       user, and the write-intent bitmap is NOT used to
 385                       optimise the process.
 386
 387      This file is writable, and each of the strings that could be
 388      read are meaningful for writing.
 389
 390       'idle' will stop an active resync/recovery etc.  There is no
 391           guarantee that another resync/recovery may not be automatically
 392           started again, though some event will be needed to trigger
 393           this.
 394        'resync' or 'recovery' can be used to restart the
 395           corresponding operation if it was stopped with 'idle'.
 396        'check' and 'repair' will start the appropriate process
 397           providing the current state is 'idle'.
 398
 399      This file responds to select/poll.  Any important change in the value
 400      triggers a poll event.  Sometimes the value will briefly be
 401      "recover" if a recovery seems to be needed, but cannot be
 402      achieved. In that case, the transition to "recover" isn't
 403      notified, but the transition away is.
 404
 405   degraded
 406      This contains a count of the number of devices by which the
 407      arrays is degraded.  So an optimal array with show '0'.  A
 408      single failed/missing drive will show '1', etc.
 409      This file responds to select/poll, any increase or decrease
 410      in the count of missing devices will trigger an event.
 411
 412   mismatch_count
 413      When performing 'check' and 'repair', and possibly when
 414      performing 'resync', md will count the number of errors that are
 415      found.  The count in 'mismatch_cnt' is the number of sectors
 416      that were re-written, or (for 'check') would have been
 417      re-written.  As most raid levels work in units of pages rather
 418      than sectors, this my be larger than the number of actual errors
 419      by a factor of the number of sectors in a page.
 420
 421   bitmap_set_bits
 422      If the array has a write-intent bitmap, then writing to this
 423      attribute can set bits in the bitmap, indicating that a resync
 424      would need to check the corresponding blocks. Either individual
 425      numbers or start-end pairs can be written.  Multiple numbers
 426      can be separated by a space.
 427      Note that the numbers are 'bit' numbers, not 'block' numbers.
 428      They should be scaled by the bitmap_chunksize.
 429
 430   sync_speed_min
 431   sync_speed_max
 432     This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
 433     however they only apply to the particular array.
 434     If no value has been written to these, of if the word 'system'
 435     is written, then the system-wide value is used.  If a value,
 436     in kibibytes-per-second is written, then it is used.
 437     When the files are read, they show the currently active value
 438     followed by "(local)" or "(system)" depending on whether it is
 439     a locally set or system-wide value.
 440
 441   sync_completed
 442     This shows the number of sectors that have been completed of
 443     whatever the current sync_action is, followed by the number of
 444     sectors in total that could need to be processed.  The two
 445     numbers are separated by a '/'  thus effectively showing one
 446     value, a fraction of the process that is complete.
 447     A 'select' on this attribute will return when resync completes,
 448     when it reaches the current sync_max (below) and possibly at
 449     other times.
 450
 451   sync_max
 452     This is a number of sectors at which point a resync/recovery
 453     process will pause.  When a resync is active, the value can
 454     only ever be increased, never decreased.  The value of 'max'
 455     effectively disables the limit.
 456
 457
 458   sync_speed
 459     This shows the current actual speed, in K/sec, of the current
 460     sync_action.  It is averaged over the last 30 seconds.
 461
 462   suspend_lo
 463   suspend_hi
 464     The two values, given as numbers of sectors, indicate a range
 465     within the array where IO will be blocked.  This is currently
 466     only supported for raid4/5/6.
 467
 468
 469Each active md device may also have attributes specific to the
 470personality module that manages it.
 471These are specific to the implementation of the module and could
 472change substantially if the implementation changes.
 473
 474These currently include
 475
 476  stripe_cache_size  (currently raid5 only)
 477      number of entries in the stripe cache.  This is writable, but
 478      there are upper and lower limits (32768, 16).  Default is 128.
 479  strip_cache_active (currently raid5 only)
 480      number of active entries in the stripe cache
 481  preread_bypass_threshold (currently raid5 only)
 482      number of times a stripe requiring preread will be bypassed by
 483      a stripe that does not require preread.  For fairness defaults
 484      to 1.  Setting this to 0 disables bypass accounting and
 485      requires preread stripes to wait until all full-width stripe-
 486      writes are complete.  Valid values are 0 to stripe_cache_size.
 487