Provided by: mdadm_3.3-2ubuntu7_i386 bug

NAME

       md - Multiple Device driver aka Linux Software RAID

SYNOPSIS

       /dev/mdn
       /dev/md/n
       /dev/md/name

DESCRIPTION

       The  md  driver  provides  virtual devices that are created from one or
       more independent underlying  devices.   This  array  of  devices  often
       contains  redundancy  and  the devices are often disk drives, hence the
       acronym RAID which stands for a Redundant Array of Independent Disks.

       md supports RAID levels 1 (mirroring), 4  (striped  array  with  parity
       device),  5  (striped  array  with  distributed  parity information), 6
       (striped array with distributed dual redundancy  information),  and  10
       (striped  and  mirrored).   If  some number of underlying devices fails
       while using one of these levels, the array will continue  to  function;
       this  number  is one for RAID levels 4 and 5, two for RAID level 6, and
       all but one (N-1) for RAID level 1, and dependent on configuration  for
       level 10.

       md also supports a number of pseudo RAID (non-redundant) configurations
       including RAID0 (striped array), LINEAR (catenated array), MULTIPATH (a
       set  of  different  interfaces to the same device), and FAULTY (a layer
       over a single device into which errors can be injected).

   MD METADATA
       Each device in an array may have some metadata stored  in  the  device.
       This  metadata  is sometimes called a superblock.  The metadata records
       information about the structure and state of the  array.   This  allows
       the array to be reliably re-assembled after a shutdown.

       From Linux kernel version 2.6.10, md provides support for two different
       formats of metadata, and other formats can be  added.   Prior  to  this
       release, only one format is supported.

       The common format — known as version 0.90 — has a superblock that is 4K
       long and is written into a 64K aligned block that starts at  least  64K
       and  less than 128K from the end of the device (i.e. to get the address
       of the superblock round the size of the device down to  a  multiple  of
       64K  and  then subtract 64K).  The available size of each device is the
       amount of space before the super block, so between 64K and 128K is lost
       when a device in incorporated into an MD array.  This superblock stores
       multi-byte fields in a processor-dependent  manner,  so  arrays  cannot
       easily be moved between computers with different processors.

       The new format — known as version 1 — has a superblock that is normally
       1K long, but can be longer.  It is normally stored between 8K  and  12K
       from  the end of the device, on a 4K boundary, though variations can be
       stored at the start of the device (version 1.1) or 4K from the start of
       the  device  (version 1.2).  This metadata format stores multibyte data
       in a processor-independent  format  and  supports  up  to  hundreds  of
       component devices (version 0.90 only supports 28).

       The metadata contains, among other things:

       LEVEL  The  manner  in  which  the  devices are arranged into the array
              (LINEAR, RAID0, RAID1, RAID4, RAID5, RAID10, MULTIPATH).

       UUID   a 128 bit Universally  Unique  Identifier  that  identifies  the
              array that contains this device.

       When  a version 0.90 array is being reshaped (e.g. adding extra devices
       to a RAID5), the version number  is  temporarily  set  to  0.91.   This
       ensures that if the reshape process is stopped in the middle (e.g. by a
       system crash) and the machine boots into an older kernel that does  not
       support  reshaping,  then  the array will not be assembled (which would
       cause data corruption) but will be left untouched until a  kernel  that
       can complete the reshape processes is used.

   ARRAYS WITHOUT METADATA
       While it is usually best to create arrays with superblocks so that they
       can be assembled reliably, there are some circumstances when  an  array
       without superblocks is preferred.  These include:

       LEGACY ARRAYS
              Early  versions of the md driver only supported LINEAR and RAID0
              configurations and did not  use  a  superblock  (which  is  less
              critical  with  these configurations).  While such arrays should
              be rebuilt with superblocks if possible, md continues to support
              them.

       FAULTY Being  a  largely transparent layer over a different device, the
              FAULTY  personality  doesn't  gain  anything   from   having   a
              superblock.

       MULTIPATH
              It is often possible to detect devices which are different paths
              to the same storage directly rather than  having  a  distinctive
              superblock  written to the device and searched for on all paths.
              In this case, a MULTIPATH array with no superblock makes sense.

       RAID1  In some configurations it might be desired  to  create  a  RAID1
              configuration  that  does  not use a superblock, and to maintain
              the state of the array  elsewhere.   While  not  encouraged  for
              general use, it does have special-purpose uses and is supported.

