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MD(4)                     Kernel Interfaces Manual                     MD(4)

NAME         top

       md - Multiple Device driver aka Linux Software RAID

SYNOPSIS         top

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

DESCRIPTION         top

       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         top

       /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         top

       mdadm(8),

COLOPHON         top

       This page is part of the mdadm (Tool for managing md arrays in Linux)
       project.  Information about the project can be found at 
       ⟨http://neil.brown.name/blog/mdadm⟩.  If you have a bug report for
       this manual page, send it to linux-raid@vger.kernl.org.  This page
       was obtained from the project's upstream Git repository 
       ⟨git://neil.brown.name/mdadm⟩ on 2017-04-25.  If you discover any ren‐
       dering problems in this HTML version of the page, or you believe
       there is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

                                                                       MD(4)

Pages that refer to this page: mdadm.conf(5)mdadm(8)mdassemble(8)mdmon(8)raid6check(8)xfs_growfs(8)