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.TH MD 4
.SH NAME
md \- Multiple Device driver aka Linux Software Raid
.SH SYNOPSIS
.BI /dev/md n
.br
.BI /dev/md/ n
.SH DESCRIPTION
The
.B md
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 Independant Devices.
.PP
.B md
support RAID levels 1 (mirroring) 4 (striped array with parity device) and 5
(striped array with distributed parity information. If a single underlying
device fails while using one of these level, the array will continue
to function.
.PP
.B md
also supports a number of pseudo RAID (non-redundant) configuations
including RAID0 (striped array), LINEAR (catenated array) and
MULTIPATH (a set of different interfaces to the same device).
.SS MD SUPER BLOCK
With the exception of Legacy Arrays described below, each device that
is incorportated into an MD array has a
.I super block
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:
.TP
LEVEL
The manner in which the devices are arranged into the array
(linear, raid0, raid1, raid4, raid5, multipath).
.TP
UUID
a 128 bit Universally Unique Identifier that identifies the array that
this device is part of.
.SS LEGACY ARRAYS
Early versions of the
.B md
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 assembled
properly, the
.B md
driver still supports legacy linear and raid0 md arrays that
do not have a superblock.
.SS LINEAR
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.
.SS RAID0
A RAID0 array (which has zero redundancy) is also known as a
striped array.
.SS RAID1
.SS RAID4
.SS RAID5
.SS MUTIPATH
.SS REBUILD/RESYNC
.SH FILES
.TP
.B /proc/mdstat
Contains information about the status of currently running array.
.TP
.B /proc/sys/dev/raid/speed_limit_min
A readable and writable file that reflects the current 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.
.TP
.B /proc/sys/dev/raid/speed_limit_max
A readable and writable file that reflects the current goal rebuild
speed for times when no non-rebuild activity is current on an array.
The default is 100,000.
.SH SEE ALSO
.BR mdadm (8),
.BR mkraid (8).
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