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           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
           acronym RAID which stands for a Redundant Array of Independent Disks.
           md supports RAID levels 1 (mirroring), 4  (striped  array  with  parity
           device),  5  (striped  array  with  distributed  parity information), 6
           (striped array with distributed dual redundancy  information),  and  10
           (striped  and  mirrored).   If  some number of underlying devices fails
           while using one of these levels, the array will continue  to  function;
           this  number  is one for RAID levels 4 and 5, two for RAID level 6, and
           all but one (N-1) for RAID level 1, and dependent on configuration  for
           level 10.
           md also supports a number of pseudo RAID (non-redundant) configurations
           including RAID0 (striped array), LINEAR (catenated array), MULTIPATH (a
           set  of  different  interfaces to the same device), and FAULTY (a layer
           over a single device into which errors can be injected).
           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-
           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:
           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
           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
           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.
           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
           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
           drivers 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.
           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.
           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
           Firstly, after an unclean shutdown, the resync process will consult the
           bitmap and only resync those blocks that  correspond  to  bits  in  the
           bitmap that are set.  This can dramatically reduce resync time.
           Secondly,  when  a  drive fails and is removed from the array, md stops
           clearing bits in the intent log.  If that same drive is re-added to the
           array,  md  will notice and will only recover the sections of the drive
           that are covered by bits in the intent log  that  are  set.   This  can
           allow a device to be temporarily removed and reinserted without causing
           an enormous recovery cost.
           The intent log can be stored in a file on a separate device, or it  can
           be stored near the superblocks of an array which has superblocks.
           It  is  possible  to add an intent log to an active array, or remove an
           intent log if one is present.
           In 2.6.13, intent bitmaps are only supported with RAID1.  Other  levels
           with redundancy are supported from 2.6.15.
           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
           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 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
                  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.


           mdadm(8), mkraid(8).

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