md(4) — Linux manual page


MD(4)                   Kernel Interfaces Manual                   MD(4)

NAME         top

       md - Multiple Device driver aka Linux Software RAID

SYNOPSIS         top


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).

       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

       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,

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

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

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

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

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

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

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

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

       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.

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

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

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

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

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

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

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

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

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

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

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

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

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

       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

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

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

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

       A RAID4 array is like a RAID0 array with an extra device for
       storing parity. This device is the last of the active devices in
       the array. Unlike RAID0, RAID4 also requires that all stripes
       span all drives, so extra space on devices that are larger than
       the smallest is 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 is very similar to RAID4.  The difference is that the
       parity blocks for each stripe, instead of being on a single
       device, are distributed across all devices.  This allows more
       parallelism when writing, as two different block updates will
       quite possibly affect parity blocks on different devices so there
       is less contention.

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

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

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

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

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

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

       For simplicity it is assumed that the size of the chunks equals
       the size of the blocks of the underlying devices as well as those
       of the RAID10 device exported by the kernel (for example
       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
                     │ Device #1 │ Device #2 │ Device #3 │ Device #4 │
              │ 0x00 │     0     │     0     │     1     │     1     │
              │ 0x01 │     2     │     2     │     3     │     3     │
              │ ...  │    ...    │    ...    │    ...    │    ...    │
              │  :   │     :     │     :     │     :     │     :     │
              │ ...  │    ...    │    ...    │    ...    │    ...    │
              │ 0x80 │    254    │    254    │    255    │    255    │
                       \---------v---------/   \---------v---------/
                               RAID1                   RAID1

              Example with 2 copies per chunk and an odd number (5) of
                     │ 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
                     │ 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
                     │ 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
              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
                     │ 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
                     │ 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

              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

       If, for example, an array is created with 5 devices and 2
       replicas, then space equivalent to 2.5 of the devices will be
       available, and every block will be stored on two different

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

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

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

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

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

       The FAULTY module may be requested to simulate faults to allow
       testing of other md levels or of filesystems.  Faults can be
       chosen to trigger on read requests or write requests, and can be
       transient (a subsequent read/write at the address will probably
       succeed) or persistent (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.

       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

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

       If the md driver detects a write error on a device in a RAID1,
       RAID4, RAID5, RAID6, or RAID10 array, it immediately disables
       that device (marking it as faulty) and continues operation on the
       remaining devices.  If there are spare drives, the driver will
       start recreating on one of the spare drives the data which was on
       that failed drive, either by copying a working drive in a RAID1
       configuration, or by doing calculations with the parity block on
       RAID4, RAID5 or RAID6, or by finding and copying originals for

       In kernels prior to about 2.6.15, a read error would cause the
       same effect as a write error.  In later kernels, a read-error
       will instead cause md to attempt a recovery by overwriting the
       bad block. i.e. it will find the correct data from elsewhere,
       write it over the block that failed, and then try to read it back
       again.  If either the write or the re-read fail, md will treat
       the error the same way that a write error is treated, and will
       fail the whole device.

       While this recovery process is happening, the md driver will
       monitor accesses to the array and will slow down the rate of
       recovery if other activity is happening, so that normal access to
       the array will not be unduly affected.  When no other activity is
       happening, the recovery process proceeds at full speed.  The
       actual speed targets for the two different situations can be
       controlled by the speed_limit_min and speed_limit_max control
       files mentioned below.

       As storage devices can develop bad blocks at any time it is
       valuable to regularly read all blocks on all devices in an array
       so as to catch such bad blocks early.  This process is called

       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

       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.

       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

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

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

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

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

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

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

       The list is particularly useful when recovering to a spare.  If a
       few blocks cannot be read from the other devices, the bulk of the
       recovery can complete and those few bad blocks will be recorded
       in the bad block list.

       Due to non-atomicity nature of RAID write operations,
       interruption of write operations (system crash, etc.) to RAID456
       array can lead to inconsistent parity and data loss (so called
       RAID-5 write hole).  To plug the write hole md supports two
       mechanisms described below.

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

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

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

              This mechanism is documented more fully in the file

       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

       If a write-intent bitmap is also provided, write requests to
       write-mostly devices will be treated as write-behind requests and
       md will not wait for writes to those requests to complete before
       reporting the write as complete to the filesystem.

       This allows for a RAID1 with WRITE-BEHIND to be used to mirror
       data over a slow link to a remote computer (providing the link
       isn't too slow).  The extra latency of the remote link will not
       slow down normal operations, but the remote system will still
       have a reasonably up-to-date copy of all data.

       From Linux 4.10, md supports FAILFAST for RAID1 and RAID10
       arrays.  This is a flag that can be set on individual drives,
       though it is usually set on all drives, or no drives.

       When md sends an I/O request to a drive that is marked as
       FAILFAST, and when the array could survive the loss of that drive
       without losing data, md will request that the underlying device
       does not perform any retries.  This means that a failure will be
       reported to md promptly, and it can mark the device as faulty and
       continue using the other device(s).  md cannot control the
       timeout that the underlying devices use to determine failure.
       Any changes desired to that timeout must be set 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, 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

       •   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.

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

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

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

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

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

              A count of problems found will be stored in

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

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

              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 *

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       The md driver recognised several different kernel parameters.

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


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


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


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


              This tells the md driver to assemble /dev/md n from the
              listed devices.  It is only necessary to start the device
              holding the root filesystem this way.  Other arrays are
              best started once the system is booted.

              In 2.6 kernels, the d immediately after the = indicates
              that a partitionable device (e.g.  /dev/md/d0) should be
              created rather than the original non-partitionable device.

              This tells the md driver to assemble a legacy RAID0 or
              LINEAR array without a superblock.  n gives the md device
              number, l gives the level, 0 for RAID0 or -1 for LINEAR, c
              gives the chunk size as a base-2 logarithm offset by
              twelve, so 0 means 4K, 1 means 8K.  i is ignored (legacy

FILES         top

              Contains information about the status of currently running

              A readable and writable file that reflects the current
              "goal" rebuild speed for times when non-rebuild activity
              is current on an array.  The speed is in Kibibytes per
              second, and is a per-device rate, not a per-array rate
              (which means that an array with more disks will shuffle
              more data for a given speed).   The default is 1000.

              A readable and writable file that reflects the current
              "goal" rebuild speed for times when no non-rebuild
              activity is current on an array.  The default is 200,000.

SEE ALSO         top


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 
       ⟨⟩.  If you have a bug report for
       this manual page, send it to  This
       page was obtained from the project's upstream Git repository
       ⟨⟩ on
       2023-12-22.  (At that time, the date of the most recent commit
       that was found in the repository was 2023-12-19.)  If you
       discover any rendering 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


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