   ARRAYS WITH EXTERNAL METADATA
       From  release  2.6.28,  the  md  driver supports arrays with externally
       managed metadata.  That is, the metadata is not managed by  the  kernel
       but  rather  by  a  user-space program which is external to the kernel.
       This  allows  support  for  a  variety  of  metadata  formats   without
       cluttering the kernel with lots of details.

       md  is  able to communicate with the user-space program through various
       sysfs attributes so  that  it  can  make  appropriate  changes  to  the
       metadata  - for example to mark a device as faulty.  When necessary, md
       will wait for the program to acknowledge the  event  by  writing  to  a
       sysfs  attribute.   The  manual  page for mdmon(8) contains more detail
       about this interaction.

   CONTAINERS
       Many metadata formats use a single block  of  metadata  to  describe  a
       number  of  different arrays which all use the same set of devices.  In
       this case it is helpful for the kernel to know about the  full  set  of
       devices  as  a  whole.   This  set  is  known  to md as a container.  A
       container is an md array with  externally  managed  metadata  and  with
       device  offset and size so that it just covers the metadata part of the
       devices.  The remainder of each device is available to be  incorporated
       into various arrays.

   LINEAR
       A  LINEAR  array  simply catenates the available space on each drive to
       form one large virtual drive.

       One  advantage  of  this  arrangement  over  the  more   common   RAID0
       arrangement  is that the array may be reconfigured at a later time with
       an extra drive, so the array is made bigger without disturbing the data
       that is on the array.  This can even be done on a live array.

       If  a  chunksize is given with a LINEAR array, the usable space on each
       device is rounded down to a multiple of this chunksize.

   RAID0
       A RAID0 array (which has zero redundancy) is also known  as  a  striped
       array.  A RAID0 array is configured at creation with a Chunk Size which
       must be a power of  two  (prior  to  Linux  2.6.31),  and  at  least  4
       kibibytes.

       The  RAID0  driver  assigns  the  first chunk of the array to the first
       device, the second chunk to the second device,  and  so  on  until  all
       drives have been assigned one chunk.  This collection of chunks forms a
       stripe.  Further chunks are gathered into stripes in the same way,  and
       are assigned to the remaining space in the drives.

       If  devices  in  the  array  are  not  all the same size, then once the
       smallest device has been exhausted, the RAID0 driver starts  collecting
       chunks  into smaller stripes that only span the drives which still have
       remaining space.

   RAID1
       A RAID1 array is also known as a mirrored set (though mirrors  tend  to
       provide reflected images, which RAID1 does not) or a plex.

       Once  initialised,  each  device  in a RAID1 array contains exactly the
       same data.  Changes are written to all devices in  parallel.   Data  is
       read  from  any  one  device.   The  driver attempts to distribute read
       requests across all devices to maximise performance.

       All devices in a RAID1 array should be the same size.  If they are not,
       then  only the amount of space available on the smallest device is used
       (any extra space on other devices is wasted).

       Note that the read balancing done by the driver does not make the RAID1
       performance  profile  be  the  same  as  for  RAID0; a single stream of
       sequential input will not  be  accelerated  (e.g.  a  single  dd),  but
       multiple sequential streams or a random workload will use more than one
       spindle. In theory, having an N-disk  RAID1  will  allow  N  sequential
       threads to read from all disks.

       Individual  devices  in a RAID1 can be marked as "write-mostly".  These
       drives are excluded from the normal read balancing  and  will  only  be
       read  from  when  there  is  no  other  option.  This can be useful for
       devices connected over a slow link.

   RAID4
       A RAID4 array is like a RAID0 array with an extra  device  for  storing
       parity.  This  device  is  the last of the active devices in the array.
       Unlike RAID0, RAID4 also requires that all stripes span all drives,  so
       extra space on devices that are larger than the smallest is wasted.

       When  any block in a RAID4 array is modified, the parity block for that
       stripe (i.e. the block in the parity device at the same  device  offset
       as  the  stripe)  is  also  modified  so  that  the parity block always
       contains the "parity" for  the  whole  stripe.   I.e.  its  content  is
       equivalent  to  the  result  of  performing  an  exclusive-or operation
       between all the data blocks in the stripe.

       This allows the array to continue to function if one device fails.  The
       data  that  was  on  that  device  can be calculated as needed from the
       parity block and the other data blocks.

