md - Multiple Device driver aka Linux Software RAID




   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).

   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

   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.

   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:

          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

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

          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.

   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.

   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.

   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.

   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

   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.

   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.

   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 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  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  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 usually
   be 2) and whether their layout should  be  "near",  "far"  or  "offset"
   (with "offset" being available since Linux 2.6.18).

   About the RAID10 Layout Examples:
   The  examples  below visualise the chunk distribution on the underlying
   devices for the respective layout.

   For simplicity it is assumed that the size of  the  chunks  equals  the
   size  of  the  blocks of the underlying devices as well as those of the
   RAID10 device exported by the kernel (for example /dev/md/name).
   Therefore the chunks / chunk numbers map directly to the  blocks /block
   addresses of the exported RAID10 device.

   Decimal  numbers (0, 1, 2, ...) are the chunks of the RAID10 and due to
   the above assumption  also  the  blocks  and  block  addresses  of  the
   exported RAID10 device.
   Repeated numbers mean copies of a chunk / block (obviously on different
   underlying devices).
   Hexadecimal numbers (0x00, 0x01, 0x02, ...) are the block addresses  of
   the underlying devices.

    "near" Layout
          When  "near" replicas are chosen, the multiple copies of a given
          chunk are laid out consecutively ("as close  to  each  other  as
          possible") across the stripes of the array.

          With  an  even  number of devices, they will likely (unless some
          misalignment is present) lay at the  very  same  offset  on  the
          different devices.
          This is as the "classic" RAID1+0; that is two groups of mirrored
          devices (in the example  below  the  groups  Device #1 / #2  and
          Device #3 / #4  are each a RAID1) both in turn forming a striped

          Example with 2 copies  per  chunk  and  an  even  number (4)  of

                 Device #1  Device #2  Device #3  Device #4 
          0x00      0          0          1          1     
          0x01      2          2          3          3     
          ...      ...        ...        ...        ...    
           :        :          :          :          :     
          ...      ...        ...        ...        ...    
          0x80     254        254        255        255    
                  \---------v---------/   \---------v---------/
                          RAID1                   RAID1

          Example  with  2 copies  per  chunk  and  an  odd  number (5) of

                 Dev #1  Dev #2  Dev #3  Dev #4  Dev #5 
          0x00    0       0       1       1       2    
          0x01    2       3       3       4       4    
          ...    ...     ...     ...     ...     ...   
           :      :       :       :       :       :    
          ...    ...     ...     ...     ...     ...   
          0x80   317     318     318     319     319   

    "far" Layout
          When "far" replicas are chosen, the multiple copies of  a  given
          chunk  are  laid  out  quite  distant  ("as  far  as  reasonably
          possible") from each other.

          First a complete sequence of all data blocks (that  is  all  the
          data  one  sees  on the exported RAID10 block device) is striped
          over the  devices.  Then  another  (though  "shifted")  complete
          sequence of all data blocks; and so on (in the case of more than
          2 copies per chunk).

          The "shift" needed to prevent placing copies of the same  chunks
          on  the  same  devices  is  actually  a  cyclic permutation with
          offset 1 of each of the stripes within a  complete  sequence  of
          The  offset 1  is  relative to the previous complete sequence of
          chunks, so in case of more than 2 copies per chunk one gets  the
          following offsets:
          1. complete sequence of chunks: offset =  0
          2. complete sequence of chunks: offset =  1
          3. complete sequence of chunks: offset =  2
          n. complete sequence of chunks: offset = n-1

          Example  with  2 copies  per  chunk  and  an  even number (4) of

                 Device #1  Device #2  Device #3  Device #4 
          0x00      0          1          2          3      \
          0x01      4          5          6          7      > [#]
          ...      ...        ...        ...        ...     :
           :        :          :          :          :      :
          ...      ...        ...        ...        ...     :
          0x40     252        253        254        255     /
          0x41      3          0          1          2      \
          0x42      7          4          5          6      > [#]~
          ...      ...        ...        ...        ...     :
           :        :          :          :          :      :
          ...      ...        ...        ...        ...     :
          0x80     255        252        253        254     /

          Example with  2 copies  per  chunk  and  an  odd  number (5)  of

                 Dev #1  Dev #2  Dev #3  Dev #4  Dev #5 
          0x00    0       1       2       3       4     \
          0x01    5       6       7       8       9     > [#]
          ...    ...     ...     ...     ...     ...    :
           :      :       :       :       :       :     :
          ...    ...     ...     ...     ...     ...    :
          0x40   315     316     317     318     319    /
          0x41    4       0       1       2       3     \
          0x42    9       5       6       7       8     > [#]~
          ...    ...     ...     ...     ...     ...    :
           :      :       :       :       :       :     :
          ...    ...     ...     ...     ...     ...    :
          0x80   319     315     316     317     318    /

          With  [#] being  the  complete  sequence  of chunks and [#]~ the
          cyclic permutation with offset 1 thereof (in the  case  of  more
          than     2     copies     per     chunk     there    would    be
          ([#]~)~, (([#]~)~)~, ...).

