MD(4)                                                       MD(4)


NNAAMMEE
       md - Multiple Device driver aka Linux Software Raid

SSYYNNOOPPSSIISS
       //ddeevv//mmdd_n
       //ddeevv//mmdd//_n

DDEESSCCRRIIPPTTIIOONN
       The  mmdd  driver  provides virtual devices that are created
       from one or more  independant  underlying  devices.   This
       array  of devices often contains redundancy, and hence the
       acronym RAID which stands for a Redundant Array  of  Inde-
       pendant Devices.

       mmdd 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.

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


   MMDD SSUUPPEERR BBLLOOCCKK
       With the exception of Legacy Arrays described below,  each
       device  that is incorportated 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.


   LLEEGGAACCYY AARRRRAAYYSS
       Early  versions of the mmdd 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 mmdd driver still supports legacy  linear
       and raid0 md arrays that do not have a superblock.


   LLIINNEEAARR
       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.


   RRAAIIDD00
       A RAID0 array (which has zero redundancy) is also known as
       a striped array.

   RRAAIIDD11
   RRAAIIDD44
   RRAAIIDD55
   MMUUTTIIPPAATTHH
   RREEBBUUIILLDD//RREESSYYNNCC
FFIILLEESS
       //pprroocc//mmddssttaatt
              Contains  information about the status of currently
              running array.

       //pprroocc//ssyyss//ddeevv//rraaiidd//ssppeeeedd__lliimmiitt__mmiinn
              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.


       //pprroocc//ssyyss//ddeevv//rraaiidd//ssppeeeedd__lliimmiitt__mmaaxx
              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.


SSEEEE AALLSSOO
       mmddaaddmm(8), mmkkrraaiidd(8).


                                                            MD(4)