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MD(4)                      Kernel Interfaces Manual                      MD(4)

       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  con-
       tains  redundancy, and hence the acronym RAID which stands for a Redun-
       dant Array of Independent Devices.

       md supports RAID levels 1 (mirroring)  4  (striped  array  with  parity
       device),  5  (striped  array with distributed parity information) and 6
       (striped array with distributed dual  redundancy  information.)   If  a
       some  number  of underlying devices fails while using one of these lev-
       els, 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.

       md also supports a number of pseudo RAID (non-redundant) configurations
       including RAID0 (striped array), LINEAR (catenated array) and MULTIPATH
       (a set of different interfaces to the same device).

       With the exception of Legacy Arrays described below, each  device  that
       is  incorporated into an MD array has a super block written towards the
       end of the device.   This  superblock  records  information  about  the
       structure  and state of the array so that the array can be reliably re-
       assembled after a shutdown.

       The superblock 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.

       The superblock contains, among other things:

       LEVEL  The  manner  in  which  the  devices are arranged into the array
              (linear, raid0, raid1, raid4, raid5, multipath).

       UUID   a 128 bit Universally  Unique  Identifier  that  identifies  the
              array that this device is part of.

       Early versions of the md driver only supported Linear and Raid0 config-
       urations and so did not use an MD superblock (as there is no state that
       needs to be recorded).  While it is strongly recommended that all newly
       created arrays utilise a superblock to help ensure that they are assem-
       bled  properly, the md driver still supports legacy linear and raid0 md
       arrays that do not have a superblock.

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

       One  advantage  of this arrangement over the more common RAID0 arrange-
       ment is that the array may be reconfigured at  a  later  time  with  an
       extra drive and so the array is made bigger without disturbing the data
       that is on the array.  However this cannot be done on a live array.

       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 which
       are assigned to the remaining space in the drives.

       If devices in the array are not all the same size, then once the small-
       est  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.

       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  con-
       tains  the "parity" for the whole stripe.  i.e. its contents is equiva-
       lent 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  par-
       ity 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  dis-
       tributed 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.

       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 logical 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 to attempt to redirect requests to another  inter-

       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 filesystem.  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 persistant (subse-
       quent 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 request of the relevant
       time.  For example if persistent read faults have a period of 100, then
       ever  100th  read request would generate a fault, and the faulty sector
       would be recorded so that subsequent reads on that  sector  would  also

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

       It  list of faulty sectors can be flushed, and the active list of fail-
       ure modes can be cleared.

       When changes are made to a RAID1, RAID4, RAID5 or RAID6 array there  is
       a possibility of inconsistency for short periods of time as each update
       requires are least two block to be written to  different  devices,  and
       these  writes probably wont 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  inconsis-
       tency.   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 par-
       ity block has the correct data.  This process, known as  "resynchronis-
       ing"  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) when it is restarted after an unclean shutdown, it cannot recal-
       culate parity, and so it is possible that data  might  be  undetectably
       corrupted.   The 2.4 md driver does not alert the operator to this con-
       dition.  The 2.5 md driver will fail to start an array in  this  condi-
       tion without manual intervention.

       If the md driver detects any error on a device in a RAID1, RAID4, RAID5
       or RAID6 array, it immediately disables  that  device  (marking  it  as
       faulty)  and continues operation on the remaining devices.  If there is
       a spare drive, the driver will start recreating on  one  of  the  spare
       drives  the  data  what  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.

       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.

       The md driver recognised three 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  par-
              titions,  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 paramenter  disables  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.

              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 disc 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).