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MD(4)									 MD(4)

       md - Multiple Device driver aka Linux Software RAID


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

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

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

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

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

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

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

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

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

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

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

       From release 2.6.28, the md driver supports arrays with externally man‐
       aged 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	 meta‐
       data - for example to mark a device as faulty.  When necessary, md will
       wait for the program to acknowledge the event by	 writing  to  a	 sysfs
       attribute.   The	 manual	 page  for mdmon(8) contains more detail about
       this interaction.

       Many metadata formats use a single block of metadata to describe a num‐
       ber 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

       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  arrange‐
       ment  is	 that  the  array  may be reconfigured at a later time with an
       extra drive, so the array is made bigger without	 disturbing  the  data
       that is on the array.  This can even be done on a live array.

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

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

       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 small‐
       est device has been  exhausted,	the  RAID0  driver  starts  collecting
       chunks  into smaller stripes that only span the drives which still have
       remaining space.

       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 multi‐
       ple  sequential	streams	 or  a	random workload will use more than one
       spindle. In theory, having an N-disk  RAID1  will  allow	 N  sequential
       threads to read from all disks.

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

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

       When  any block in a RAID4 array is modified, the parity block for that
       stripe (i.e. the block in the parity device at the same	device	offset
       as  the	stripe)	 is also modified so that the parity block always con‐
       tains the "parity" for the whole stripe.	 I.e. its content  is  equiva‐
       lent  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 par‐
       ity block and the other data blocks.

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

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

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

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

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

       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.	Effec‐
       tively 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

       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 differ‐
       ent drive.  This is an artifact of the implementation and  is  unlikely
       to be of real value.

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

       A MULTIPATH array is  composed  of  a  number  of  logically  different
       devices,	 often	fibre  channel interfaces, that all refer the the same
       real device. If one of these interfaces fails (e.g. due to cable	 prob‐
       lems),  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 driv‐
       ers should be preferred for new installations.

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

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

       The list of faulty sectors can be flushed, and the active list of fail‐
       ure modes can be cleared.

       When  changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array
       there is a possibility of inconsistency for short periods  of  time  as
       each  update  requires  at  least  two block to be written to different
       devices, and these writes probably won't happen	at  exactly  the  same
       time.   Thus  if	 a  system with one of these arrays is shutdown in the
       middle of a write operation (e.g. due to power failure), the array  may
       not be consistent.

       To  handle  this	 situation,  the  md  driver marks an array as "dirty"
       before writing any data to it, and marks it as "clean" when  the	 array
       is  being  disabled, e.g. at shutdown.  If the md driver finds an array
       to be dirty at startup, it proceeds to correct any  possibly  inconsis‐
       tency.	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 par‐
       ity 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.

       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.

       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

       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 possi‐
       bly 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 mis‐
       match occurs is being treated by the swap system as  being  empty,  and
       the data will never be read from that block.

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

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

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

       This bitmap is used for two optimisations.

       Firstly, after an unclean shutdown, the resync process will consult the
       bitmap and only resync those blocks that correspond to bits in the bit‐
       map 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.

       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  differ‐
       ent 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

       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, 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 chang‐
       ing 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  dif‐
       ferent layout or chunk size.  It can also convert between these differ‐
       ent 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 increas‐
       ing 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 num‐
       ber 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  sec‐
       tion' 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 inter‐

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

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

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

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

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

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

       This  interface	is  documented	more  fully  in	 the  file  Documenta‐
       tion/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.

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

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

	      This  can	 be  used  to  monitor and control the resync/recovery
	      process of MD.  In particular, writing "check" here  will	 cause
	      the array to read all data block and check that they are consis‐
	      tent (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.

	      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  mem‐
	      ory" condition for the system.

	      memory_consumed	  =	system_page_size    *	 nr_disks    *

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

       The md driver recognised several different kernel parameters.

	      This will disable the normal detection of md arrays that happens
	      at  boot time.  If a drive is partitioned with MS-DOS style par‐
	      titions, 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


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


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


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


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

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

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

	      Contains	information  about  the	 status	 of  currently running

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

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



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