FreeBSD/Linux Kernel Cross Reference
sys/Documentation/md.txt

     Tools that manage md devices can be found at
        http://www.kernel.org/pub/linux/utils/raid/ 
     
     
     Boot time assembly of RAID arrays
     ---------------------------------
     
     You can boot with your md device with the following kernel command
     lines:
    
    for old raid arrays without persistent superblocks:
      md=<md device no.>,<raid level>,<chunk size factor>,<fault level>,dev0,dev1,...,devn
    
    for raid arrays with persistent superblocks
      md=<md device no.>,dev0,dev1,...,devn
    or, to assemble a partitionable array:
      md=d<md device no.>,dev0,dev1,...,devn
      
    md device no. = the number of the md device ... 
                  0 means md0, 
                  1 md1,
                  2 md2,
                  3 md3,
                  4 md4
    
    raid level = -1 linear mode
                  0 striped mode
                  other modes are only supported with persistent super blocks
    
    chunk size factor = (raid-0 and raid-1 only)
                  Set  the chunk size as 4k << n.
                  
    fault level = totally ignored
                                
    dev0-devn: e.g. /dev/hda1,/dev/hdc1,/dev/sda1,/dev/sdb1
                                
    A possible loadlin line (Harald Hoyer <HarryH@Royal.Net>)  looks like this:
    
    e:\loadlin\loadlin e:\zimage root=/dev/md0 md=0,0,4,0,/dev/hdb2,/dev/hdc3 ro
    
    
    Boot time autodetection of RAID arrays
    --------------------------------------
    
    When md is compiled into the kernel (not as module), partitions of
    type 0xfd are scanned and automatically assembled into RAID arrays.
    This autodetection may be suppressed with the kernel parameter
    "raid=noautodetect".  As of kernel 2.6.9, only drives with a type 0
    superblock can be autodetected and run at boot time.
    
    The kernel parameter "raid=partitionable" (or "raid=part") means
    that all auto-detected arrays are assembled as partitionable.
    
    Boot time assembly of degraded/dirty arrays
    -------------------------------------------
    
    If a raid5 or raid6 array is both dirty and degraded, it could have
    undetectable data corruption.  This is because the fact that it is
    'dirty' means that the parity cannot be trusted, and the fact that it
    is degraded means that some datablocks are missing and cannot reliably
    be reconstructed (due to no parity).
    
    For this reason, md will normally refuse to start such an array.  This
    requires the sysadmin to take action to explicitly start the array
    despite possible corruption.  This is normally done with
       mdadm --assemble --force ....
    
    This option is not really available if the array has the root
    filesystem on it.  In order to support this booting from such an
    array, md supports a module parameter "start_dirty_degraded" which,
    when set to 1, bypassed the checks and will allows dirty degraded
    arrays to be started.
    
    So, to boot with a root filesystem of a dirty degraded raid[56], use
    
       md-mod.start_dirty_degraded=1
    
    
    Superblock formats
    ------------------
    
    The md driver can support a variety of different superblock formats.
    Currently, it supports superblock formats "0.90.0" and the "md-1" format
    introduced in the 2.5 development series.
    
    The kernel will autodetect which format superblock is being used.
    
    Superblock format '0' is treated differently to others for legacy
    reasons - it is the original superblock format.
    
    
    General Rules - apply for all superblock formats
    ------------------------------------------------
    
    An array is 'created' by writing appropriate superblocks to all
    devices.
    
    It is 'assembled' by associating each of these devices with an
    particular md virtual device.  Once it is completely assembled, it can
   be accessed.
   
   An array should be created by a user-space tool.  This will write
   superblocks to all devices.  It will usually mark the array as
   'unclean', or with some devices missing so that the kernel md driver
   can create appropriate redundancy (copying in raid1, parity
   calculation in raid4/5).
   
   When an array is assembled, it is first initialized with the
   SET_ARRAY_INFO ioctl.  This contains, in particular, a major and minor
   version number.  The major version number selects which superblock
   format is to be used.  The minor number might be used to tune handling
   of the format, such as suggesting where on each device to look for the
   superblock.
   
   Then each device is added using the ADD_NEW_DISK ioctl.  This
   provides, in particular, a major and minor number identifying the
   device to add.
   
   The array is started with the RUN_ARRAY ioctl.
   
   Once started, new devices can be added.  They should have an
   appropriate superblock written to them, and then passed be in with
   ADD_NEW_DISK.
   
   Devices that have failed or are not yet active can be detached from an
   array using HOT_REMOVE_DISK.
   
