Provided by: mdadm_4.0-2ubuntu1_amd64 bug

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
       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).

   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
       metadata, and other formats can be added.  Prior to  this  release,  only  one  format  is
       supported.

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

       The new format — known as version 1 — has a superblock that is 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
              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.

   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
       interaction.

   CONTAINERS
       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.

   LINEAR
       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.

   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.

   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
       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.

   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.
       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
       devices).
       Hexadecimal  numbers  (0x00, 0x01,  0x02, ...)  are  the block addresses of the underlying
       devices.

        "near" Layout
              When "near" replicas are chosen, the multiple copies of a given chunk are laid  out
              consecutively  ("as  close  to  each  other as possible") across the stripes of the
              array.

              With an even number of devices, they  will  likely  (unless  some  misalignment  is
              present) lay at the very same offset on the 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
              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
       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
       installations.

   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
       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
       parameter.

   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
       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.

   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
       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
       block.

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

       Thus  the  mismatch_cnt  value  can  not  be interpreted very reliably on RAID1 or RAID10,
       especially when the device is used for swap.

   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
       period  of  time  with  no  writes  to an area of the array, the corresponding bit will be
       cleared.

       This bitmap is used for two optimisations.

       Firstly, after an unclean shutdown, the resync process will consult the  bitmap  and  only
       resync  those  blocks  that  correspond  to  bits  in  the  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
       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
       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.

   RAID456 WRITE JOURNAL
       Due to non-atomicity nature of RAID write operations,  interruption  of  write  operations
       (system  crash,  etc.)  to RAID456 array can lead to inconsistent parity and data loss (so
       called RAID-5 write hole).

       To plug the write hole, from Linux 4.4 (to be confirmed), md supports write ahead  journal
       for  RAID456.  When the array is created, an 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.

   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
       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.

   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
       underlying device does not perform any  retries.   This  means  that  a  failure  will  be
       reported to md promptly, and it can mark the device as faulty and continue using the other
       device(s).  md cannot control the timeout that the underlying  devices  use  to  determine
       failure.   Any  changes  desired  to  that timeout must be set explictly on the underlying
       device, separately from using mdadm.

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

       FAILFAST  is  appropriate  for storage arrays that have a low probability of true failure,
       but will sometimes introduce unacceptable delays to I/O requests while performing internal
       maintenance.   The  value  of 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
       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
       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
       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
       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
           crash.

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

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

   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/md.txt  which  is
       distributed   with   the   kernel  sources.   That  file  should  be  consulted  for  full
       documentation.  The following are just a selection of attribute files that are available.

       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
              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.

       md/stripe_cache_size
              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

       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.

   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
              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.

       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
              device.

       md=n,l,c,i,dev...
              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).

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
              array 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)