Provided by: mdadm_4.2-0ubuntu2_amd64 bug


       md - Multiple Device driver aka Linux Software RAID




       The  md  driver  provides  virtual  devices  that are created from one or more independent
       underlying devices.  This array of devices often contains redundancy and the  devices  are
       often  disk  drives,  hence  the  acronym  RAID  which  stands  for  a  Redundant Array of
       Independent Disks.

       md supports RAID levels 1 (mirroring), 4 (striped array with parity  device),  5  (striped
       array  with  distributed  parity  information),  6  (striped  array  with distributed dual
       redundancy information), and 10 (striped and mirrored).   If  some  number  of  underlying
       devices  fails  while using one of these levels, the array will continue to function; this
       number is one for RAID levels 4 and 5, two for RAID level 6, and all  but  one  (N-1)  for
       RAID level 1, and dependent on configuration for level 10.

       md  also  supports  a number of pseudo RAID (non-redundant) configurations including RAID0
       (striped array), LINEAR (catenated array), MULTIPATH (a set of different interfaces to the
       same device), and FAULTY (a layer over a single device into which errors can be injected).

       Each  device  in  an  array may have some metadata stored in the device.  This metadata is
       sometimes called a superblock.  The metadata records information about the  structure  and
       state of the array.  This allows the array to be reliably re-assembled after a shutdown.

       From  Linux  kernel  version  2.6.10,  md  provides  support  for two different formats of
       metadata, and other formats can be added.  Prior to  this  release,  only  one  format  is

       The  common  format  —  known  as  version  0.90 — has a superblock that is 4K long and is
       written into a 64K aligned block that starts at least 64K and less than 128K from the  end
       of the device (i.e. to get the address of the superblock round the size of the device down
       to a multiple of 64K and then subtract 64K).  The available size of  each  device  is  the
       amount  of  space before the super block, so between 64K and 128K is lost when a device in
       incorporated into an MD array.  This superblock stores multi-byte fields in  a  processor-
       dependent  manner,  so  arrays  cannot  easily  be  moved between computers with different

       The new format — known as version 1 — has a superblock that is normally 1K long,  but  can
       be  longer.   It is normally stored between 8K and 12K from the end of the device, on a 4K
       boundary, though variations can be stored at the start of the device (version 1.1)  or  4K
       from the start of the device (version 1.2).  This metadata format stores multibyte data in
       a processor-independent format and supports up to hundreds of component  devices  (version
       0.90 only supports 28).

       The metadata contains, among other things:

       LEVEL  The  manner in which the devices are arranged into the array (LINEAR, RAID0, RAID1,
              RAID4, RAID5, RAID10, MULTIPATH).

       UUID   a 128 bit Universally Unique Identifier that identifies  the  array  that  contains
              this device.

       When  a  version  0.90 array is being reshaped (e.g. adding extra devices to a RAID5), the
       version number is temporarily set to 0.91.  This ensures that if the  reshape  process  is
       stopped  in the middle (e.g. by a system crash) and the machine boots into an older kernel
       that does not support reshaping, then the array will not be assembled (which  would  cause
       data  corruption)  but will be left untouched until a kernel that can complete the reshape
       processes is used.

       While it is usually best to create arrays with superblocks so that they can  be  assembled
       reliably,  there  are  some  circumstances when an array without superblocks is preferred.
       These include:

              Early versions of the md driver only supported LINEAR and RAID0 configurations  and
              did not use a superblock (which is less critical with these configurations).  While
              such arrays should be rebuilt with superblocks if possible, md continues to support

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

              It is often possible to detect devices  which  are  different  paths  to  the  same
              storage  directly rather than having a distinctive superblock written to the device
              and searched for on all paths.  In this case, a MULTIPATH array with no  superblock
              makes sense.

       RAID1  In  some  configurations  it  might be desired to create a RAID1 configuration that
              does not use a superblock, and to maintain the state of the array elsewhere.  While
              not encouraged for general use, it does have special-purpose uses and is supported.