   
   Specific Rules that apply to format-0 super block arrays, and
          arrays with no superblock (non-persistent).
   -------------------------------------------------------------
   
   An array can be 'created' by describing the array (level, chunksize
   etc) in a SET_ARRAY_INFO ioctl.  This must has major_version==0 and
   raid_disks != 0.
   
   Then uninitialized devices can be added with ADD_NEW_DISK.  The
   structure passed to ADD_NEW_DISK must specify the state of the device
   and its role in the array.
   
   Once started with RUN_ARRAY, uninitialized spares can be added with
   HOT_ADD_DISK.
   
   
   
   MD devices in sysfs
   -------------------
   md devices appear in sysfs (/sys) as regular block devices,
   e.g.
      /sys/block/md0
   
   Each 'md' device will contain a subdirectory called 'md' which
   contains further md-specific information about the device.
   
   All md devices contain:
     level
        a text file indicating the 'raid level'. e.g. raid0, raid1,
        raid5, linear, multipath, faulty.
        If no raid level has been set yet (array is still being
        assembled), the value will reflect whatever has been written
        to it, which may be a name like the above, or may be a number
        such as '0', '5', etc.
   
     raid_disks
        a text file with a simple number indicating the number of devices
        in a fully functional array.  If this is not yet known, the file
        will be empty.  If an array is being resized this will contain
        the new number of devices.
        Some raid levels allow this value to be set while the array is
        active.  This will reconfigure the array.   Otherwise it can only
        be set while assembling an array.
        A change to this attribute will not be permitted if it would
        reduce the size of the array.  To reduce the number of drives
        in an e.g. raid5, the array size must first be reduced by
        setting the 'array_size' attribute.
   
     chunk_size
        This is the size in bytes for 'chunks' and is only relevant to
        raid levels that involve striping (0,4,5,6,10). The address space
        of the array is conceptually divided into chunks and consecutive
        chunks are striped onto neighbouring devices.
        The size should be at least PAGE_SIZE (4k) and should be a power
        of 2.  This can only be set while assembling an array
   
     layout
        The "layout" for the array for the particular level.  This is
        simply a number that is interpretted differently by different
        levels.  It can be written while assembling an array.
   
     array_size
        This can be used to artificially constrain the available space in
        the array to be less than is actually available on the combined
        devices.  Writing a number (in Kilobytes) which is less than
        the available size will set the size.  Any reconfiguration of the
        array (e.g. adding devices) will not cause the size to change.
        Writing the word 'default' will cause the effective size of the
        array to be whatever size is actually available based on
        'level', 'chunk_size' and 'component_size'.
   
        This can be used to reduce the size of the array before reducing
        the number of devices in a raid4/5/6, or to support external
        metadata formats which mandate such clipping.
   
     reshape_position
        This is either "none" or a sector number within the devices of
        the array where "reshape" is up to.  If this is set, the three
        attributes mentioned above (raid_disks, chunk_size, layout) can
        potentially have 2 values, an old and a new value.  If these
        values differ, reading the attribute returns
           new (old)
        and writing will effect the 'new' value, leaving the 'old'
        unchanged.
   
     component_size
        For arrays with data redundancy (i.e. not raid0, linear, faulty,
        multipath), all components must be the same size - or at least
        there must a size that they all provide space for.  This is a key
        part or the geometry of the array.  It is measured in sectors
        and can be read from here.  Writing to this value may resize
        the array if the personality supports it (raid1, raid5, raid6),
        and if the component drives are large enough.
   
     metadata_version
        This indicates the format that is being used to record metadata
        about the array.  It can be 0.90 (traditional format), 1.0, 1.1,
        1.2 (newer format in varying locations) or "none" indicating that
        the kernel isn't managing metadata at all.
        Alternately it can be "external:" followed by a string which
        is set by user-space.  This indicates that metadata is managed
        by a user-space program.  Any device failure or other event that
        requires a metadata update will cause array activity to be
        suspended until the event is acknowledged.
   
     resync_start
        The point at which resync should start.  If no resync is needed,
        this will be a very large number (or 'none' since 2.6.30-rc1).  At
        array creation it will default to 0, though starting the array as
        'clean' will set it much larger.
   
      new_dev
        This file can be written but not read.  The value written should
        be a block device number as major:minor.  e.g. 8:0
        This will cause that device to be attached to the array, if it is
        available.  It will then appear at md/dev-XXX (depending on the
        name of the device) and further configuration is then possible.
   
      safe_mode_delay
        When an md array has seen no write requests for a certain period
        of time, it will be marked as 'clean'.  When another write
        request arrives, the array is marked as 'dirty' before the write
        commences.  This is known as 'safe_mode'.
        The 'certain period' is controlled by this file which stores the
        period as a number of seconds.  The default is 200msec (0.200).
        Writing a value of 0 disables safemode.
   
      array_state
        This file contains a single word which describes the current
        state of the array.  In many cases, the state can be set by
        writing the word for the desired state, however some states
        cannot be explicitly set, and some transitions are not allowed.
   
