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 the devices are often disk drives, hence the acro-
       nym 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 multi-
       ple  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 proces-

       The new format -- known as version 1 -- has a superblock that  is  nor-
       mally 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.

       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 crit-
              ical 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 gen-
              eral use, it does have special-purpose uses and is supported.

       From release 2.6.28, the md driver supports arrays with externally man-
       aged  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 meta-
       data - 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 num-
       ber 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

       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 arrange-
       ment 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 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).

       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 multi-
       ple 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  con-
       tains  the  "parity" for the whole stripe.  I.e. its content 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 writ-
       ing, 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  multi-
       ple 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 dif-
              ferent 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  possi-
              ble") 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  off-
              set 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  off-
              set 1  of each of the striped copies of <number of devices> con-
              secutive 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 differ-
       ent  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 prob-
       lems), 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 driv-
       ers 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 (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 requests of the rele-
       vant 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  tran-

       The 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, 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 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.  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 possi-
       bly 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 mis-
       match  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 bit-
       map 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 differ-
       ent devices can be faulty without taking the whole array out of action.

       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  addi-
       tional  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

       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.

       From  Linux  4.10,  md  supports  FAILFAST for RAID1 and RAID10 arrays.
       This is a flag that can be set on individual drives, though it is  usu-
       ally set on all drives, or no drives.

       When md sends an I/O request to a drive that is marked as FAILFAST, and
       when the array could survive the loss  of  that  drive  without  losing
       data,  md  will request that the underlying device does not perform any
       retries.  This means that a failure will be reported  to  md  promptly,
       and  it  can  mark  the  device  as faulty and continue using the other
       device(s).  md cannot control the timeout that the  underlying  devices
       use  to determine failure.  Any changes desired to that timeout must be
       set explictly on the underlying device, separately from using mdadm.

       If a FAILFAST request does fail, and if it is still safe  to  mark  the
       device  as  faulty  without  data loss, that will be done and the array
       will continue functioning on a reduced number of devices.  If it is not
       possible to safely mark the device as faulty, md will retry the request
       without disabling retries in the underlying device.  In  any  case,  md
       will  not  attempt to repair read errors on a device marked as FAILFAST
       by writing out the correct.  It will just mark the device as faulty.

       FAILFAST is appropriate for storage arrays that have a low  probability
       of  true  failure,  but will sometimes introduce unacceptable delays to
       I/O requests while performing internal maintenance.  The value of  set-
       ting  FAILFAST  involves  a  trade-off.  The gain is that the chance of
       unacceptable delays is substantially reduced.  The  cost  is  that  the
       unlikely  event  of  data-loss on one device is slightly more likely to
       result in data-loss for the array.

       When a device in an array using FAILFAST is marked as faulty,  it  will
       usually  become  usable again in a short while.  mdadm makes no attempt
       to detect that possibility.  Some separate mechanism, tuned to the spe-
       cific  details  of  the  expected failure modes, needs to be created to
       monitor devices to see when they return to full functionality,  and  to
       then re-add them to the array.  In order of this "re-add" functionality
       to be effective, an array using FAILFAST should always  have  a  write-
       intent bitmap.

       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 chang-
       ing  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 dif-
       ferent layout or chunk size.  It can also convert between these differ-
       ent  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  increas-
       ing  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 num-
       ber  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 sec-
       tion' 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

       o   Disable writes to that section of the array (using the sysfs inter-

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

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

       o   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 subdi-
       rectory called md which contains various files for providing access  to
       information about the array.

       This  interface  is  documented  more  fully  in  the  file  Documenta-
       tion/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 consis-
              tent  (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 mem-
              ory" 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 fair-
              ness  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  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  parameter  disables  this


              These  are  available in 2.6 and later kernels only.  They indi-
              cate that autodetected MD arrays should be created as partition-
              able  arrays, with a different major device number to the origi-
              nal 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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