       From release 2.6.28, the md driver supports arrays with externally managed metadata.  That
       is, the metadata is not managed by the kernel but rather by a user-space program which  is
       external  to  the  kernel.   This allows support for a variety of metadata formats without
       cluttering the kernel with lots of details.

       md is able to communicate with the user-space program through various sysfs attributes  so
       that  it  can  make  appropriate changes to the metadata - for example to mark a device as
       faulty.  When necessary, md will wait for the program to acknowledge the event by  writing
       to  a  sysfs  attribute.   The  manual  page  for mdmon(8) contains more detail about this

       Many metadata formats use a single block of metadata to describe  a  number  of  different
       arrays  which  all use the same set of devices.  In this case it is helpful for the kernel
       to know about the full set of devices as a whole.  This set is known to md as a container.
       A  container  is  an  md array with externally managed metadata and with device offset and
       size so that it just covers the metadata part of  the  devices.   The  remainder  of  each
       device is available to be incorporated into various arrays.

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

       One advantage of this arrangement over the more common RAID0 arrangement is that the array
       may  be  reconfigured  at  a  later  time with an extra drive, so the array is made bigger
       without disturbing the data that is on the array.  This can even be done on a live array.

       If a chunksize is given with a LINEAR array, the usable space on each  device  is  rounded
       down to a multiple of this chunksize.

       A RAID0 array (which has zero redundancy) is also known as a striped array.  A RAID0 array
       is configured at creation with a Chunk Size which must be a power of two (prior  to  Linux
       2.6.31), and at least 4 kibibytes.

       The  RAID0  driver  assigns  the  first chunk of the array to the first device, the second
       chunk to the second device, and so on until all drives have been assigned one chunk.  This
       collection of chunks forms a stripe.  Further chunks are gathered into stripes in the same
       way, and are assigned to the remaining space in the drives.

       If devices in the array are not all the same size, then once the smallest device has  been
       exhausted,  the  RAID0 driver starts collecting chunks into smaller stripes that only span
       the drives which still have remaining space.

       A 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 parameter, use either
       the --update=layout-original option or the --update=layout-alternate option.

       Once you have updated the layout you will not be able to  mount  the  array  on  an  older
       kernel.   If  you  need to revert to an older kernel, the layout information can be erased
       with the --update=layout-unspecificed option.  If you use this option to --assemble  while
       running  a  newer  kernel, the array will NOT assemble, but the metadata will be update so
       that it can be assembled on an older kernel.

       No that setting the layout to "unspecified" removes protections against this bug, and  you
       must be sure that the kernel you use matches the layout of the array.

       A  RAID1  array  is also known as a mirrored set (though mirrors tend to provide reflected
       images, which RAID1 does not) or a plex.

       Once initialised, each device in a RAID1 array contains exactly the  same  data.   Changes
       are  written  to  all  devices in parallel.  Data is read from any one device.  The driver
       attempts to distribute read requests across all devices to maximise performance.

       All devices in a RAID1 array should be the same size.  If they  are  not,  then  only  the
       amount of space available on the smallest device is used (any extra space on other devices
       is wasted).

       Note that the read balancing done by the  driver  does  not  make  the  RAID1  performance
       profile  be  the  same  as  for  RAID0;  a  single  stream of sequential input will not be
       accelerated (e.g. a single dd), but multiple sequential streams or a random workload  will
       use  more  than  one  spindle.  In  theory, having an N-disk RAID1 will allow N sequential
       threads to read from all disks.

       Individual devices in a RAID1 can be marked as "write-mostly".  These drives are  excluded
       from  the  normal read balancing and will only be read from when there is no other option.
       This can be useful for devices connected over a slow link.

       A RAID4 array is like a RAID0 array with an extra device for storing parity.  This  device
       is the last of the active devices in the array. Unlike RAID0, RAID4 also requires that all
       stripes span all drives, so extra space on devices that are larger than  the  smallest  is

       When  any  block  in a RAID4 array is modified, the parity block for that stripe (i.e. the
       block in the parity device at the same device offset as the stripe) is  also  modified  so
       that the parity block always contains the "parity" for the whole stripe.  I.e. its content
       is equivalent to the result of performing an exclusive-or operation between all  the  data
       blocks in the stripe.