        Select/poll works on this file.  All changes except between
           active_idle and active (which can be frequent and are not
           very interesting) are notified.  active->active_idle is
           reported if the metadata is externally managed.
   
        clear
            No devices, no size, no level
            Writing is equivalent to STOP_ARRAY ioctl
        inactive
            May have some settings, but array is not active
               all IO results in error
            When written, doesn't tear down array, but just stops it
        suspended (not supported yet)
            All IO requests will block. The array can be reconfigured.
            Writing this, if accepted, will block until array is quiessent
        readonly
            no resync can happen.  no superblocks get written.
            write requests fail
        read-auto
            like readonly, but behaves like 'clean' on a write request.
   
        clean - no pending writes, but otherwise active.
            When written to inactive array, starts without resync
            If a write request arrives then
              if metadata is known, mark 'dirty' and switch to 'active'.
              if not known, block and switch to write-pending
            If written to an active array that has pending writes, then fails.
        active
            fully active: IO and resync can be happening.
            When written to inactive array, starts with resync
   
        write-pending
            clean, but writes are blocked waiting for 'active' to be written.
   
        active-idle
            like active, but no writes have been seen for a while (safe_mode_delay).
   
     bitmap/location
        This indicates where the write-intent bitmap for the array is
        stored.
        It can be one of "none", "file" or "[+-]N".
        "file" may later be extended to "file:/file/name"
        "[+-]N" means that many sectors from the start of the metadata.
          This is replicated on all devices.  For arrays with externally
          managed metadata, the offset is from the beginning of the
          device.
     bitmap/chunksize
        The size, in bytes, of the chunk which will be represented by a
        single bit.  For RAID456, it is a portion of an individual
        device. For RAID10, it is a portion of the array.  For RAID1, it
        is both (they come to the same thing).
     bitmap/time_base
        The time, in seconds, between looking for bits in the bitmap to
        be cleared. In the current implementation, a bit will be cleared
        between 2 and 3 times "time_base" after all the covered blocks
        are known to be in-sync.
     bitmap/backlog
        When write-mostly devices are active in a RAID1, write requests
        to those devices proceed in the background - the filesystem (or
        other user of the device) does not have to wait for them.
        'backlog' sets a limit on the number of concurrent background
        writes.  If there are more than this, new writes will by
        synchronous.
     bitmap/metadata
        This can be either 'internal' or 'external'.
        'internal' is the default and means the metadata for the bitmap
        is stored in the first 256 bytes of the allocated space and is
        managed by the md module.
        'external' means that bitmap metadata is managed externally to
        the kernel (i.e. by some userspace program)
     bitmap/can_clear
        This is either 'true' or 'false'.  If 'true', then bits in the
        bitmap will be cleared when the corresponding blocks are thought
        to be in-sync.  If 'false', bits will never be cleared.
        This is automatically set to 'false' if a write happens on a
        degraded array, or if the array becomes degraded during a write.
        When metadata is managed externally, it should be set to true
        once the array becomes non-degraded, and this fact has been
        recorded in the metadata.
        
        
        
   
   As component devices are added to an md array, they appear in the 'md'
   directory as new directories named
         dev-XXX
   where XXX is a name that the kernel knows for the device, e.g. hdb1.
   Each directory contains:
   
         block
           a symlink to the block device in /sys/block, e.g.
                /sys/block/md0/md/dev-hdb1/block -> ../../../../block/hdb/hdb1
   
         super
           A file containing an image of the superblock read from, or
           written to, that device.
   
         state
           A file recording the current state of the device in the array
           which can be a comma separated list of
                 faulty   - device has been kicked from active use due to
                            a detected fault, or it has unacknowledged bad
                            blocks
                 in_sync  - device is a fully in-sync member of the array
                 writemostly - device will only be subject to read
                            requests if there are no other options.
                            This applies only to raid1 arrays.
                 blocked  - device has failed, and the failure hasn't been
                            acknowledged yet by the metadata handler.
                            Writes that would write to this device if
                            it were not faulty are blocked.
                 spare    - device is working, but not a full member.
                            This includes spares that are in the process
                            of being recovered to
                 write_error - device has ever seen a write error.
                 want_replacement - device is (mostly) working but probably
                            should be replaced, either due to errors or
                            due to user request.
                 replacement - device is a replacement for another active
                            device with same raid_disk.
   
