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MD(4)											    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
       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 pro-
       cessors.

       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 us, 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 by 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 make 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".  This 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 determin 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  sin-
       gle 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.

   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 2.6.17, md can reshape a raid5 array to have more devices.  Other possibili-
       ties 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.

       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 criti-
       cal  section.  In this case, the restripe will need to progress in stages, as a section is
       suspended, backed up, restriped, and released; this is not yet implemented.

   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/spool_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), mkraid(8).

											    MD(4)
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