       This  allows  the array to continue to function if one device fails.  The data that was on
       that device can be calculated as needed from the parity block and the other data blocks.

       RAID5 is very similar to RAID4.  The difference is that the parity blocks for each stripe,
       instead of being on a single device, are distributed across all devices.  This allows more
       parallelism when writing, as two different block updates will quite possibly affect parity
       blocks on different devices so there is less contention.

       This  also allows more parallelism when reading, as read requests are distributed over all
       the devices in the array instead of all but one.

       RAID6 is similar to RAID5, but can handle the loss of any two devices without  data  loss.
       Accordingly, it requires N+2 drives to store N drives worth of data.

       The  performance  for  RAID6  is slightly lower but comparable to RAID5 in normal mode and
       single disk failure mode.  It is very slow in dual disk failure mode, however.

       RAID10 provides a combination of RAID1 and RAID0,  and  is  sometimes  known  as  RAID1+0.
       Every  datablock  is  duplicated  some  number  of  times, and the resulting collection of
       datablocks are distributed over multiple drives.

       When configuring a RAID10 array, it is necessary to specify the number of replicas of each
       data  block  that are required (this will usually be 2) and whether their layout should be
       "near", "far" or "offset" (with "offset" being available since Linux 2.6.18).

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

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

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

        "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

              With  an  even  number  of  devices,  they will likely (unless some misalignment is
              present) lay at the very same offset on the different devices.
              This is as the "classic" RAID1+0; that is two groups of mirrored  devices  (in  the
              example  below  the groups Device #1 / #2 and Device #3 / #4 are each a RAID1) both
              in turn forming a striped RAID0.

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

                    │ 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 devices:

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

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

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

              The "shift" needed to prevent placing copies of the same chunks on the same devices
              is  actually  a  cyclic  permutation  with offset 1 of each of the stripes within a
              complete sequence of chunks.
              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 devices:

                    │ 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 devices:

                    │ 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 slower.

       "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 blocks).

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

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

                    │ 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 devices:

                    │ 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

       Finally, it is possible to have an array with both "near" and "far" copies.  If  an  array
       is  configured with 2 near copies and 2 far copies, then there will be a total of 4 copies
       of each block, each on a different drive.  This is an artifact of the  implementation  and
       is unlikely to be of real value.

       MULTIPATH  is  not really a RAID at all as there is only one real device in a MULTIPATH md
       array.  However there are multiple access points (paths) to this device, and one of  these
       paths might fail, so there are some similarities.

       A  MULTIPATH  array  is  composed  of a number of logically different devices, often fibre
       channel interfaces, that all refer the the same real device. If one  of  these  interfaces
       fails (e.g. due to cable problems), the MULTIPATH driver will attempt to redirect requests
       to another interface.

       The MULTIPATH drive is not receiving any ongoing development and should  be  considered  a
       legacy  driver.   The  device-mapper  based  multipath drivers should be preferred for new

       The FAULTY md module is provided for testing purposes.  A FAULTY  array  has  exactly  one
       component  device  and is normally assembled without a superblock, so the md array created
       provides direct access to all of the data in the component device.

       The FAULTY module may be requested to simulate faults to allow testing of other md  levels
       or  of  filesystems.   Faults can be chosen to trigger on read requests or write requests,
       and can be transient (a subsequent read/write at the address  will  probably  succeed)  or
       persistent  (subsequent  read/write  of the same address will fail).  Further, read faults
       can be "fixable" meaning that they persist until a write request at the same address.

       Fault types can be requested with a period.  In this case, the fault will recur repeatedly
       after  the  given number of requests of the relevant type.  For example if persistent read
       faults have a period of 100, then every 100th read request would generate a fault, and the
       faulty sector would be recorded so that subsequent reads on that sector would also fail.

