Provided by: mdadm_3.2.3-2ubuntu1_i386 bug

NAME

       md - Multiple Device driver aka Linux Software RAID

SYNOPSIS

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

DESCRIPTION

       The  md  driver  provides  virtual devices that are created from one or
       more independent underlying  devices.   This  array  of  devices  often
       contains  redundancy  and  the devices are often disk drives, hence the
       acronym RAID which stands for a Redundant Array of Independent Disks.

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

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

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

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

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

       The new format -- known as version  1  --  has  a  superblock  that  is
       normally  1K long, but can be longer.  It is normally stored between 8K
       and 12K from the end of the device, on a 4K boundary, though variations
       can  be  stored at the start of the device (version 1.1) or 4K from the
       start of  the  device  (version  1.2).   This  metadata  format  stores
       multibyte  data  in  a  processor-independent format and supports up to
       hundreds of component devices (version 0.90 only supports 28).

       The metadata contains, among other things:

       LEVEL  The manner in which the devices  are  arranged  into  the  array
              (linear, raid0, raid1, raid4, raid5, raid10, multipath).

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

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

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

       LEGACY ARRAYS
              Early versions of the md driver only supported Linear and  Raid0
              configurations  and  did  not  use  a  superblock (which is less
              critical with these configurations).  While such  arrays  should
              be rebuilt with superblocks if possible, md continues to support
              them.

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

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

       RAID1  In  some  configurations  it  might be desired to create a raid1
              configuration that does not use a superblock,  and  to  maintain
              the  state  of  the  array  elsewhere.  While not encouraged for
              general use, it does have special-purpose uses and is supported.

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

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

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

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

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

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

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

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

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

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

       Once initialised, each device in a RAID1  array  contains  exactly  the
       same  data.   Changes  are written to all devices in parallel.  Data is
       read from any one device.   The  driver  attempts  to  distribute  read
       requests across all devices to maximise performance.

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

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

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

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

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

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

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

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

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

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

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

       When  configuring a RAID10 array, it is necessary to specify the number
       of replicas of each data block that are required (this will 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 consecutively 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  possibility of inconsistency for short periods of time as
       each update requires at least two block  to  be  written  to  different
       devices,  and  these  writes  probably won't happen at exactly the same
       time.  Thus if a system with one of these arrays  is  shutdown  in  the
       middle  of a write operation (e.g. due to power failure), the array may
       not be consistent.

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

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

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

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

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

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

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

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

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

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

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

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

       If  an  array is created by mdadm with --assume-clean then a subsequent
       check could be expected to find some mismatches.

       On a truly clean RAID5 or RAID6 array, any mismatches should indicate a
       hardware  problem  at  some  level - software issues should never cause
       such a mismatch.

       However on RAID1 and RAID10 it is possible for software issues to cause
       a  mismatch  to  be  reported.  This does not necessarily mean that the
       data on the array is corrupted.  It could simply  be  that  the  system
       does  not  care what is stored on that part of the array - it is unused
       space.

       The most likely cause for an unexpected mismatch  on  RAID1  or  RAID10
       occurs if a swap partition or swap file is stored on the array.

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

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

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

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

   BITMAP WRITE-INTENT LOGGING
       From Linux 2.6.13, md supports a bitmap  based  write-intent  log.   If
       configured,  the bitmap is used to record which blocks of the array may
       be out of sync.  Before any write request is  honoured,  md  will  make
       sure  that  the corresponding bit in the log is set.  After a period of
       time with no writes to an area of the array, the corresponding bit will
       be cleared.

       This bitmap is used for two optimisations.

       Firstly, after an unclean shutdown, the resync process will consult the
       bitmap and only resync those blocks that  correspond  to  bits  in  the
       bitmap that are set.  This can dramatically reduce resync time.

       Secondly,  when  a  drive fails and is removed from the array, md stops
       clearing bits in the intent log.  If that same drive is re-added to the
       array,  md  will notice and will only recover the sections of the drive
       that are covered by bits in the intent log  that  are  set.   This  can
       allow a device to be temporarily removed and reinserted without causing
       an enormous recovery cost.

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

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

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

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

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

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

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

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

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

       During  any  stripe  process there is a 'critical section' during which
       live  data  is  being  overwritten  on  disk.   For  the  operation  of
       increasing  the  number  of  drives  in  a raid5, this critical section
       covers the first few stripes (the number being the product of  the  old
       and  new  number  of  devices).  After this critical section is passed,
       data is only written to areas of the array which no  longer  hold  live
       data -- the live data has already been located away.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

              memory_consumed    =    system_page_size    *     nr_disks     *
              stripe_cache_size

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

   KERNEL PARAMETERS
       The md driver recognised several different kernel parameters.

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

       raid=partitionable

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

       md_mod.start_ro=1

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

       md_mod.start_dirty_degraded=1

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

       md=n,dev,dev,...

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

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

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

FILES

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

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

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

SEE ALSO

       mdadm(8), mkraid(8).

                                                                         MD(4)