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MD(4)											    MD(4)

NAME
       md - Multiple Device driver aka Linux Software Raid

SYNOPSIS
       /dev/mdn
       /dev/md/n

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 hence the  acro-
       nym RAID which stands for a Redundant Array of Independent Devices.

       md  supports RAID levels 1 (mirroring) 4 (striped array with parity device) and 5 (striped
       array with distributed parity information).  If a single  underlying  device  fails  while
       using one of these levels, the array will continue to function.

       md  also  supports  a number of pseudo RAID (non-redundant) configurations including RAID0
       (striped array), LINEAR (catenated array) and MULTIPATH (a set of different interfaces  to
       the same device).

   MD SUPER BLOCK
       With the exception of Legacy Arrays described below, each device that is incorporated into
       an MD array has a super block written towards the end  of  the  device.	 This  superblock
       records	information  about  the structure and state of the array so that the array can be
       reliably re-assembled after a shutdown.

       The superblock 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 avail-
       able 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.

       The superblock contains, among other things:

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

       UUID   a  128 bit Universally Unique Identifier that identifies the array that this device
	      is part of.

   LEGACY ARRAYS
       Early versions of the md driver only supported Linear and Raid0 configurations and so  did
       not  use  an  MD superblock (as there is no state that needs to be recorded).  While it is
       strongly recommended that all newly created arrays utilise a  superblock  to  help  ensure
       that  they are assembled properly, the md driver still supports legacy linear and raid0 md
       arrays that do not have a superblock.

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

       One advantage of this arrangement over the more common RAID0 arrangement is that the array
       may be reconfigured at a later time with an extra drive and so the array  is  made  bigger
       without	disturbing  the data that is on the array.  However this cannot be done on a live
       array.

   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, 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 which 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 reflect
       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.

   RAID4
       A RAID4 array is like a RAID0 array with an  extra  device  for	storing  parity.   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  con-
       tents  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.

   MUTIPATH
       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 logical different devices, often fibre chan-
       nel interfaces, that all refer the the same real device. If one of these interfaces  fails
       (e.g.  due  to  cable  problems),  the multipath driver to attempt to redirect requests to
       another interface.

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

       To handle this situation, the md driver marks an array as "dirty" before writing any  data
       to it, and marks it as "clean" when the array is being disabled, e.g. at shutdown.  If the
       md driver finds an array to be dirty at startup,  it  proceeds  to  correct  any  possibly
       inconsistency.	For RAID1, this involves copying the contents of the first drive onto all
       other drives.  For RAID4 or RAID5 this involves recalculating the parity for  each  stripe
       and making sure that the parity block has the correct data.

       If  a  RAID4  or RAID5 array is degraded (missing one drive) when it is restarted after an
       unclean shutdown, it cannot recalculate parity, and so it is possible that data	might  be
       undetectably  corrupted.  The md driver currently does not alert the operator to this con-
       dition.	It should probably fail to start an array in this condition without manual inter-
       vention.

   RECOVERY
       If the md driver detects any error on a device in a RAID1, RAID4, or RAID5 array, it imme-
       diately disables that device (marking it as faulty) and continues operation on the remain-
       ing  devices.   If  there is a spare drive, the driver will start recreating on one of the
       spare drives the data what 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 and RAID5.

       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.

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 disc will shuffle more data for a given speed).   The default is 100.

       /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 100,000.

SEE ALSO
       mdadm(8), mkraid(8).

											    MD(4)
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