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, they 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 RAID 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 an be reliably re-assembled after a shutdown.
  34
  35 The superblock is 4K long and is written into a 64K aligned block that
  36 start 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 ammount 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
  47 .TP
  48 UUID
  49 a 128 bit Universally Unique Identifier that identifies the array that
  50 this device is part of.
  51
  52 .SS LINEAR
  53 .SS RAID0
  54 .SS RAID1
  55 .SS RAID4
  56 .SS RAID5
  57 .SS REBUILD/RESYNC
  58 .SH FILES
  59 .TP
  60 .B /proc/mdstat
  61 Contains information about the status of currently running array.
  62 .TP
  63 .B /proc/sys/dev/raid/speed_limit_min
  64 A readable and writable file that reflects the current goal rebuild
  65 speed for times when non-rebuild activity is current on an array.
  66 The speed is in Kibibytes per second, and is a per-device rate, not a
  67 per-array rate (which means that an array with more disc will shuffle
  68 more data for a given speed).   The default is 100.
  69
  70 .TP
  71 .B /proc/sys/dev/raid/speed_limit_max
  72 A readable and writable file that reflects the current goal rebuild
  73 speed for times when no non-rebuild activity is current on an array.
  74 The default is 100,000.
  75
  76 .SH SEE ALSO
  77 .BR mdadm (8),
  78 .BR mkraid (8).