md.man

   1 MD(4)                                                       MD(4)
   2
   3
   4
   5 N\bNA\bAM\bME\bE
   6        md - Multiple Device driver aka Linux Software Raid
   7
   8 S\bSY\bYN\bNO\bOP\bPS\bSI\bIS\bS
   9        /\b/d\bde\bev\bv/\b/m\bmd\bd_\bn
  10        /\b/d\bde\bev\bv/\b/m\bmd\bd/\b/_\bn
  11
  12 D\bDE\bES\bSC\bCR\bRI\bIP\bPT\bTI\bIO\bON\bN
  13        The  m\bmd\bd  driver  provides virtual devices that are created
  14        from one or more  independent  underlying  devices.   This
  15        array  of devices often contains redundancy, and hence the
  16        acronym RAID which stands for a Redundant Array  of  Inde-
  17        pendent Devices.
  18
  19        m\bmd\bd support RAID levels 1 (mirroring) 4 (striped array with
  20        parity device) and 5 (striped array with distributed  par-
  21        ity  information.   If  a  single  underlying device fails
  22        while using one of these level, the array will continue to
  23        function.
  24
  25        m\bmd\bd  also  supports a number of pseudo RAID (non-redundant)
  26        configurations including  RAID0  (striped  array),  LINEAR
  27        (catenated array) and MULTIPATH (a set of different inter-
  28        faces to the same device).
  29
  30
  31    M\bMD\bD S\bSU\bUP\bPE\bER\bR B\bBL\bLO\bOC\bCK\bK
  32        With the exception of Legacy Arrays described below,  each
  33        device  that  is incorporated into an MD array has a _\bs_\bu_\bp_\be_\br
  34        _\bb_\bl_\bo_\bc_\bk  written  towards  the  end  of  the  device.   This
  35        superblock  records  information  about  the structure and
  36        state of the array so that the array can be  reliably  re-
  37        assembled after a shutdown.
  38
  39        The  superblock  is  4K  long  and  is  written into a 64K
  40        aligned block that starts at least 64K and less than  128K
  41        from the end of the device (i.e. to get the address of the
  42        superblock round the size of the device down to a multiple
  43        of 64K and then subtract 64K).  The available size of each
  44        device is the amount of space before the super  block,  so
  45        between 64K and 128K is lost when a device in incorporated
  46        into an MD array.
  47
  48        The superblock contains, among other things:
  49
  50        LEVEL  The manner in which the devices are  arranged  into
  51               the array (linear, raid0, raid1, raid4, raid5, mul-
  52               tipath).
  53
  54        UUID   a 128 bit Universally Unique Identifier that  iden-
  55               tifies the array that this device is part of.
  56
  57
  58    L\bLE\bEG\bGA\bAC\bCY\bY A\bAR\bRR\bRA\bAY\bYS\bS
  59        Early  versions of the m\bmd\bd driver only supported Linear and
  60        Raid0 configurations and so did not use an  MD  superblock
  61        (as  there is not state that needs to be recorded).  While
  62        it is strongly recommended that all newly  created  arrays
  63        utilise  a  superblock to help ensure that they are assem-
  64        bled properly, the m\bmd\bd driver still supports legacy  linear
  65        and raid0 md arrays that do not have a superblock.
  66
  67
  68    L\bLI\bIN\bNE\bEA\bAR\bR
  69        A  linear  array  simply  catenates the available space on
  70        each drive together to form one large virtual drive.
  71
  72        One advantage of this arrangement  over  the  more  common
  73        RAID0 arrangement is that the array may be reconfigured at
  74        a later time with an extra drive and so the array is  made
  75        bigger  without  disturbing the data that is on the array.
  76        However this cannot be done on a live array.
  77
  78
  79
  80    R\bRA\bAI\bID\bD0\b0
  81        A RAID0 array (which has zero redundancy) is also known as
  82        a  striped array.  A RAID0 array is configured at creation
  83        with a C\bCh\bhu\bun\bnk\bk S\bSi\biz\bze\be which must be a multiple of 4 kibibytes.