   RAID5
       RAID5 is very similar to RAID4.  The  difference  is  that  the  parity
       blocks  for  each  stripe,  instead  of  being  on a single device, are
       distributed across all devices.   This  allows  more  parallelism  when
       writing,  as  two  different  block  updates will quite possibly affect
       parity blocks on different devices so there is less contention.

       This also allows more parallelism when reading, as  read  requests  are
       distributed over all the devices in the array instead of all but one.

   RAID6
       RAID6  is  similar to RAID5, but can handle the loss of any two devices
       without data loss.  Accordingly, it requires  N+2  drives  to  store  N
       drives worth of data.

       The  performance for RAID6 is slightly lower but comparable to RAID5 in
       normal mode and single disk failure mode.  It is very slow in dual disk
       failure mode, however.

   RAID10
       RAID10  provides  a  combination  of  RAID1 and RAID0, and is sometimes
       known as RAID1+0.  Every datablock is duplicated some number of  times,
       and  the  resulting  collection  of  datablocks  are  distributed  over
       multiple drives.

       When configuring a RAID10 array, it is necessary to specify the  number
       of replicas of each data block that are required (this will normally be
       2) and whether the replicas should be 'near', 'offset' or 'far'.  (Note
       that the 'offset' layout is only available from 2.6.18).

       When  'near'  replicas are chosen, the multiple copies of a given chunk
       are laid out consecutively across the stripes of the array, so the  two
       copies of a datablock will likely be at the same offset on two adjacent
       devices.

       When 'far' replicas are chosen, the multiple copies of  a  given  chunk
       are laid out quite distant from each other.  The first copy of all data
       blocks will be striped across the early part of  all  drives  in  RAID0
       fashion,  and then the next copy of all blocks will be striped across a
       later section of all drives, always ensuring that  all  copies  of  any
       given block are on different drives.

       The  'far'  arrangement  can  give sequential read performance equal to
       that of a RAID0 array, but at the cost of reduced write performance.

       When 'offset' replicas are chosen, the multiple copies of a given chunk
       are  laid  out  on  consecutive  drives  and  at  consecutive  offsets.
       Effectively each stripe is duplicated and the copies are offset by  one
       device.    This  should give similar read characteristics to 'far' if a
       suitably large chunk size is used, but  without  as  much  seeking  for
       writes.

       It  should  be  noted that the number of devices in a RAID10 array need
       not be a multiple of the number of replica of each data block; however,
       there must be at least as many devices as replicas.

       If,  for  example,  an  array is created with 5 devices and 2 replicas,
       then space equivalent to 2.5 of the  devices  will  be  available,  and
       every block will be stored on two different devices.

       Finally,  it  is  possible  to have an array with both 'near' and 'far'
       copies.  If an array is configured with 2 near copies and 2 far copies,
       then  there  will  be  a  total  of  4  copies of each block, each on a
       different drive.  This is an artifact  of  the  implementation  and  is
       unlikely to be of real value.

   MULTIPATH
       MULTIPATH  is not really a RAID at all as there is only one real device
       in a MULTIPATH md array.  However  there  are  multiple  access  points
       (paths) to this device, and one of these paths might fail, so there are
       some similarities.

       A MULTIPATH array is  composed  of  a  number  of  logically  different
       devices,  often  fibre  channel interfaces, that all refer the the same
       real device. If one of  these  interfaces  fails  (e.g.  due  to  cable
       problems),  the  MULTIPATH  driver will attempt to redirect requests to
       another interface.

       The MULTIPATH drive is not receiving any ongoing development and should
       be  considered  a  legacy  driver.   The  device-mapper based multipath
       drivers should be preferred for new installations.

   FAULTY
       The FAULTY md module is provided for testing purposes.  A FAULTY  array
       has  exactly  one  component device and is normally assembled without a
       superblock, so the md array created provides direct access  to  all  of
       the data in the component device.

       The  FAULTY module may be requested to simulate faults to allow testing
       of other md levels or of filesystems.  Faults can be chosen to  trigger
       on  read requests or write requests, and can be transient (a subsequent
       read/write  at  the  address  will  probably  succeed)  or   persistent
       (subsequent  read/write  of the same address will fail).  Further, read
       faults can be "fixable" meaning that they persist until a write request
       at the same address.

       Fault  types  can  be requested with a period.  In this case, the fault
       will recur repeatedly  after  the  given  number  of  requests  of  the
       relevant  type.  For example if persistent read faults have a period of
       100, then every 100th read request would  generate  a  fault,  and  the
       faulty sector would be recorded so that subsequent reads on that sector
       would also fail.

