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MD(4) MD(4)
NNAAMMEE
md - Multiple Device driver aka Linux Software Raid
SSYYNNOOPPSSIISS
//ddeevv//mmdd_n
//ddeevv//mmdd//_n
DDEESSCCRRIIPPTTIIOONN
The mmdd 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.
mmdd 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.
mmdd 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).
MMDD SSUUPPEERR BBLLOOCCKK
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.
LLEEGGAACCYY AARRRRAAYYSS
Early versions of the mmdd 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 mmdd driver still supports legacy linear
and raid0 md arrays that do not have a superblock.
LLIINNEEAARR
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.
RRAAIIDD00
A RAID0 array (which has zero redundancy) is also known as
a striped array.
RRAAIIDD11
RRAAIIDD44
RRAAIIDD55
MMUUTTIIPPAATTHH
RREEBBUUIILLDD//RREESSYYNNCC
FFIILLEESS
//pprroocc//mmddssttaatt
Contains information about the status of currently
running array.
//pprroocc//ssyyss//ddeevv//rraaiidd//ssppeeeedd__lliimmiitt__mmiinn
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.
//pprroocc//ssyyss//ddeevv//rraaiidd//ssppeeeedd__lliimmiitt__mmaaxx
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.
SSEEEE AALLSSOO
mmddaaddmm(8), mmkkrraaiidd(8).
MD(4)
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