MD(4)                                                       MD(4)



NNAAMMEE
       md - Multiple Device driver aka Linux Software Raid

SSYYNNOOPPSSIISS
       //ddeevv//mmdd_n
       //ddeevv//mmdd//_n

DDEESSCCRRIIPPTTIIOONN
       The  mmdd  driver  provides virtual devices that are created
       from one or more  independent  underlying  devices.   This
       array  of devices often contains redundancy, and hence the
       acronym RAID which stands for a Redundant Array  of  Inde-
       pendent Devices.

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

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


   MMDD SSUUPPEERR BBLLOOCCKK
       With the exception of Legacy Arrays described below,  each
       device  that  is incorporated into an MD array has a _s_u_p_e_r
       _b_l_o_c_k  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 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.

       The superblock contains, among other things:

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

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


   LLEEGGAACCYY AARRRRAAYYSS
       Early  versions of the mmdd driver only supported Linear and
       Raid0 configurations and so did not use an  MD  superblock
       (as  there is not 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 assem-
       bled properly, the mmdd driver still supports legacy  linear
       and raid0 md arrays that do not have a superblock.


   LLIINNEEAARR
       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.



   RRAAIIDD00
       A RAID0 array (which has zero redundancy) is also known as
       a  striped array.  A RAID0 array is configured at creation
       with a CChhuunnkk SSiizzee which must be a multiple of 4 kibibytes.

       The  RAID0  driver  places 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 chuck.
       This collection of chunks forms a ssttrriippee.  Further  chunks
       are  gathered  into  stripes  in  the  same  way which are
       assigned to the remaining space in the drives.

       If device in the array are not all  the  same  size,  then
       once  the  smallest  devices has been exhausted, the RAID0
       driver starts collecting chunks into smaller stripes  that
       only span the drives which still have remaining space.



   RRAAIIDD11
       A RAID1 array is also known as a mirrored set (though mir-
       rors 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.


   RRAAIIDD44
       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 contents 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.


   RRAAIIDD55
       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.


   MMUUTTIIPPAATTHH
       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  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
       to attempt to redirect requests to another interface.



   UUNNCCLLEEAANN SSHHUUTTDDOOWWNN
       When changes are made to an RAID1, RAID4, or  RAID5  array
       there  is a possibility of inconsistency for short periods
       of time as each update requires are least two block to  be
       written  to  different  devices, and these writes probably
       wont happen at exactly the same time.  This  is  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.

       The 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 shut-
       down.  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 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  ddooeess
       nnoott alert the operator to this condition.  It should prob-
       ably fail to start an array in this condition without man-
       ual intervention.


   RREECCOOVVEERRYY
       If the md driver detects any error on a device in a RAID1,
       RAID4, or RAID5 array, it immediately disables that device
       (marking  it  as  faulty)  and  continues operation on the
       remaining 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.

       Why 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 dif-
       ferent situations can be controlled by the ssppeeeedd__lliimmiitt__mmiinn
       and ssppeeeedd__lliimmiitt__mmaaxx control files mentioned below.



FFIILLEESS
       //pprroocc//mmddssttaatt
              Contains  information about the status of currently
              running array.

       //pprroocc//ssyyss//ddeevv//rraaiidd//ssppeeeedd__lliimmiitt__mmiinn
              A readable and writable file that reflects the cur-
              rent  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.


       //pprroocc//ssyyss//ddeevv//rraaiidd//ssppeeeedd__lliimmiitt__mmaaxx
              A readable and writable file that reflects the cur-
              rent  goal  rebuild  speed  for  times when no non-
              rebuild activity  is  current  on  an  array.   The
              default is 100,000.


SSEEEE AALLSSOO
       mmddaaddmm(8), mmkkrraaiidd(8).



                                                            MD(4)