       There is a limit to the number of faulty sectors that  are  remembered.
       Faults   generated  after  this  limit  is  exhausted  are  treated  as
       transient.

       The list of faulty sectors can be  flushed,  and  the  active  list  of
       failure modes can be cleared.

   UNCLEAN SHUTDOWN
       When  changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array
       there is a possibility of inconsistency for short periods  of  time  as
       each  update  requires  at  least  two block to be written to different
       devices, and these writes probably won't happen  at  exactly  the  same
       time.   Thus  if  a  system with one of these arrays is shutdown in the
       middle of a write operation (e.g. due to power failure), the array  may
       not be consistent.

       To  handle  this  situation,  the  md  driver marks an array as "dirty"
       before writing any data to it, and marks it as "clean" when  the  array
       is  being  disabled, e.g. at shutdown.  If the md driver finds an array
       to  be  dirty  at  startup,  it  proceeds  to  correct   any   possibly
       inconsistency.   For  RAID1,  this involves copying the contents of the
       first drive onto all other drives.  For RAID4,  RAID5  and  RAID6  this
       involves  recalculating the parity for each stripe and making sure that
       the parity block has the correct data.  For RAID10 it involves  copying
       one  of  the replicas of each block onto all the others.  This process,
       known as "resynchronising" or "resync" is performed in the  background.
       The array can still be used, though possibly with reduced performance.

       If  a  RAID4,  RAID5  or  RAID6 array is degraded (missing at least one
       drive, two for RAID6) when it is restarted after an  unclean  shutdown,
       it  cannot recalculate parity, and so it is possible that data might be
       undetectably corrupted.  The 2.4 md driver does not alert the  operator
       to  this  condition.   The 2.6 md driver will fail to start an array in
       this condition without manual intervention, though this  behaviour  can
       be overridden by a kernel parameter.

   RECOVERY
       If  the  md driver detects a write error on a device in a RAID1, RAID4,
       RAID5, RAID6, or RAID10 array,  it  immediately  disables  that  device
       (marking  it  as  faulty)  and  continues  operation  on  the remaining
       devices.  If there are spare drives, the driver will  start  recreating
       on  one  of  the  spare drives the data which was on that failed drive,
       either by copying a working drive in a RAID1 configuration, or by doing
       calculations  with  the  parity  block  on RAID4, RAID5 or RAID6, or by
       finding and copying originals for RAID10.

       In kernels prior to about 2.6.15, a read error  would  cause  the  same
       effect  as  a write error.  In later kernels, a read-error will instead
       cause md to attempt a recovery by overwriting the bad  block.  i.e.  it
       will find the correct data from elsewhere, write it over the block that
       failed, and then try to read it back again.  If either the write or the
       re-read  fail,  md will treat the error the same way that a write error
       is treated, and will fail the whole device.

       While this recovery process is happening, the md  driver  will  monitor
       accesses  to the array and will slow down the rate of recovery if other
       activity is happening, so that normal access to the array will  not  be
       unduly  affected.   When  no  other activity is happening, the recovery
       process proceeds at full speed.  The actual speed targets for  the  two
       different  situations  can  be  controlled  by  the speed_limit_min and
       speed_limit_max control files mentioned below.

   SCRUBBING AND MISMATCHES
       As storage devices can develop bad blocks at any time it is valuable to
       regularly  read  all  blocks  on all devices in an array so as to catch
       such bad blocks early.  This process is called scrubbing.

       md arrays can be scrubbed by writing either check or repair to the file
       md/sync_action in the sysfs directory for the device.

       Requesting a scrub will cause md to read every block on every device in
       the array, and check that  the  data  is  consistent.   For  RAID1  and
       RAID10,  this means checking that the copies are identical.  For RAID4,
       RAID5, RAID6 this means checking that the parity block  is  (or  blocks
       are) correct.

       If  a read error is detected during this process, the normal read-error
       handling causes correct data to be found from other devices and  to  be
       written  back to the faulty device.  In many case this will effectively
       fix the bad block.

       If all blocks read successfully but are found  to  not  be  consistent,
       then this is regarded as a mismatch.

       If  check  was used, then no action is taken to handle the mismatch, it
       is simply recorded.  If repair  was  used,  then  a  mismatch  will  be
       repaired  in  the same way that resync repairs arrays.  For RAID5/RAID6
       new parity blocks are written.  For RAID1/RAID10, all but one block are
       overwritten with the content of that one block.