          The advantage of this  layout  is  that  MD  can  easily  spread
          sequential  reads over the devices, making them similar to RAID0
          in terms of speed.
          The cost is more seeking for writes, making  them  substantially

   "offset" Layout
          When  "offset"  replicas  are  chosen, all the copies of a given
          chunk are striped consecutively ("offset by  the  stripe  length
          after each other") over the devices.

          Explained  in detail, <number of devices> consecutive chunks are
          striped over the devices, immediately followed  by  a  "shifted"
          copy  of  these  chunks (and by further such "shifted" copies in
          the case of more than 2 copies per chunk).
          This pattern repeats for all further consecutive chunks  of  the
          exported  RAID10  device  (in  other  words:  all  further  data

          The "shift" needed to prevent placing copies of the same  chunks
          on  the  same  devices  is  actually  a  cyclic permutation with
          offset 1 of each of the striped copies of  <number  of  devices>
          consecutive chunks.
          The offset 1 is relative to the previous striped copy of <number
          of devices> consecutive chunks, so in case of more than 2 copies
          per chunk one gets the following offsets:
          1. <number of devices> consecutive chunks: offset =  0
          2. <number of devices> consecutive chunks: offset =  1
          3. <number of devices> consecutive chunks: offset =  2
          n. <number of devices> consecutive chunks: offset = n-1

          Example  with  2 copies  per  chunk  and  an  even number (4) of

                 Device #1  Device #2  Device #3  Device #4 
          0x00      0          1          2          3      ) AA
          0x01      3          0          1          2      ) AA~
          0x02      4          5          6          7      ) AB
          0x03      7          4          5          6      ) AB~
          ...      ...        ...        ...        ...     ) ...
           :        :          :          :          :        :
          ...      ...        ...        ...        ...     ) ...
          0x79     251        252        253        254     ) EX
          0x80     254        251        252        253     ) EX~

          Example with  2 copies  per  chunk  and  an  odd  number (5)  of

                 Dev #1  Dev #2  Dev #3  Dev #4  Dev #5 
          0x00    0       1       2       3       4     ) AA
          0x01    4       0       1       2       3     ) AA~
          0x02    5       6       7       8       9     ) AB
          0x03    9       5       6       7       8     ) AB~
          ...    ...     ...     ...     ...     ...    ) ...
           :      :       :       :       :       :       :
          ...    ...     ...     ...     ...     ...    ) ...
          0x79   314     315     316     317     318    ) EX
          0x80   318     314     315     316     317    ) EX~

          With  AA, AB, ...,  AZ, BA, ...  being  the  sets  of <number of
          devices> consecutive chunks and AA~, AB~, ..., AZ~, BA~, ... the
          cyclic  permutations  with offset 1 thereof (in the case of more
          than 2 copies per chunk there would be (AA~)~, ...  as  well  as
          ((AA~)~)~, ... and so on).

          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  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.

   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

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

   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.

   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.

   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

   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.

   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.

   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 list.  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

   The list 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

   Due  to  non-atomicity nature of RAID write operations, interruption of
   write operations (system crash, etc.) to  RAID456  array  can  lead  to
   inconsistent parity and data loss (so called RAID-5 write hole).

   To  plug  the write hole, from Linux 4.4 (to be confirmed), md supports
   write ahead  journal  for  RAID456.  When  the  array  is  created,  an
   additional  journal  device  can  be  added to the array through write-
   journal option. The RAID write journal works  similar  to  file  system
   journals.   Before  writing  to  the  data  disks, md persists data AND
   parity of the stripe to the journal device. After crashes, md  searches
   the  journal device for incomplete write operations, and replay them to
   the data disks.

   When the journal device fails, the RAID array is forced to run in read-
   only mode.

   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,  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

   *   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.

   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.

          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.

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

          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.

          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     *

          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.

   The md driver recognised several different kernel parameters.

          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


          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.


          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.


          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


          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.

          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).


          Contains information  about  the  status  of  currently  running

          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.

          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.





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