md.4

   1 .TH MD 4
   2 .SH NAME
   3 md \- Multiple Device driver aka Linux Software Raid
   4 .SH SYNOPSIS
   5 .BI /dev/md n
   6 .br
   7 .BI /dev/md/ n
   8 .SH DESCRIPTION
   9 The
  10 .B md
  11 driver provides virtual devices that are created from one or more
  12 independant underlying devices.  This array of devices often contains
  13 redundancy, and hence the acronym RAID which stands for a Redundant
  14 Array of Independant Devices.
  15 .PP
  16 .B md
  17 support RAID levels 1 (mirroring) 4 (striped array with parity device) and 5
  18 (striped array with distributed parity information.  If a single underlying
  19 device fails while using one of these level, the array will continue
  20 to function.
  21 .PP
  22 .B md
  23 also supports a number of pseudo RAID (non-redundant) configuations
  24 including RAID0 (striped array), LINEAR (catenated array) and
  25 MULTIPATH (a set of different interfaces to the same device).
  26
  27 .SS MD SUPER BLOCK
  28 With the exception of Legacy Arrays described below, each device that
  29 is incorportated into an MD array has a
  30 .I super block
  31 written towards the end of the device.  This superblock records
  32 information about the structure and state of the array so that the
  33 array can be reliably re-assembled after a shutdown.
  34
  35 The superblock is 4K long and is written into a 64K aligned block that
  36 starts at least 64K and less than 128K from the end of the device
  37 (i.e. to get the address of the superblock round the size of the
  38 device down to a multiple of 64K and then subtract 64K).
  39 The available size of each device is the amount of space before the
  40 super block, so between 64K and 128K is lost when a device in
  41 incorporated into an MD array.
  42
  43 The superblock contains, among other things:
  44 .TP
  45 LEVEL
  46 The manner in which the devices are arranged into the array
  47 (linear, raid0, raid1, raid4, raid5, multipath).
  48 .TP
  49 UUID
  50 a 128 bit Universally Unique Identifier that identifies the array that
  51 this device is part of.
  52
  53 .SS LEGACY ARRAYS
  54 Early versions of the
  55 .B md
  56 driver only supported Linear and Raid0 configurations and so
  57 did not use an MD superblock (as there is not state that needs to be
  58 recorded).  While it is strongly recommended that all newly created
  59 arrays utilise a superblock to help ensure that they are assembled
  60 properly, the
  61 .B md
  62 driver still supports legacy linear and raid0 md arrays that
  63 do not have a superblock.
  64
  65 .SS LINEAR
  66
  67 A linear array simply catenates the available space on each
  68 drive together to form one large virtual drive.
  69
  70 One advantage of this arrangement over the more common RAID0
  71 arrangement is that the array may be reconfigured at a later time with
  72 an extra drive and so the array is made bigger without disturbing the
  73 data that is on the array.  However this cannot be done on a live
  74 array.
  75
  76
  77
  78 .SS RAID0
  79
  80 A RAID0 array (which has zero redundancy) is also known as a
  81 striped array.
  82 .SS RAID1
  83 .SS RAID4
  84 .SS RAID5
  85 .SS MUTIPATH
  86 .SS REBUILD/RESYNC
  87 .SH FILES
  88 .TP
  89 .B /proc/mdstat
  90 Contains information about the status of currently running array.
  91 .TP
  92 .B /proc/sys/dev/raid/speed_limit_min
  93 A readable and writable file that reflects the current goal rebuild
  94 speed for times when non-rebuild activity is current on an array.
  95 The speed is in Kibibytes per second, and is a per-device rate, not a
  96 per-array rate (which means that an array with more disc will shuffle
  97 more data for a given speed).   The default is 100.
  98
  99 .TP
 100 .B /proc/sys/dev/raid/speed_limit_max
 101 A readable and writable file that reflects the current goal rebuild
 102 speed for times when no non-rebuild activity is current on an array.
 103 The default is 100,000.
 104
 105 .SH SEE ALSO
 106 .BR mdadm (8),
 107 .BR mkraid (8).