       A  count  of  mismatches is recorded in the sysfs file md/mismatch_cnt.
       This is set to zero when a scrub starts and is incremented  whenever  a
       sector  is  found  that is a mismatch.  md normally works in units much
       larger than a single sector and when it finds a mismatch, it  does  not
       determine exactly how many actual sectors were affected but simply adds
       the number of sectors in the IO unit that was used.  So a value of  128
       could  simply  mean  that  a  single  64KB  check found an error (128 x
       512bytes = 64KB).

       If an array is created by mdadm with --assume-clean then  a  subsequent
       check could be expected to find some mismatches.

       On a truly clean RAID5 or RAID6 array, any mismatches should indicate a
       hardware problem at some level - software  issues  should  never  cause
       such a mismatch.

       However on RAID1 and RAID10 it is possible for software issues to cause
       a mismatch to be reported.  This does not  necessarily  mean  that  the
       data  on  the  array  is corrupted.  It could simply be that the system
       does not care what is stored on that part of the array - it  is  unused
       space.

       The  most  likely  cause  for an unexpected mismatch on RAID1 or RAID10
       occurs if a swap partition or swap file is stored on the array.

       When the swap subsystem wants to write a page of memory out,  it  flags
       the  page as 'clean' in the memory manager and requests the swap device
       to write it out.  It is quite possible that the memory will be  changed
       while  the  write-out is happening.  In that case the 'clean' flag will
       be found to be clear when the write completes and so the swap subsystem
       will  simply  forget  that  the  swapout  had  been attempted, and will
       possibly choose a different page to write out.

       If the swap device was on RAID1 (or RAID10), then the data is sent from
       memory to a device twice (or more depending on the number of devices in
       the array).  Thus it is possible that the memory gets  changed  between
       the times it is sent, so different data can be written to the different
       devices in the array.  This will be detected by check  as  a  mismatch.
       However  it  does  not  reflect  any corruption as the block where this
       mismatch occurs is being treated by the swap system as being empty, and
       the data will never be read from that block.

       It  is  conceivable for a similar situation to occur on non-swap files,
       though it is less likely.

       Thus the mismatch_cnt value can not be  interpreted  very  reliably  on
       RAID1 or RAID10, especially when the device is used for swap.

   BITMAP WRITE-INTENT LOGGING
       From  Linux  2.6.13,  md  supports a bitmap based write-intent log.  If
       configured, the bitmap is used to record which blocks of the array  may
       be  out  of  sync.   Before any write request is honoured, md will make
       sure that the corresponding bit in the log is set.  After a  period  of
       time with no writes to an area of the array, the corresponding bit will
       be cleared.

       This bitmap is used for two optimisations.

       Firstly, after an unclean shutdown, the resync process will consult the
       bitmap  and  only  resync  those  blocks that correspond to bits in the
       bitmap that are set.  This can dramatically reduce resync time.

       Secondly, when a drive fails and is removed from the  array,  md  stops
       clearing bits in the intent log.  If that same drive is re-added to the
       array, md will notice and will only recover the sections of  the  drive
       that  are  covered  by  bits  in the intent log that are set.  This can
       allow a device to be temporarily removed and reinserted without causing
       an enormous recovery cost.

       The  intent log can be stored in a file on a separate device, or it can
       be stored near the superblocks of an array which has superblocks.

       It is possible to add an intent log to an active array,  or  remove  an
       intent log if one is present.

       In  2.6.13, intent bitmaps are only supported with RAID1.  Other levels
       with redundancy are supported from 2.6.15.

   BAD BLOCK LOG
       From Linux 3.5 each device in an md array can store a  list  of  known-
       bad-blocks.   This list is 4K in size and usually positioned at the end
       of the space between the superblock and the data.

       When a block cannot be read and cannot  be  repaired  by  writing  data
       recovered from other devices, the address of the block is stored in the
       bad block log.  Similarly if an attempt to write  a  block  fails,  the
       address  will  be recorded as a bad block.  If attempting to record the
       bad block fails, the whole device will be marked faulty.

       Attempting to read from a known bad block  will  cause  a  read  error.
       Attempting  to  write to a known bad block will be ignored if any write
       errors have been reported by the device.  If there have been  no  write
       errors then the data will be written to the known bad block and if that
       succeeds, the address will be removed from the list.

