md <root
MD(4)                                Kernel Interfaces Manual                               MD(4)

NAME
       md - Multiple Device driver aka Linux Software RAID

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

DESCRIPTION
       The  md  driver  provides  virtual  devices  that are created from one or more independent
       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 Indepen‐
       dent Disks.

       md supports RAID levels 1 (mirroring), 4 (striped array with parity  device),  5  (striped
       array  with distributed parity information), 6 (striped array with distributed dual redun‐
       dancy information), and 10 (striped and mirrored).  If some number of  underlying  devices
       fails while using one of these levels, the array will continue to function; this number is
       one for RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for RAID level 1,
       and dependent on configuration for level 10.

       md  also  supports  a number of pseudo RAID (non-redundant) configurations including RAID0
       (striped array), LINEAR (catenated array), MULTIPATH (a set of different interfaces to the
       same device), and FAULTY (a layer over a single device into which errors can be injected).

   MD METADATA
       Each  device  in  an  array may have some metadata stored in the device.  This metadata is
       sometimes called a superblock.  The metadata records information about the  structure  and
       state of the array.  This allows the array to be reliably re-assembled after a shutdown.

       From  Linux  kernel version 2.6.10, md provides support for two different formats of meta‐
       data, and other formats can be added.  Prior to this release,  only  one  format  is  sup‐
       ported.

       The  common format — known as version 0.90 — has a superblock that is 4K long and is writ‐
       ten 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 incorpo‐
       rated 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 stor‐
              age directly rather than having a distinctive superblock written to the device  and
              searched  for  on  all  paths.   In this case, a MULTIPATH array with no superblock
              makes sense.

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

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

       md  is able to communicate with the user-space program through various sysfs attributes so
       that it can make appropriate changes to the metadata - for example to  mark  a  device  as
       faulty.   When necessary, md will wait for the program to acknowledge the event by writing
       to a sysfs attribute.  The manual page for mdmon(8) contains more detail about this inter‐
       action.

   CONTAINERS
       Many  metadata  formats  use  a single block of metadata to describe a number of different
       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  vir‐
       tual drive.

       One advantage of this arrangement over the more common RAID0 arrangement is that the array
       may be reconfigured at a later time with an extra drive, so the array is made bigger with‐
       out disturbing the data that is on the array.  This can even be done on a live array.

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

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

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

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

   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  pro‐
       file be the same as for RAID0; a single stream of sequential input will not be accelerated
       (e.g. a single dd), but multiple sequential streams or a random  workload  will  use  more
       than  one  spindle.  In  theory, having an N-disk RAID1 will allow N sequential threads to
       read from all disks.

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

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

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

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

   RAID5
       RAID5 is very similar to RAID4.  The difference is that the parity blocks for each stripe,
       instead of being on a single device, are distributed across all devices.  This allows more
       parallelism when writing, as two different block updates will quite possibly affect parity
       blocks on different devices so there is less contention.

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

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

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

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

       When configuring a RAID10 array, it is necessary to specify the number of replicas of each
       data  block that are required (this will normally be 2) and whether the replicas should be
       'near', 'offset' or 'far'.  (Note that the 'offset' layout is only available from 2.6.18).

       When 'near' replicas are chosen, the multiple copies of a given chunk are laid out consec‐
       utively  across  the stripes of the array, so the two copies of a datablock will likely be
       at the same offset on two adjacent devices.

       When 'far' replicas are chosen, the multiple copies of a given chunk are  laid  out  quite
       distant  from  each  other.   The first copy of all data blocks will be striped across the
       early part of all drives in RAID0 fashion, and then the next copy of all  blocks  will  be
       striped across a later section of all drives, always ensuring that all copies of any given
       block are on different drives.

       The 'far' arrangement can give sequential read performance equal to that of a RAID0 array,
       but at the cost of reduced write performance.

       When  'offset'  replicas  are chosen, the multiple copies of a given chunk are laid out on
       consecutive drives and at consecutive offsets.  Effectively each stripe is duplicated  and
       the  copies  are  offset by one device.   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 possi‐
       bility of inconsistency for short periods of time as each update  requires  at  least  two
       block  to  be  written  to  different  devices,  and these writes probably won't happen at
       exactly the same time.  Thus if a system with one of these arrays is shutdown in the  mid‐
       dle of a write operation (e.g. due to power failure), the array may not be consistent.

       To  handle this situation, the md driver marks an array as "dirty" before writing any data
       to it, and marks it as "clean" when the array is being disabled, e.g. at shutdown.  If the
       md  driver  finds  an  array  to  be dirty at startup, it proceeds to correct any possibly
       inconsistency.  For RAID1, this involves copying the contents of the first drive onto  all
       other  drives.  For RAID4, RAID5 and RAID6 this involves recalculating the parity for each
       stripe and making sure that the parity block has the correct data.  For RAID10 it involves
       copying  one  of  the  replicas of each block onto all the others.  This process, known as
       "resynchronising" or "resync" is performed in the background.   The  array  can  still  be
       used, though possibly with reduced performance.

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

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

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

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

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

       md  arrays can be scrubbed by writing either check or repair to the file md/sync_action in
       the sysfs directory for the device.

       Requesting a scrub will cause md to read every block on every device  in  the  array,  and
       check  that  the  data  is consistent.  For RAID1 and RAID10, this means checking that the
       copies are identical.  For RAID4, RAID5, RAID6 this means checking that the  parity  block
       is (or blocks are) correct.

       If  a  read  error  is detected during this process, the normal read-error handling causes
       correct data to be found from other devices and to be written back to the  faulty  device.
       In many case this will effectively fix the bad block.

