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MD(4)                                Kernel Interfaces Manual                               MD(4)

NAME
       md - Multiple Device driver aka Linux Software RAID

SYNOPSIS
       /dev/mdn
       /dev/md/n
       /dev/md/name

DESCRIPTION
       The  md  driver provides virtual devices that are created from one or more independent un-
       derlying devices.  This array of devices often contains redundancy and the devices are of-
       ten  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 redun-
       dancy 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).

   MD METADATA
       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 meta-
       data, and other formats can be added.  Prior to this release,  only  one  format  is  sup-
       ported.

       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 pro-
       cessors.

       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.

   ARRAYS WITHOUT METADATA
       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:

       LEGACY ARRAYS
              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
              them.

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

       MULTIPATH
              It is often possible to detect devices which are different paths to the same  stor-
              age  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.

   ARRAYS WITH EXTERNAL METADATA
       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 inter-
       action.

   CONTAINERS
       Many metadata formats use a single block of metadata to describe a number of different ar-
       rays  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.

   LINEAR
       A  LINEAR  array simply catenates the available space on each drive to form one large vir-
       tual 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 with-
       out 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.

   RAID0
       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 ker-
       nels 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  ker-
       nel.   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  run-
       ning  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.

   RAID1
       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 at-
       tempts 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  pro-
       file 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.

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

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

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

   RAID5
       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
       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
       RAID10 provides a combination of RAID1 and RAID0, and is sometimes known as RAID1+0.   Ev-
       ery  datablock  is  duplicated  some number of times, and the resulting collection of dat-
       ablocks 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  re-
       spective 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  assump-
       tion also the blocks and block addresses of the exported RAID10 device.
       Repeated  numbers  mean  copies  of a chunk / block (obviously on different underlying de-
       vices).
       Hexadecimal numbers (0x00, 0x01, 0x02, ...) are the block addresses of the underlying  de-
       vices.

        "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  ar-
              ray.

              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
                      \---------------------v---------------------/
                                          RAID0

              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 com-
              plete 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 con-
              secutively ("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 de-
              vice (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>  con-
              secutive  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
       devices.

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

   MULTIPATH
       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 in-
       stallations.

   FAULTY
       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
       cleared.

   UNCLEAN SHUTDOWN
       When  changes  are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array there is a possi-
       bility 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 ex-
       actly 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 in-
       consistency.  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  condi-
       tion  without manual intervention, though this behaviour can be overridden by a kernel pa-
       rameter.

   RECOVERY
       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 op-
       eration on the remaining devices.  If there are spare drives, the driver will start recre-
       ating  on one of the spare drives the data which was on that failed drive, either by copy-
       ing 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 er-
       ror.  In later kernels, a read-error will instead cause md to attempt a recovery by  over-
       writing  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 ac-
       cess 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 men-
       tioned below.

   SCRUBBING AND MISMATCHES
       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  num-
       ber  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  ex-
       pected 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 par-
       tition 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 differ-
       ent 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.   How-
       ever  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
       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, espe-
       cially when the device is used for swap.

   BITMAP WRITE-INTENT LOGGING
       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 pe-
       riod of time with no writes to an area  of  the  array,  the  corresponding  bit  will  be
       cleared.

       This bitmap is used for two optimisations.

       Firstly,  after  an  unclean shutdown, the resync process will consult the bitmap and only
       resync those blocks that correspond to bits in the bitmap that are set.  This can dramati-
       cally 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 re-
       cover  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  enor-
       mous 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.

   BAD BLOCK LIST
       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 at-
       tempt 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.

   RAID WRITE HOLE
       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
       below.

       DIRTY STRIPE JOURNAL
              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-jour-
              nal 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  opera-
              tions, and replay them to the data disks.

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

       PARTIAL PARITY LOG
              From  Linux  4.12 md supports Partial Parity Log (PPL) for RAID5 arrays only.  Par-
              tial 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 addi-
              tional 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
              data.

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

   WRITE-BEHIND
       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 com-
       plete 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.

   FAILFAST
       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  under-
       lying  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,  sepa-
       rately 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
       array.

       When  a  device in an array using FAILFAST is marked as faulty, it will usually become us-
       able 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 ar-
       ray using FAILFAST should always have a write-intent bitmap.

   RESTRIPING
       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  shal-
       lower),  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 num-
       ber  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
       kernels.

       During any stripe process there is a 'critical section' during which live  data  is  being
       overwritten  on  disk.   For  the operation of increasing the number of drives in a RAID5,
       this critical section covers the first few stripes (the number being the  product  of  the
       old and new number of devices).  After this critical section is passed, data is only writ-
       ten 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

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

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

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

       o   provide  for  restoring  the  critical data before restarting the array after a system
           crash.

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

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

   SYSFS INTERFACE
       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 documenta-
       tion.  The following are just a selection of attribute files that are available.

       md/sync_speed_min
              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.

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

       md/sync_action
              This can be used to monitor and control the resync/recovery process of MD.  In par-
              ticular, 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 per-
              formed, but any errors will be corrected.

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

       md/stripe_cache_size
              This is only available on RAID5 and RAID6.  It records the size (in pages  per  de-
              vice)  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

       md/preread_bypass_threshold
              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.

       md/bitmap/backlog
              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.

       md/bitmap/can_clear
              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  meta-
              data and writes 'true'.

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

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

       md/bitmap/location
              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_BIT-
              MAP_FILE ioctl.

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

       md/bitmap/max_backlog_used
              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.

       md/bitmap/metadata
              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.

       md/bitmap/space
              This shows the space (in sectors) which is available at md/bitmap/location, and al-
              lows the kernel to know when it is safe to resize the bitmap to match a resized ar-
              ray. 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  be-
              yond superblock. For other metadata versions, assume no change is possible.

       md/bitmap/time_base
              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.

   KERNEL PARAMETERS
       The md driver recognised several different kernel parameters.

       raid=noautodetect
              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 par-
              titions has a partition type of 0xFD, then that  partition  will  normally  be  in-
              spected 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.

       raid=partitionable

       raid=part
              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.

       md_mod.start_ro=1

       /sys/module/md_mod/parameters/start_ro
              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.

       md_mod.start_dirty_degraded=1

       /sys/module/md_mod/parameters/start_dirty_degraded
              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.

       md=n,dev,dev,...

       md=dn,dev,dev,...
              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 de-
              vice.

       md=n,l,c,i,dev...
              This tells the md driver to assemble a legacy RAID0 or LINEAR array without  a  su-
              perblock.   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).

FILES
       /proc/mdstat
              Contains information about the status of currently running array.

       /proc/sys/dev/raid/speed_limit_min
              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 ar-
              ray with more disks will shuffle more data for a given  speed).    The  default  is
              1000.

       /proc/sys/dev/raid/speed_limit_max
              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.

SEE ALSO
       mdadm(8),

                                                                                            MD(4)

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