  84
  85        The  RAID0  driver  places the first chunk of the array to
  86        the first device, the second chunk to the  second  device,
  87        and  so  on until all drives have been assigned one chuck.
  88        This collection of chunks forms a s\bst\btr\bri\bip\bpe\be.  Further  chunks
  89        are  gathered  into  stripes  in  the  same  way which are
  90        assigned to the remaining space in the drives.
  91
  92        If device in the array are not all  the  same  size,  then
  93        once  the  smallest  devices has been exhausted, the RAID0
  94        driver starts collecting chunks into smaller stripes  that
  95        only span the drives which still have remaining space.
  96
  97
  98
  99    R\bRA\bAI\bID\bD1\b1
 100        A RAID1 array is also known as a mirrored set (though mir-
 101        rors tend to provide reflect images, which RAID1 does not)
 102        or a plex.
 103
 104        Once  initialised,  each  device in a RAID1 array contains
 105        exactly the same data.  Changes are written to all devices
 106        in  parallel.   Data  is  read  from  any one device.  The
 107        driver attempts to distribute  read  requests  across  all
 108        devices to maximise performance.
 109
 110        All  devices in a RAID1 array should be the same size.  If
 111        they are not, then only the amount of space  available  on
 112        the  smallest  device  is  used.  Any extra space on other
 113        devices is wasted.
 114
 115
 116    R\bRA\bAI\bID\bD4\b4
 117        A RAID4 array is like a RAID0 array with an  extra  device
 118        for  storing  parity.   Unlike  RAID0, RAID4 also requires
 119        that all stripes  span  all  drives,  so  extra  space  on
 120        devices that are larger than the smallest is wasted.
 121
 122        When  any  block  in  a RAID4 array is modified the parity
 123        block for that stripe (i.e. the block in the parity device
 124        at  the same device offset as the stripe) is also modified
 125        so that the parity block always contains the "parity"  for
 126        the  whole stripe.  i.e. its contents is equivalent to the
 127        result of performing an exclusive-or operation between all
 128        the data blocks in the stripe.
 129
 130        This  allows  the  array  to  continue  to function if one
 131        device fails.  The data that was on  that  device  can  be
 132        calculated  as  needed from the parity block and the other
 133        data blocks.
 134
 135
 136    R\bRA\bAI\bID\bD5\b5
 137        RAID5 is very similar to RAID4.  The  difference  is  that
 138        the  parity  blocks for each stripe, instead of being on a
 139        single device, are distributed across all  devices.   This
 140        allows  more  parallelism  when  writing  as two different
 141        block updates will quite possibly affect parity blocks  on
 142        different devices so there is less contention.
 143
 144        This  also  allows  more  parallelism when reading as read
 145        requests are distributed over all the devices in the array
 146        instead of all but one.
 147
 148
 149    M\bMU\bUT\bTI\bIP\bPA\bAT\bTH\bH
 150        MULTIPATH is not really a RAID at all as there is only one
 151        real device in a MULTIPATH md array.   However  there  are
 152        multiple  access points (paths) to this device, and one of
 153        these paths might fail, so there are some similarities.
 154
 155        A MULTIPATH array is composed of  a  number  of  different
 156        devices,  often  fibre  channel interfaces, that all refer
 157        the the same real device.   If  one  of  these  interfaces
 158        fails  (e.g.  due to cable problems), the multipath driver
 159        to attempt to redirect requests to another interface.
 160
 161
 162
 163    U\bUN\bNC\bCL\bLE\bEA\bAN\bN S\bSH\bHU\bUT\bTD\bDO\bOW\bWN\bN
 164        When changes are made to an RAID1, RAID4, or  RAID5  array
 165        there  is a possibility of inconsistency for short periods
 166        of time as each update requires are least two block to  be
 167        written  to  different  devices, and these writes probably
 168        wont happen at exactly the same time.  This  is  a  system
 169        with  one  of  these arrays is shutdown in the middle of a
 170        write operation (e.g. due to power failure), the array may
 171        not be consistent.