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

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

       When  changes  are  made  to  a  RAID1,  RAID4,  RAID5,  RAID6, or RAID10 array there is a
       possibility of inconsistency for short periods of time as each update  requires  at  least
       two  block  to  be written to different devices, and these writes probably won't happen at
       exactly the same time.  Thus if a system with one of  these  arrays  is  shutdown  in  the
       middle of a write operation (e.g. due to power failure), the array may not be consistent.

       To  handle this situation, the md driver marks an array as "dirty" before writing any data
       to it, and marks it as "clean" when the array is being disabled, e.g. at shutdown.  If the
       md  driver  finds  an  array  to  be dirty at startup, it proceeds to correct any possibly
       inconsistency.  For RAID1, this involves copying the contents of the first drive onto  all
       other  drives.  For RAID4, RAID5 and RAID6 this involves recalculating the parity for each
       stripe and making sure that the parity block has the correct data.  For RAID10 it involves
       copying  one  of  the  replicas of each block onto all the others.  This process, known as
       "resynchronising" or "resync" is performed in the background.   The  array  can  still  be
       used, though possibly with reduced performance.

       If  a  RAID4, RAID5 or RAID6 array is degraded (missing at least one drive, two for RAID6)
       when it is restarted after an unclean shutdown, it cannot recalculate parity, and so it is
       possible  that data might be undetectably corrupted.  The 2.4 md driver does not alert the
       operator to this condition.  The 2.6 md driver  will  fail  to  start  an  array  in  this
       condition without manual intervention, though this behaviour can be overridden by a kernel

       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 space.

       The  most  likely  cause  for  an  unexpected mismatch on RAID1 or RAID10 occurs if a swap
       partition or swap file is stored on the array.

       When the swap subsystem wants to write a page of memory out, it flags the page as  'clean'
       in  the memory manager and requests the swap device to write it out.  It is quite possible
       that the memory will be changed while the  write-out  is  happening.   In  that  case  the
       'clean'  flag will be found to be clear when the write completes and so the swap subsystem
       will simply forget that the swapout  had  been  attempted,  and  will  possibly  choose  a
       different page to write out.

       If the swap device was on RAID1 (or RAID10), then the data is sent from memory to a device
       twice (or more depending on the number of devices in the array).  Thus it is possible that
       the  memory gets changed between the times it is sent, so different data can be written to
       the different devices in the array.  This  will  be  detected  by  check  as  a  mismatch.
       However  it  does  not  reflect  any corruption as the block where this mismatch occurs is
       being treated by the swap system as being empty, and the data will never be read from that

       It  is  conceivable  for a similar situation to occur on non-swap files, though it is less

       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

       This bitmap is used for two optimisations.

       Firstly,  after  an  unclean shutdown, the resync process will consult the bitmap and only
       resync those blocks that correspond to  bits  in  the  bitmap  that  are  set.   This  can
       dramatically reduce resync time.

       Secondly,  when a drive fails and is removed from the array, md stops clearing bits in the
       intent log.  If that same drive is re-added to the array, md will  notice  and  will  only
       recover the sections of the drive that are covered by bits in the intent log that are set.
       This can allow a device to be  temporarily  removed  and  reinserted  without  causing  an
       enormous recovery cost.

       The  intent log can be stored in a file on a separate device, or it can be stored near the
       superblocks of an array which has superblocks.

       It is possible to add an intent log to an active array, or remove an intent log if one  is

       In 2.6.13, intent bitmaps are only supported with RAID1.  Other levels with redundancy are
       supported from 2.6.15.

       From Linux 3.5 each device in an md array can store a list of known-bad-blocks.  This list
       is  4K  in  size and usually positioned at the end of the space between the superblock and
       the data.

       When a block cannot be read and cannot be repaired by writing data  recovered  from  other
       devices,  the  address  of  the  block  is  stored in the bad block list.  Similarly if an
       attempt to write a block fails,  the  address  will  be  recorded  as  a  bad  block.   If
       attempting to record the bad block fails, the whole device will be marked faulty.

       Attempting to read from a known bad block will cause a read error.  Attempting to write to
       a known bad block will be ignored if any write errors have been reported  by  the  device.
       If  there  have  been no write errors then the data will be written to the known bad block
       and if that succeeds, the address will be removed from the list.