   
           This list may grow in future.
           This can be written to.
           Writing "faulty"  simulates a failure on the device.
           Writing "remove" removes the device from the array.
           Writing "writemostly" sets the writemostly flag.
           Writing "-writemostly" clears the writemostly flag.
           Writing "blocked" sets the "blocked" flag.
           Writing "-blocked" clears the "blocked" flags and allows writes
                   to complete and possibly simulates an error.
           Writing "in_sync" sets the in_sync flag.
           Writing "write_error" sets writeerrorseen flag.
           Writing "-write_error" clears writeerrorseen flag.
           Writing "want_replacement" is allowed at any time except to a
                   replacement device or a spare.  It sets the flag.
           Writing "-want_replacement" is allowed at any time.  It clears
                   the flag.
           Writing "replacement" or "-replacement" is only allowed before
                   starting the array.  It sets or clears the flag.
   
   
           This file responds to select/poll. Any change to 'faulty'
           or 'blocked' causes an event.
   
         errors
           An approximate count of read errors that have been detected on
           this device but have not caused the device to be evicted from
           the array (either because they were corrected or because they
           happened while the array was read-only).  When using version-1
           metadata, this value persists across restarts of the array.
   
           This value can be written while assembling an array thus
           providing an ongoing count for arrays with metadata managed by
           userspace.
   
         slot
           This gives the role that the device has in the array.  It will
           either be 'none' if the device is not active in the array
           (i.e. is a spare or has failed) or an integer less than the
           'raid_disks' number for the array indicating which position
           it currently fills.  This can only be set while assembling an
           array.  A device for which this is set is assumed to be working.
   
         offset
           This gives the location in the device (in sectors from the
           start) where data from the array will be stored.  Any part of
           the device before this offset us not touched, unless it is
           used for storing metadata (Formats 1.1 and 1.2).
   
         size
           The amount of the device, after the offset, that can be used
           for storage of data.  This will normally be the same as the
           component_size.  This can be written while assembling an
           array.  If a value less than the current component_size is
           written, it will be rejected.
   
         recovery_start
           When the device is not 'in_sync', this records the number of
           sectors from the start of the device which are known to be
           correct.  This is normally zero, but during a recovery
           operation is will steadily increase, and if the recovery is
           interrupted, restoring this value can cause recovery to
           avoid repeating the earlier blocks.  With v1.x metadata, this
           value is saved and restored automatically.
   
           This can be set whenever the device is not an active member of
           the array, either before the array is activated, or before
           the 'slot' is set.
   
           Setting this to 'none' is equivalent to setting 'in_sync'.
           Setting to any other value also clears the 'in_sync' flag.
           
         bad_blocks
           This gives the list of all known bad blocks in the form of
           start address and length (in sectors respectively). If output
           is too big to fit in a page, it will be truncated. Writing
           "sector length" to this file adds new acknowledged (i.e.
           recorded to disk safely) bad blocks.
   
         unacknowledged_bad_blocks
           This gives the list of known-but-not-yet-saved-to-disk bad
           blocks in the same form of 'bad_blocks'. If output is too big
           to fit in a page, it will be truncated. Writing to this file
           adds bad blocks without acknowledging them. This is largely
           for testing.
   
   
   
   An active md device will also contain and entry for each active device
   in the array.  These are named
   
       rdNN
   
   where 'NN' is the position in the array, starting from 0.
   So for a 3 drive array there will be rd0, rd1, rd2.
   These are symbolic links to the appropriate 'dev-XXX' entry.
   Thus, for example,
          cat /sys/block/md*/md/rd*/state
   will show 'in_sync' on every line.
   
   
   
   Active md devices for levels that support data redundancy (1,4,5,6)
   also have
   
      sync_action
        a text file that can be used to monitor and control the rebuild
        process.  It contains one word which can be one of:
          resync        - redundancy is being recalculated after unclean
                          shutdown or creation
          recover       - a hot spare is being built to replace a
                          failed/missing device
          idle          - nothing is happening
          check         - A full check of redundancy was requested and is
                          happening.  This reads all block and checks
                          them. A repair may also happen for some raid
                          levels.
          repair        - A full check and repair is happening.  This is
                          similar to 'resync', but was requested by the
                          user, and the write-intent bitmap is NOT used to
                          optimise the process.
   
