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       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 a superblock which 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  this superblock, 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 superblock format stores multibyte data
       in  a  processor-independent  format  and  supports  up  to hundreds of
       component devices (version 0.90 only supports 28).

       The superblock 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.

       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 us, it does have special-purpose uses and is  supported.

       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, 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.

       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.

       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 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

       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 degraded 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

       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 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.

       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 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 2.6.17, md can reshape a raid5 array to have more devices.
       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.

       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; this is not yet implemented.

       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/spool_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 read and write operations to the  array.   The
              default is 128.  Increasing this number can increase performance
              in some situations, at some cost in system memory.

       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 100.

              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 100,000.


       mdadm(8), mkraid(8).