       This allows an array to fail more gracefully - a few blocks on different  devices  can  be
       faulty without taking the whole array out of 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 list.

       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 md supports two mechanisms described

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

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

              From  Linux  4.12  md  supports  Partial  Parity  Log  (PPL) for RAID5 arrays only.
              Partial parity for a write operation is the XOR of stripe data chunks not  modified
              by  the  write.  PPL  is  stored  in  the metadata region of RAID member drives, no
              additional journal drive is needed.  After crashes, if one of the not modified data
              disks  of  the  stripe  is  missing, this updated parity can be used to recover its

              This mechanism is documented more fully in the file Documentation/md/raid5-ppl.rst

       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 usually 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  explicitly  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 setting 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

       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 specific 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  changing  the  arrangement of data and parity (possibly changing the RAID
       level, e.g. 1 to 5 or 5 to 6).

       As of Linux 2.6.35, md can reshape a RAID4, RAID5, or RAID6  array  to  have  a  different
       number  of  devices  (more or fewer) and to have a different layout or chunk size.  It can
       also convert between these different RAID levels.  It can also convert between  RAID0  and
       RAID10,  and  between  RAID0 and RAID4 or RAID5.  Other possibilities may follow in future

       During any stripe process there is a 'critical section' during which live  data  is  being
       overwritten  on  disk.   For  the operation of increasing the number of drives in a RAID5,
       this critical section covers the first few stripes (the number being the  product  of  the
       old  and  new  number  of  devices).   After this critical section is passed, data is only
       written to areas of the array which no longer hold live data — the live data  has  already
       been located away.

       For a reshape which reduces the number of devices, the 'critical section' is at the end of
       the reshape process.

       md is not able to ensure data preservation if there is a crash (e.g. power failure) during
       the  critical  section.   If  md is asked to start an array which failed during a critical
       section of restriping, it will fail to start the array.

       To deal with this possibility, a user-space program must

       •   Disable writes to that section of the array (using the sysfs interface),

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

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

       •   provide  for  restoring  the  critical data before restarting the array after a system

       mdadm versions from 2.4 do this for growing a RAID5 array.

       For operations that do not change the size of the  array,  like  simply  increasing  chunk
       size,  or  converting  RAID5  to  RAID6  with  one extra device, the entire process is the
       critical section.  In this case, the restripe will  need  to  progress  in  stages,  as  a
       section is suspended, backed up, restriped, and released.

       Each block device appears as a directory in sysfs (which is usually mounted at /sys).  For
       MD devices, this directory will contain a subdirectory called md  which  contains  various
       files for providing access to information about the array.

       This interface is documented more fully in the file Documentation/admin-guide/md.rst which
       is distributed  with  the  kernel  sources.   That  file  should  be  consulted  for  full
       documentation.  The following are just a selection of attribute files that are available.

              This     value,     if     set,    overrides    the    system-wide    setting    in
              /proc/sys/dev/raid/speed_limit_min for this array only.  Writing the  value  system
              to this file will cause the system-wide setting to have effect.

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

              This can be used to monitor and control the  resync/recovery  process  of  MD.   In
              particular,  writing  "check"  here will cause the array to read all data block and
              check that they are consistent (e.g. parity is correct, or all mirror replicas  are
              the same).  Any discrepancies found are NOT corrected.

              A count of problems found will be stored in md/mismatch_count.

              Alternately,  "repair"  can  be  written  which  will  cause  the  same check to be
              performed, but any errors will be corrected.

              Finally, "idle" can be written to stop the check/repair process.