 172
 173        The handle this situation, the md driver marks an array as
 174        "dirty" before writing any data to it,  and  marks  it  as
 175        "clean"  when  the  array is being disabled, e.g. at shut-
 176        down.  If the md driver finds an  array  to  be  dirty  at
 177        startup,  it  proceeds  to  correct any possibly inconsis-
 178        tency.  For RAID1, this involves copying the  contents  of
 179        the first drive onto all other drives.  For RAID4 or RAID5
 180        this involves recalculating the parity for each stripe and
 181        making sure that the parity block has the correct data.
 182
 183        If  a RAID4 or RAID5 array is degraded (missing one drive)
 184        when it is restarted after an unclean shutdown, it  cannot
 185        recalculate  parity, and so it is possible that data might
 186        be undetectably corrupted.  The md driver  currently  d\bdo\boe\bes\bs
 187        n\bno\bot\bt alert the operator to this condition.  It should prob-
 188        ably fail to start an array in this condition without man-
 189        ual intervention.
 190
 191
 192    R\bRE\bEC\bCO\bOV\bVE\bER\bRY\bY
 193        If the md driver detects any error on a device in a RAID1,
 194        RAID4, or RAID5 array, it immediately disables that device
 195        (marking  it  as  faulty)  and  continues operation on the
 196        remaining devices.  If there is a spare drive, the  driver
 197        will  start recreating on one of the spare drives the data
 198        what was on that failed drive, either by copying a working
 199        drive  in  a RAID1 configuration, or by doing calculations
 200        with the parity block on RAID4 and RAID5.
 201
 202        Why this recovery process is happening, the md driver will
 203        monitor  accesses to the array and will slow down the rate
 204        of recovery if other activity is happening, so that normal
 205        access  to the array will not be unduly affected.  When no
 206        other activity is happening, the recovery process proceeds
 207        at  full speed.  The actual speed targets for the two dif-
 208        ferent situations can be controlled by the s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bmi\bin\bn
 209        and s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bma\bax\bx control files mentioned below.
 210
 211
 212
 213 F\bFI\bIL\bLE\bES\bS
 214        /\b/p\bpr\bro\boc\bc/\b/m\bmd\bds\bst\bta\bat\bt
 215               Contains  information about the status of currently
 216               running array.
 217
 218        /\b/p\bpr\bro\boc\bc/\b/s\bsy\bys\bs/\b/d\bde\bev\bv/\b/r\bra\bai\bid\bd/\b/s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bmi\bin\bn
 219               A readable and writable file that reflects the cur-
 220               rent  goal rebuild speed for times when non-rebuild
 221               activity is current on an array.  The speed  is  in
 222               Kibibytes per second, and is a per-device rate, not
 223               a per-array rate (which means that  an  array  with
 224               more  disc  will  shuffle  more  data  for  a given
 225               speed).   The default is 100.
 226
 227
 228        /\b/p\bpr\bro\boc\bc/\b/s\bsy\bys\bs/\b/d\bde\bev\bv/\b/r\bra\bai\bid\bd/\b/s\bsp\bpe\bee\bed\bd_\b_l\bli\bim\bmi\bit\bt_\b_m\bma\bax\bx
 229               A readable and writable file that reflects the cur-
 230               rent  goal  rebuild  speed  for  times when no non-
 231               rebuild activity  is  current  on  an  array.   The
 232               default is 100,000.
 233
 234
 235 S\bSE\bEE\bE A\bAL\bLS\bSO\bO
 236        m\bmd\bda\bad\bdm\bm(8), m\bmk\bkr\bra\bai\bid\bd(8).
 237
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 239
 240                                                             MD(4)