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 de-
       vice),  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 re-
       lease, 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 ad-
       dress 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 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  ar-
              ray 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  en-
       sures  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  su-

              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 at-
       tribute.   The manual page for mdmon(8) contains more detail about this

       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 re-
       mainder of each device is available to be incorporated into various ar-

       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 ex-
       tra 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 de-
       vice, 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 bug was introduced in linux 3.14 which changed the layout  of  blocks
       in  a  RAID0  beyond the region that is striped over all devices.  This
       bug does not affect an array with all devices the same  size,  but  can
       affect other RAID0 arrays.

       Linux  5.4 (and some stable kernels to which the change was backported)
       will not normally assemble such an array as it cannot know which layout
       to  use.   There is a module parameter "raid0.default_layout" which can
       be set to "1" to force the kernel to use the pre-3.14 layout or to  "2"
       to  force  it  to  use  the 3.14-and-later layout.  when creating a new
       RAID0 array, mdadm will record the chosen layout in the metadata  in  a
       way  that  allows newer kernels to assemble the array without needing a
       module parameter.

       To assemble an old array on a new kernel without using the  module  pa-
       rameter,  use  either  the --update=layout-original option or the --up-
       date=layout-alternate option.

       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 re-
       quests 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 se-
       quential 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 de-
       vices 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. Un-
       like 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  ex-
       ported 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  De-
              vice #3 / #4  are  each  a RAID1) both in turn forming a striped

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

                    | 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 de-

                    | 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  se-
              quence  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 de-

                    | 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  de-

                    | 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  se-
              quential 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 de-

                    | 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  de-

                    | 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 de-
              vices> 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  ev-
       ery 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  de-
       vices, 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 in-

       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 de-
       vices, 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

       To  handle  this situation, the md driver marks an array as "dirty" be-
       fore 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 recalculat-
       ing  the  parity  for each stripe and making sure that the parity block
       has the correct data.  For RAID10 it involves copying one of the repli-
       cas  of each block onto all the others.  This process, known as "resyn-
       chronising" 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 de-
       vices.  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 calcu-
       lations  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 ef-
       fect  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  re-
       paired 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 oc-
       curs 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 al-
       low 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  re-
       covered  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.  At-
       tempting to write to a known bad block will be ignored if any write er-
       rors have been reported by the device.  If there have been no write er-
       rors  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 in-
       consistent 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  de-
       vice(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 un-
       acceptable  delays  is substantially reduced.  The cost is that the un-
       likely event of data-loss on one device is slightly more likely to  re-
       sult 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-in-
       tent 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 be-
       tween  RAID0 and RAID4 or RAID5.  Other possibilities may follow in fu-
       ture 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  de-
       vice,  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 op-
              erations  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 ar-

              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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