# md

## NAME



           md - Multiple Device driver aka Linux Software RAID
    


## SYNOPSIS



           **/dev/md** _n_
           **/dev/md/**_n_
           **/dev/md/** name
    


## DESCRIPTION



           The **md** driver provides virtual devices that are created from one
           or more independent underlying devices.  This array of devices
           often contains redundancy and the devices are often disk drives,
           hence the acronym RAID which stands for a Redundant Array of
           Independent Disks.
    
           **md** supports RAID levels 1 (mirroring), 4 (striped array with
           parity device), 5 (striped array with distributed parity
           information), 6 (striped array with distributed dual redundancy
           information), and 10 (striped and mirrored).  If some number of
           underlying devices fails while using one of these levels, the
           array will continue to function; this number is one for RAID
           levels 4 and 5, two for RAID level 6, and all but one (N-1) for
           RAID level 1, and dependent on configuration for level 10.
    
           **md** also supports a number of pseudo RAID (non-redundant)
           configurations including RAID0 (striped array), LINEAR (catenated
           array), MULTIPATH (a set of different interfaces to the same
           device), and FAULTY (a layer over a single device into which
           errors can be injected).
    
       **MD METADATA**
           Each device in an array may have some _metadata_ stored in the
           device.  This metadata is sometimes called a **superblock**.  The
           metadata records information about the structure and state of the
           array.  This allows the array to be reliably re-assembled after a
           shutdown.
    
           **md** provides support for two different formats of metadata, and
           other formats can be added.
    
           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**
           _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 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.
    
           Note that setting the layout to "unspecified" removes protections
           against this bug, and you must be sure that the kernel you use
           matches the layout of the array.
    
       **RAID1**
           A RAID1 array is also known as a mirrored set (though mirrors tend
           to provide reflected images, which RAID1 does not) or a plex.
    
           Once initialised, each device in a RAID1 array contains exactly
           the same data.  Changes are written to all devices in parallel.
           Data is read from any one device.  The driver 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".
    
           **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 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.
    
           A read-error will 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**
           _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.
    
           All raid levels with redundancy are supported.
    
       **BAD BLOCK LIST**
           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.
    
       **RAID WRITE HOLE**
           Due to non-atomicity nature of RAID write operations, interruption
           of write operations (system crash, etc.) to RAID456 array can lead
           to inconsistent parity and data loss (so called RAID-5 write
           hole).  To plug the write hole md supports two mechanisms
           described below.
    
           DIRTY STRIPE JOURNAL
                  From Linux 4.4, md supports write ahead journal for
                  RAID456.  When the array is created, an additional journal
                  device can be added to the array through write-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.
    
           PARTIAL PARITY LOG
                  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.
    
                  See Documentation/driver-api/md/raid5-ppl.rst for
                  implementation details.
    
       **WRITE-BEHIND**
           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).
    
           _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** 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/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.
    
           **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.
    
           **md/bitmap/backlog**
                  The value stored in the file only has any effect on RAID1
                  when write-mostly devices are active, and write requests to
                  those devices are proceed in the background.
    
                  This variable sets a limit on the number of concurrent
                  background writes, the valid values are 0 to 16383, 0 means
                  that write-behind is not allowed, while any other number
                  means it can happen.  If there are more write requests than
                  the number, new writes will by synchronous.
    
           **md/bitmap/can_clear**
                  This is for externally managed bitmaps, where the kernel
                  writes the bitmap itself, but metadata describing the
                  bitmap is managed by mdmon or similar.
    
                  When the array is degraded, bits mustn't be cleared. When
                  the array becomes optimal again, bit can be cleared, but
                  first the metadata needs to record the current event count.
                  So md sets this to 'false' and notifies mdmon, then mdmon
                  updates the metadata and writes 'true'.
    
                  There is no code in mdmon to actually do this, so maybe it
                  doesn't even work.
    
           **md/bitmap/chunksize**
                  The bitmap chunksize can only be changed when no bitmap is
                  active, and the value should be power of 2 and at least
                  512.
    
           **md/bitmap/location**
                  This indicates where the write-intent bitmap for the array
                  is stored.  It can be "none" or "file" or a signed offset
                  from the array metadata - measured in sectors. You cannot
                  set a file by writing here - that can only be done with the
                  SET_BITMAP_FILE ioctl.
    
                  Write 'none' to 'bitmap/location' will clear bitmap, and
                  the previous location value must be write to it to restore
                  bitmap.
    
           **md/bitmap/max_backlog_used**
                  This keeps track of the maximum number of concurrent write-
                  behind requests for an md array, writing any value to this
                  file will clear it.
    
           **md/bitmap/metadata**
                  This can be 'internal' or 'clustered' or 'external'.
                  'internal' is set by default, which means the metadata for
                  bitmap is stored in the first 256 bytes of the bitmap
                  space. 'clustered' means separate bitmap metadata are used
                  for each cluster node. 'external' means that bitmap
                  metadata is managed externally to the kernel.
    
           **md/bitmap/space**
                  This shows the space (in sectors) which is available at
                  md/bitmap/location, and allows the kernel to know when it
                  is safe to resize the bitmap to match a resized array. It
                  should big enough to contain the total bytes in the bitmap.
    
                  For 1.0 metadata, assume we can use up to the superblock if
                  before, else to 4K beyond superblock. For other metadata
                  versions, assume no change is possible.
    
           **md/bitmap/time_base**
                  This shows the time (in seconds) between disk flushes, and
                  is used to looking for bits in the bitmap to be cleared.
    
                  The default value is 5 seconds, and it should be an
                  unsigned long value.
    
       **KERNEL PARAMETERS**
           The md driver recognised several different kernel parameters.
    
           **raid=noautodetect**
                  This will disable the normal detection of md arrays that
                  happens at boot time.  If a drive is partitioned with MS-
                  DOS style partitions, then if any of the 4 main 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.
    
           **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=d** _n_**,**_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.
    
           **md=**_n_**,**_l_**,**_c_**,**_i_**,**_dev..._
                  This tells the md driver to assemble a legacy RAID0 or
                  LINEAR array without a superblock.  _n_ gives the md device
                  number, _l_ gives the level, 0 for RAID0 or -1 for LINEAR, _c_
                  gives the chunk size as a base-2 logarithm offset by
                  twelve, so 0 means 4K, 1 means 8K.  _i_ is ignored (legacy
                  support).
    


## FILES



           **/proc/mdstat**
                  Contains information about the status of currently running
                  array.
    
           **/proc/sys/dev/raid/speed_limit_min**
                  A readable and writable file that reflects the current
                  "goal" rebuild speed for times when non-rebuild activity is
                  current on an array.  The speed is in Kibibytes per second,
                  and is a per-device rate, not a per-array rate (which means
                  that an array with more disks will shuffle more data for a
                  given speed).   The default is 1000.
    
           **/proc/sys/dev/raid/speed_limit_max**
                  A readable and writable file that reflects the current
                  "goal" rebuild speed for times when no non-rebuild activity
                  is current on an array.  The default is 200,000.
    


* * *