         This file is writable, and each of the strings that could be
         read are meaningful for writing.
   
          'idle' will stop an active resync/recovery etc.  There is no
              guarantee that another resync/recovery may not be automatically
              started again, though some event will be needed to trigger
              this.
           'resync' or 'recovery' can be used to restart the
              corresponding operation if it was stopped with 'idle'.
           'check' and 'repair' will start the appropriate process
              providing the current state is 'idle'.
   
         This file responds to select/poll.  Any important change in the value
         triggers a poll event.  Sometimes the value will briefly be
         "recover" if a recovery seems to be needed, but cannot be
         achieved. In that case, the transition to "recover" isn't
         notified, but the transition away is.
   
      degraded
         This contains a count of the number of devices by which the
         arrays is degraded.  So an optimal array with show '0'.  A
         single failed/missing drive will show '1', etc.
         This file responds to select/poll, any increase or decrease
         in the count of missing devices will trigger an event.
   
      mismatch_count
         When performing 'check' and 'repair', and possibly when
         performing 'resync', md will count the number of errors that are
         found.  The count in 'mismatch_cnt' is the number of sectors
         that were re-written, or (for 'check') would have been
         re-written.  As most raid levels work in units of pages rather
         than sectors, this my be larger than the number of actual errors
         by a factor of the number of sectors in a page.
   
      bitmap_set_bits
         If the array has a write-intent bitmap, then writing to this
         attribute can set bits in the bitmap, indicating that a resync
         would need to check the corresponding blocks. Either individual
         numbers or start-end pairs can be written.  Multiple numbers
         can be separated by a space.
         Note that the numbers are 'bit' numbers, not 'block' numbers.
         They should be scaled by the bitmap_chunksize.
   
      sync_speed_min
      sync_speed_max
        This are similar to /proc/sys/dev/raid/speed_limit_{min,max}
        however they only apply to the particular array.
        If no value has been written to these, of if the word 'system'
        is written, then the system-wide value is used.  If a value,
        in kibibytes-per-second is written, then it is used.
        When the files are read, they show the currently active value
        followed by "(local)" or "(system)" depending on whether it is
        a locally set or system-wide value.
   
      sync_completed
        This shows the number of sectors that have been completed of
        whatever the current sync_action is, followed by the number of
        sectors in total that could need to be processed.  The two
        numbers are separated by a '/'  thus effectively showing one
        value, a fraction of the process that is complete.
        A 'select' on this attribute will return when resync completes,
        when it reaches the current sync_max (below) and possibly at
        other times.
   
      sync_max
        This is a number of sectors at which point a resync/recovery
        process will pause.  When a resync is active, the value can
        only ever be increased, never decreased.  The value of 'max'
        effectively disables the limit.
   
   
      sync_speed
        This shows the current actual speed, in K/sec, of the current
        sync_action.  It is averaged over the last 30 seconds.
   
      suspend_lo
      suspend_hi
        The two values, given as numbers of sectors, indicate a range
        within the array where IO will be blocked.  This is currently
        only supported for raid4/5/6.
   
      sync_min
      sync_max
        The two values, given as numbers of sectors, indicate a range
        within the array where 'check'/'repair' will operate. Must be
        a multiple of chunk_size. When it reaches "sync_max" it will
        pause, rather than complete.
        You can use 'select' or 'poll' on "sync_completed" to wait for
        that number to reach sync_max.  Then you can either increase
        "sync_max", or can write 'idle' to "sync_action".
   
   
   Each active md device may also have attributes specific to the
   personality module that manages it.
   These are specific to the implementation of the module and could
   change substantially if the implementation changes.
   
   These currently include
   
     stripe_cache_size  (currently raid5 only)
         number of entries in the stripe cache.  This is writable, but
         there are upper and lower limits (32768, 16).  Default is 128.
     strip_cache_active (currently raid5 only)
         number of active entries in the stripe cache
     preread_bypass_threshold (currently raid5 only)
         number of times a stripe requiring preread will be bypassed by
         a stripe that does not require preread.  For fairness defaults
         to 1.  Setting this to 0 disables bypass accounting and
         requires preread stripes to wait until all full-width stripe-
         writes are complete.  Valid values are 0 to stripe_cache_size.

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