              This is only available on RAID5 and RAID6.  It  records  the  size  (in  pages  per
              device)  of  the  stripe cache which is used for synchronising all write operations
              to the array and all read operations if the array is degraded.  The default is 256.
              Valid  values  are 17 to 32768.  Increasing this number can increase performance in
              some situations, at some cost in system memory.  Note, setting this value too  high
              can result in an "out of memory" condition for the system.

              memory_consumed = system_page_size * nr_disks * stripe_cache_size

              This  is only available on RAID5 and RAID6.  This variable sets the number of times
              MD will service a full-stripe-write before servicing a stripe  that  requires  some
              "prereading".    For   fairness  this  defaults  to  1.   Valid  values  are  0  to
              stripe_cache_size.  Setting this to 0 maximizes sequential-write throughput at  the
              cost of fairness to threads doing small or random writes.

              The value stored in the file only has any effect on RAID1 when write-mostly devices
              are active, and write requests to those devices are proceed in the background.

              This variable sets a limit on the number of concurrent background writes, the valid
              values  are  0  to 16383, 0 means that write-behind is not allowed, while any other
              number means it can happen.  If there are more write requests than the number,  new
              writes will by synchronous.

              This  is for externally managed bitmaps, where the kernel writes the bitmap itself,
              but metadata describing the bitmap is managed by mdmon or similar.

              When the array is degraded, bits mustn't be cleared. When the array becomes optimal
              again, bit can be cleared, but first the metadata needs to record the current event
              count. So md sets this to 'false'  and  notifies  mdmon,  then  mdmon  updates  the
              metadata and writes 'true'.

              There is no code in mdmon to actually do this, so maybe it doesn't even work.

              The  bitmap  chunksize  can only be changed when no bitmap is active, and the value
              should be power of 2 and at least 512.

              This indicates where the write-intent bitmap for the array is stored.   It  can  be
              "none"  or "file" or a signed offset from the array metadata - measured in sectors.
              You cannot set  a  file  by  writing  here  -  that  can  only  be  done  with  the
              SET_BITMAP_FILE ioctl.

              Write  'none'  to  'bitmap/location'  will  clear bitmap, and the previous location
              value must be write to it to restore bitmap.

              This keeps track of the maximum number of concurrent write-behind requests  for  an
              md array, writing any value to this file will clear it.

              This  can be 'internal' or 'clustered' or 'external'. 'internal' is set by default,
              which means the metadata for bitmap is stored in the first 256 bytes of the  bitmap
              space.  'clustered'  means separate bitmap metadata are used for each cluster node.
              'external' means that bitmap metadata is managed externally to the kernel.

              This shows the space (in sectors) which is  available  at  md/bitmap/location,  and
              allows  the  kernel to know when it is safe to resize the bitmap to match a resized
              array. It should big enough to contain the total bytes in the bitmap.

              For 1.0 metadata, assume we can use up to the superblock  if  before,  else  to  4K
              beyond superblock. For other metadata versions, assume no change is possible.

              This  shows  the time (in seconds) between disk flushes, and is used to looking for
              bits in the bitmap to be cleared.

              The default value is 5 seconds, and it should be an unsigned long value.

       The md driver recognised several different kernel parameters.

              This will disable the normal detection of md arrays that happens at boot time.   If
              a  drive  is  partitioned  with  MS-DOS style partitions, then if any of the 4 main
              partitions has a partition type of 0xFD,  then  that  partition  will  normally  be
              inspected  to  see  if it is part of an MD array, and if any full arrays are found,
              they are started.  This kernel parameter disables this behaviour.


              These are available in 2.6 and later kernels only.  They indicate that autodetected
              MD  arrays should be created as partitionable arrays, with a different major device
              number to the original non-partitionable md arrays.  The device number is listed as
              mdp in /proc/devices.


              This tells md to start all arrays in read-only mode.  This is a soft read-only that
              will automatically switch to read-write on the first write request.  However  until
              that  write  request, nothing is written to any device by md, and in particular, no
              resync or recovery operation is started.


              As mentioned above, md will not normally start a RAID4, RAID5,  or  RAID6  that  is
              both  dirty and degraded as this situation can imply hidden data loss.  This can be
              awkward if the root filesystem is affected.  Using  this  module  parameter  allows
              such  arrays  to  be started at boot time.  It should be understood that there is a
              real (though small) risk of data corruption in this situation.


              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

              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 array.

              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

              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.