       If  all blocks read successfully but are found to not be consistent, then this is regarded
       as a mismatch.

       If check was used, then no action is taken to handle the mismatch, it is simply  recorded.
       If  repair  was used, then a mismatch will be repaired in the same way that resync repairs
       arrays.  For RAID5/RAID6 new parity blocks are written.  For  RAID1/RAID10,  all  but  one
       block are overwritten with the content of that one block.

       A  count of mismatches is recorded in the sysfs file md/mismatch_cnt.  This is set to zero
       when a scrub starts and is incremented whenever a sector is found that is a mismatch.   md
       normally  works in units much larger than a single sector and when it finds a mismatch, it
       does not determine exactly how many actual sectors were affected but simply adds the  num‐
       ber  of  sectors in the IO unit that was used.  So a value of 128 could simply mean that a
       single 64KB check found an error (128 x 512bytes = 64KB).

       If an array is created by mdadm with --assume-clean  then  a  subsequent  check  could  be
       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 par‐
       tition or swap file is stored on the array.

       When the swap subsystem wants to write a page of memory out, it flags the page as  'clean'
       in  the memory manager and requests the swap device to write it out.  It is quite possible
       that the memory will be changed while the  write-out  is  happening.   In  that  case  the
       'clean'  flag will be found to be clear when the write completes and so the swap subsystem
       will simply forget that the swapout had been attempted, and will possibly choose a differ‐
       ent page to write out.

       If the swap device was on RAID1 (or RAID10), then the data is sent from memory to a device
       twice (or more depending on the number of devices in the array).  Thus it is possible that
       the  memory gets changed between the times it is sent, so different data can be written to
       the different devices in the array.  This will be detected by check as a  mismatch.   How‐
       ever  it  does not reflect any corruption as the block where this mismatch occurs is being
       treated by the swap system as being empty, and the data  will  never  be  read  from  that
       block.

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

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

   BITMAP WRITE-INTENT LOGGING
       From Linux 2.6.13, md supports a bitmap based write-intent log.  If configured, the bitmap
       is used to record which blocks of the array may be out of sync.  Before any write  request
       is  honoured,  md  will  make  sure that the corresponding bit in the log is set.  After a
       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 dramati‐
       cally reduce resync time.

       Secondly,  when a drive fails and is removed from the array, md stops clearing bits in the
       intent log.  If that same drive is re-added to the array, md will  notice  and  will  only
       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  enor‐
       mous recovery cost.

       The  intent log can be stored in a file on a separate device, or it can be stored near the
       superblocks of an array which has superblocks.

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

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

   BAD BLOCK LOG
       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 log.  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 log 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 log.

   WRITE-BEHIND
       From Linux 2.6.14, md supports WRITE-BEHIND on RAID1 arrays.

       This allows certain devices in the array to be flagged as write-mostly.  MD will only read
       from such devices if there is no other option.

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

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

   RESTRIPING
       Restriping,  also  known as Reshaping, is the processes of re-arranging the data stored in
       each stripe into a new layout.  This might involve changing the number of devices  in  the
       array  (so the stripes are wider), changing the chunk size (so stripes are deeper or shal‐
       lower), or changing the arrangement of data and parity (possibly changing the RAID  level,
       e.g. 1 to 5 or 5 to 6).

       As of Linux 2.6.35, md can reshape a RAID4, RAID5, or RAID6 array to have a different num‐
       ber of devices (more or fewer) and to have a different layout or chunk size.  It can  also
       convert  between  these  different  RAID  levels.   It  can also convert between RAID0 and
       RAID10, and between RAID0 and RAID4 or RAID5.  Other possibilities may  follow  in  future
       kernels.

       During  any  stripe  process there is a 'critical section' during which live data is being
       overwritten on disk.  For the operation of increasing the number of  drives  in  a  RAID5,
       this  critical  section  covers the first few stripes (the number being the product of the
       old and new number of devices).  After this critical section is passed, data is only writ‐
       ten  to areas of the array which no longer hold live data — the live data has already been
       located away.

       For a reshape which reduces the number of devices, the 'critical section' is at the end of
       the reshape process.

       md is not able to ensure data preservation if there is a crash (e.g. power failure) during
       the critical section.  If md is asked to start an array which  failed  during  a  critical
       section of restriping, it will fail to start the array.

       To deal with this possibility, a user-space program must

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

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

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

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

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

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

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

       This interface is documented more fully in the file Documentation/md.txt which is distrib‐
       uted  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 par‐
              ticular, writing "check" here will cause the array to read all data block and check
              that  they  are  consistent (e.g. parity is correct, or all mirror replicas are the
              same).  Any discrepancies found are NOT corrected.

              A count of problems found will be stored in md/mismatch_count.

              Alternately, "repair" can be written which will cause the same  check  to  be  per‐
              formed, but any errors will be corrected.

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

       md/stripe_cache_size
              This  is  only  available  on  RAID5  and RAID6.  It records the size (in pages per
              device) of the  stripe cache which is used for synchronising all  write  operations
              to the array and all read operations if the array is degraded.  The default is 256.
              Valid values are 17 to 32768.  Increasing this number can increase  performance  in
              some  situations, at some cost in system memory.  Note, setting this value too high
              can result in an "out of memory" condition for the system.

              memory_consumed = system_page_size * nr_disks * stripe_cache_size

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

   KERNEL PARAMETERS
       The md driver recognised several different kernel parameters.

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

       raid=partitionable

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

       md_mod.start_ro=1

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

       md_mod.start_dirty_degraded=1

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

       md=n,dev,dev,...

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

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

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

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

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

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

SEE ALSO
       mdadm(8),

                                                                                            MD(4)

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