       This allows an array  to  fail  more  gracefully  -  a  few  blocks  on
       different  devices  can be faulty without taking the whole array out of
       action.

       The log is particularly useful when recovering to a spare.   If  a  few
       blocks  cannot be read from the other devices, the bulk of the recovery
       can complete and those few bad blocks will be recorded in the bad block
       log.

   WRITE-BEHIND
       From Linux 2.6.14, md supports WRITE-BEHIND on RAID1 arrays.

       This allows certain devices in the array to be flagged as write-mostly.
       MD will only read from such devices if there is no other option.

       If a write-intent bitmap is also provided,  write  requests  to  write-
       mostly devices will be treated as write-behind requests and md will not
       wait for writes to those requests  to  complete  before  reporting  the
       write as complete to the filesystem.

       This  allows  for  a  RAID1 with WRITE-BEHIND to be used to mirror data
       over a slow link to a remote computer (providing  the  link  isn't  too
       slow).   The extra latency of the remote link will not slow down normal
       operations, but the remote system will still have a  reasonably  up-to-
       date copy of all data.

   RESTRIPING
       Restriping,  also  known as Reshaping, is the processes of re-arranging
       the data stored in each stripe into a new layout.  This  might  involve
       changing the number of devices in the array (so the stripes are wider),
       changing the chunk size  (so  stripes  are  deeper  or  shallower),  or
       changing the arrangement of data and parity (possibly changing the RAID
       level, e.g. 1 to 5 or 5 to 6).

       As of Linux 2.6.35, md can reshape a RAID4, RAID5, or  RAID6  array  to
       have  a  different  number  of  devices  (more  or fewer) and to have a
       different layout or chunk size.  It  can  also  convert  between  these
       different  RAID  levels.  It can also convert between RAID0 and RAID10,
       and between RAID0 and RAID4 or RAID5.  Other possibilities  may  follow
       in future kernels.

       During  any  stripe  process there is a 'critical section' during which
       live  data  is  being  overwritten  on  disk.   For  the  operation  of
       increasing  the  number  of  drives  in  a RAID5, this critical section
       covers the first few stripes (the number being the product of  the  old
       and  new  number  of  devices).  After this critical section is passed,
       data is only written to areas of the array which no  longer  hold  live
       data — the live data has already been located away.

       For  a  reshape  which  reduces  the  number  of devices, the 'critical
       section' is at the end of the reshape process.

       md is not able to ensure data preservation if there is  a  crash  (e.g.
       power failure) during the critical section.  If md is asked to start an
       array which failed during a critical section  of  restriping,  it  will
       fail to start the array.

       To deal with this possibility, a user-space program must

       ·   Disable  writes  to  that  section  of  the  array (using the sysfs
           interface),

       ·   take a copy of the data somewhere (i.e. make a backup),

       ·   allow the process to continue and invalidate the backup and restore
           write access once the critical section is passed, and

       ·   provide for restoring the critical data before restarting the array
           after a system crash.

       mdadm versions from 2.4 do this for growing a RAID5 array.

       For operations that do not change the size of the  array,  like  simply
       increasing  chunk  size,  or  converting  RAID5 to RAID6 with one extra
       device, the entire process is the critical section.  In this case,  the
       restripe  will  need  to progress in stages, as a section is suspended,
       backed up, restriped, and released.

   SYSFS INTERFACE
       Each block device appears as a directory in  sysfs  (which  is  usually
       mounted  at  /sys).   For  MD  devices,  this  directory will contain a
       subdirectory called md  which  contains  various  files  for  providing
       access to information about the array.

       This    interface    is    documented    more   fully   in   the   file
       Documentation/md.txt which is  distributed  with  the  kernel  sources.
       That  file  should  be consulted for full documentation.  The following
       are just a selection of attribute files that are available.

       md/sync_speed_min
              This  value,  if  set,  overrides  the  system-wide  setting  in
              /proc/sys/dev/raid/speed_limit_min for this array only.  Writing
              the value system to this file will cause the system-wide setting
              to have effect.

       md/sync_speed_max
              This   is   the   partner  of  md/sync_speed_min  and  overrides
              /proc/sys/dev/raid/speed_limit_max described below.

       md/sync_action
              This can be used to  monitor  and  control  the  resync/recovery
              process  of  MD.  In particular, writing "check" here will cause
              the array to read  all  data  block  and  check  that  they  are
              consistent  (e.g.  parity is correct, or all mirror replicas are
              the same).  Any discrepancies found are NOT corrected.

