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
supports RAID levels 1 (mirroring) 4 (striped array with parity
device), 5 (striped array with distributed parity information) and 6
(striped array with distributed dual redundancy information.)  If
some number of underlying devices fails while using one of these
levels, the array will continue to function; this number is one for
RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for
RAID level 1.
.PP
.B md
also supports a number of pseudo RAID (non-redundant) configurations
including RAID0 (striped array), LINEAR (catenated array),
MULTIPATH (a set of different interfaces to the same device),
and FAULTY (a layer over a single device that sythesises errors).

.SS MD SUPER BLOCK
Though it is possible to create an array without using per-device
superblocks (see below), each device in an MD array will normally have 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 NO-SUPERBLOCK ARRAYS

It is possible for some md arrays to be created without a superblock.
This allows the whole of each device to participate in the array, but
requires some external mechanism to determine what devices should be
arranged into which arrays.

FAULTY arrays are an obvious candidate for not having a superblock as
there is nothing useful to go in the superblock.  MUTIPATH arrays can also be
usefully made without superblocks as there are likely to be other ways
to detect that two paths connect to the same real devices.

Other array type can work without superblocks are RAID1, RAID0, and LINEAR.
However these should only be made without a superblock if you are sure
that you know what you are doing.

.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 yet 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 assigns 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 chunk.  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 devices in the array are not all the same size, then once the
smallest device 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 reflected 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. This device is the last of the active devices in the
array. 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 RAID6

RAID6 is similar to RAID5, but can handle the loss of any \fItwo\fP
devices without data loss.  Accordingly, it requires N+2 drives to
store N drives worth of data.

The performance for RAID6 is slightly lower but comparable to RAID5 in
normal mode and single disk failure mode.  It is very slow in dual
disk failure mode, however.

.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 logically 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 will attempt to redirect requests to
another interface. 

.SS FAULTY
The FAULTY md module is provided for testing purposes.  A faulty array
has exactly one component device and is normally assembled without a
superblock, so the md array created provides direct access to all of
the data in the component device.

The FAULTY module may be requested to simulate faults to allow testing
of other md levels or of filesystems.  Faults can be chosen to trigger
on read requests or write requests, and can be transient (a subsequent
read/write at the address will probably succeed) or persistant
(subsequent read/write of the same address will fail).  Further, read
faults can be "fixable" meaning that they persist until a write
request at the same address.

Fault types can be requested with a period.  In this case the fault
will recur repeatedly after the given number of requests of the
relevant type.  For example if persistent read faults have a period of
100, then every 100th read request would generate a fault, and the
faulty sector would be recorded so that subsequent reads on that
sector would also fail.

There is a limit to the number of faulty sectors that are remembered.
Faults generated after this limit is exhausted are treated as
transient.

The list of faulty sectors can be flushed, and the active list of
failure modes can be cleared.

.SS UNCLEAN SHUTDOWN

When changes are made to a RAID1, RAID4, RAID5 or RAID6 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 won't happen at exactly the same time.
Thus if 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.

To 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, RAID5 and RAID6 this
involves recalculating the parity for each stripe and making sure that
the parity block has the correct data.  This process, known as
"resynchronising" or "resync" is performed in the background.  The
array can still be used, though possibly with reduced performance.

In 2.6 Linux kernels, an md array is marked clean after a short period
(around 20 milliseconds) of no write activity, and then marked dirty
before any subsequent write is attempted.  This means that unclean
shutdowns are much less likely with a 2.6 kernel.

If a RAID4, RAID5 or RAID6 array is degraded (missing at least 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 2.4 md driver
.B does not
alert the operator to this condition.  The 2.5 md driver will 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,
RAID5 or RAID6 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, RAID5 or RAID6.

While 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.

.SS KERNEL PARAMETERS

The md driver recognised three different kernel parameters.
.TP
.B raid=noautodetect
This will disable the normal detection of md arrays that happens at
boot time.  If a drive is partitioned with MS-DOS style partitions,
then if any of the 4 main partitions has a partition type of 0xFD,
then that partition will normally be inspected to see if it is part of
an MD array, and if any full arrays are found, they are started.  This
kernel paramenter disables this behaviour.

.TP
.B raid=partitionable
.TP
.B raid=part
These are available in 2.6 and later kernels only.  They indicate that
autodetected MD arrays should be created as partitionable arrays, with
a different major device number to the original non-partitionable md
arrays.  The device number is listed as
.I mdp
in
.IR /proc/devices .


.TP
.BI md= n , dev , dev ,...
.TP
.BI md=d n , dev , dev ,...
This tells the md driver to assemble
.B /dev/md n
from the listed devices.  It is only necessary to start the device
holding the root filesystem this way.  Other arrays are best started
once the system is booted.

In 2.6 kernels, the
.B d
immediately after the
.B =
indicates that a partitionable device (e.g.
.BR /dev/md/d0 )
should be created rather than the original non-partitionable device.

.TP
.BI md= n , l , c , i , dev...
This tells the md driver to assemble a legacy RAID0 or LINEAR array
without a superblock.
.I n
gives the md device number,
.I l
gives the level, 0 for RAID0 or -1 for LINEAR,
.I c
gives the chunk size as a base-2 logarithm offset by twelve, so 0
means 4K, 1 means 8K.
.I i
is ignored (legacy support).

.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).