[thirdparty/mdadm.git] / md.4

.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).
Commit	Line	Data
56eb10c0 NB	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
11a3e71d	19	device fails while using one of these level, the array will continue
56eb10c0 NB	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
11a3e71d	27	.SS MD SUPER BLOCK
56eb10c0 NB	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
11a3e71d	33	array can be reliably re-assembled after a shutdown.
56eb10c0 NB	34
56eb10c0 NB	35	The superblock is 4K long and is written into a 64K aligned block that
11a3e71d	36	starts at least 64K and less than 128K from the end of the device
56eb10c0 NB	37	(i.e. to get the address of the superblock round the size of the
56eb10c0 NB	38	device down to a multiple of 64K and then subtract 64K).
11a3e71d	39	The available size of each device is the amount of space before the
56eb10c0 NB	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
11a3e71d NB	46	The manner in which the devices are arranged into the array
11a3e71d NB	47	(linear, raid0, raid1, raid4, raid5, multipath).
56eb10c0 NB	48	.TP
	49	UUID
	50	a 128 bit Universally Unique Identifier that identifies the array that
	51	this device is part of.
	52
11a3e71d NB	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
56eb10c0	65	.SS LINEAR
11a3e71d NB	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
56eb10c0	78	.SS RAID0
11a3e71d NB	79
	80	A RAID0 array (which has zero redundancy) is also known as a
	81	striped array.
56eb10c0 NB	82	.SS RAID1
	83	.SS RAID4
	84	.SS RAID5
11a3e71d	85	.SS MUTIPATH
56eb10c0 NB	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).