              A count of problems found will be stored in md/mismatch_count.

              Alternately, "repair" can be written which will cause  the  same
              check to be performed, but any errors will be corrected.

              Finally, "idle" can be written to stop the check/repair process.

       md/stripe_cache_size
              This  is only available on RAID5 and RAID6.  It records the size
              (in pages per device) of the  stripe cache  which  is  used  for
              synchronising  all  write  operations  to the array and all read
              operations if the array is degraded.  The default is 256.  Valid
              values  are  17  to  32768.  Increasing this number can increase
              performance in some situations, at some cost in  system  memory.
              Note,  setting  this  value  too  high  can result in an "out of
              memory" condition for the system.

              memory_consumed    =    system_page_size    *     nr_disks     *
              stripe_cache_size

       md/preread_bypass_threshold
              This  is  only available on RAID5 and RAID6.  This variable sets
              the number of times MD will service a  full-stripe-write  before
              servicing   a  stripe  that  requires  some  "prereading".   For
              fairness  this  defaults  to  1.   Valid   values   are   0   to
              stripe_cache_size.  Setting this to 0 maximizes sequential-write
              throughput at the cost of fairness to  threads  doing  small  or
              random writes.

   KERNEL PARAMETERS
       The md driver recognised several different kernel parameters.

       raid=noautodetect
              This will disable the normal detection of md arrays that happens
              at boot time.  If a  drive  is  partitioned  with  MS-DOS  style
              partitions, then if any of the 4 main partitions has a partition
              type of 0xFD, then that partition will normally be inspected  to
              see  if  it  is  part of an MD array, and if any full arrays are
              found, they are started.  This kernel  parameter  disables  this
              behaviour.

       raid=partitionable

       raid=part
              These  are  available  in  2.6  and  later  kernels  only.  They
              indicate that  autodetected  MD  arrays  should  be  created  as
              partitionable  arrays,  with  a different major device number to
              the original non-partitionable md arrays.  The device number  is
              listed as mdp in /proc/devices.

       md_mod.start_ro=1

       /sys/module/md_mod/parameters/start_ro
              This  tells md to start all arrays in read-only mode.  This is a
              soft read-only that will automatically switch to  read-write  on
              the  first  write  request.   However  until that write request,
              nothing is written to any device by md, and  in  particular,  no
              resync or recovery operation is started.

       md_mod.start_dirty_degraded=1

       /sys/module/md_mod/parameters/start_dirty_degraded
              As  mentioned  above, md will not normally start a RAID4, RAID5,
              or RAID6 that is both dirty and degraded as this  situation  can
              imply  hidden  data  loss.   This  can  be  awkward  if the root
              filesystem is affected.  Using this module parameter allows such
              arrays to be started at boot time.  It should be understood that
              there is a real (though small) risk of data corruption  in  this
              situation.

       md=n,dev,dev,...

       md=dn,dev,dev,...
              This  tells  the md driver to assemble /dev/md n from the listed
              devices.  It is only necessary to start the device  holding  the
              root  filesystem  this  way.  Other arrays are best started once
              the system is booted.

              In 2.6 kernels, the d immediately after the = indicates  that  a
              partitionable device (e.g.  /dev/md/d0) should be created rather
              than the original non-partitionable device.

       md=n,l,c,i,dev...
              This tells the md driver to assemble a legacy  RAID0  or  LINEAR
              array  without  a  superblock.   n gives the md device number, l
              gives the level, 0 for RAID0 or -1 for LINEAR, c gives the chunk
              size  as  a  base-2 logarithm offset by twelve, so 0 means 4K, 1
              means 8K.  i is ignored (legacy support).

FILES

       /proc/mdstat
              Contains information  about  the  status  of  currently  running
              array.

       /proc/sys/dev/raid/speed_limit_min
              A  readable  and  writable file that reflects the current "goal"
              rebuild speed for times when non-rebuild activity is current  on
              an  array.   The speed is in Kibibytes per second, and is a per-
              device rate, not a per-array rate (which  means  that  an  array
              with more disks will shuffle more data for a given speed).   The
              default is 1000.

       /proc/sys/dev/raid/speed_limit_max
              A readable and writable file that reflects  the  current  "goal"
              rebuild  speed for times when no non-rebuild activity is current
              on an array.  The default is 200,000.

SEE ALSO

       mdadm(8),

                                                                         MD(4)