md


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  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).


   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 multi-
       ple  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 proces-
       sors.

       The new format -- known as version 1 -- has a superblock that  is  nor-
       mally 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
       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 crit-
              ical 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 gen-
              eral 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 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.


   CONTAINERS
       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
       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.


   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 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.



   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 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.



       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
       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
       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  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
       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
       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
       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.


   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 (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-
       sient.

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

       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.


   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.

       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 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
       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.


   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 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

       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 inter-
           face),

       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 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.


       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

       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 mem-
              ory" 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 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.


   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  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
              behaviour.


       raid=partitionable

       raid=part
              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.


       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,
              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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