path: root/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
independent underlying devices.  This array of devices often contains
redundancy, and hence the acronym RAID which stands for a Redundant
Array of Independent 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) configurations
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 incorporated 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.
A RAID0 array is configured at creation with a
.B "Chunk Size" 
which must be a power of two, and at least 4 kibibytes.

The RAID0 driver places the first chunk of the array to the first
device, the second chunk to the second device, and so on until all
drives have been assigned one chuck.  This collection of chunks forms
a
.BR stripe .
Further chunks are gathered into stripes in the same way which are
assigned to the remaining space in the drives.

If device in the array are not all the same size, then once the
smallest devices has been exhausted, the RAID0 driver starts
collecting chunks into smaller stripes that only span the drives which
still have remaining space.


.SS RAID1

A RAID1 array is also known as a mirrored set (though mirrors tend to
provide reflect images, which RAID1 does not) or a plex.

Once initialised, each device in a RAID1 array contains exactly the
same data.  Changes are written to all devices in parallel.  Data is
read from any one device.  The driver attempts to distribute read
requests across all devices to maximise performance.

All devices in a RAID1 array should be the same size.  If they are
not, then only the amount of space available on the smallest device is
used.  Any extra space on other devices is wasted.

.SS RAID4

A RAID4 array is like a RAID0 array with an extra device for storing
parity.  Unlike RAID0, RAID4 also requires that all stripes span all
drives, so extra space on devices that are larger than the smallest is
wasted.

When any block in a RAID4 array is modified the parity block for that
stripe (i.e. the block in the parity device at the same device offset
as the stripe) is also modified so that the parity block always
contains the "parity" for the whole stripe.  i.e. its contents is
equivalent to the result of performing an exclusive-or operation
between all the data blocks in the stripe.

This allows the array to continue to function if one device fails.
The data that was on that device can be calculated as needed from the
parity block and the other data blocks.

.SS RAID5

RAID5 is very similar to RAID4.  The difference is that the parity
blocks for each stripe, instead of being on a single device, are
distributed across all devices.  This allows more parallelism when
writing as two different block updates will quite possibly affect
parity blocks on different devices so there is less contention.

This also allows more parallelism when reading as read requests are
distributed over all the devices in the array instead of all but one.

.SS MUTIPATH

MULTIPATH is not really a RAID at all as there is only one real device
in a MULTIPATH md array.  However there are multiple access points
(paths) to this device, and one of these paths might fail, so there
are some similarities.

A MULTIPATH array is composed of a number of different devices, often
fibre channel interfaces, that all refer the the same real device.
If one of these interfaces fails (e.g. due to cable problems), the
multipath driver to attempt to redirect requests to another
interface. 


.SS UNCLEAN SHUTDOWN

When changes are made to an RAID1, RAID4, or RAID5 array there is a
possibility of inconsistency for short periods of time as each update
requires are least two block to be written to different devices, and
these writes probably wont happen at exactly the same time.
This is a system with one of these arrays is shutdown in the middle of
a write operation (e.g. due to power failure), the array may not be
consistent.

The handle this situation, the md driver marks an array as "dirty"
before writing any data to it, and marks it as "clean" when the array
is being disabled, e.g. at shutdown.
If the md driver finds an array to be dirty at startup, it proceeds to
correct any possibly inconsistency.  For RAID1, this involves copying
the contents of the first drive onto all other drives.
For RAID4 or RAID5 this involves recalculating the parity for each
stripe and making sure that the parity block has the correct data.

If a RAID4 or RAID5 array is degraded (missing one drive) when it is
restarted after an unclean shutdown, it cannot recalculate parity, and
so it is possible that data might be undetectably corrupted.
The md driver currently
.B does not
alert the operator to this condition.  It should probably fail to
start an array in this condition without manual intervention.

.SS RECOVERY

If the md driver detects any error on a device in a RAID1, RAID4, or
RAID5 array, it immediately disables that device (marking it as faulty)
and continues operation on the remaining devices.  If there is a spare
drive, the driver will start recreating on one of the spare drives the
data what was on that failed drive, either by copying a working drive
in a RAID1 configuration, or by doing calculations with the parity
block on RAID4 and RAID5.

Why this recovery process is happening, the md driver will monitor
accesses to the array and will slow down the rate of recovery if other
activity is happening, so that normal access to the array will not be
unduly affected.  When no other activity is happening, the recovery
process proceeds at full speed.  The actual speed targets for the two
different situations can be controlled by the
.B speed_limit_min
and
.B speed_limit_max
control files mentioned below.


.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).