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),
6 (striped array with distributed dual redundancy information), and
10 (striped and mirrored).
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, and dependant of configuration for level 10.
.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 into which errors can be injected).

.SS MD SUPER BLOCK
Each device in an array may have a
.I superblock
which records information about the structure and state of the array.
This allows the array to be reliably re-assembled after a shutdown.

From Linux kernel version 2.6.10,
.B md
provides support for two different formats of this superblock, and
other formats can be added.  Prior to this release, only one format is
supported.

The common format - known as version 0.90 - has
a superblock that 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.
This superblock stores multi-byte fields in a processor-dependant
manner, so arrays cannot easily be moved between computers with
different processors.

The new format - known as version 1 - has a superblock that is
normally 1K long, but can be longer.  It is normally stored between 8K
and 12K from the end of the device, on a 4K boundary, though
variations can be stored at the start of the device (version 1.1) or 4K from
the start of the device (version 1.2).
This superblock format stores multibyte data in a
processor-independant format and has supports upto hundreds of
component devices (version 0.90 only supports 28).

The superblock contains, among other things:
.TP
LEVEL
The manner in which the devices are arranged into the array
(linear, raid0, raid1, raid4, raid5, raid10, multipath).
.TP
UUID
a 128 bit Universally Unique Identifier that identifies the array that
this device is part of.

.SS ARRAYS WITHOUT SUPERBLOCKS
While it is usually best to create arrays with superblocks so that
they can be assembled reliably, there are some circumstances where an
array without superblocks in preferred.  This include:
.TP
LEGACY ARRAYS
Early versions of the
.B md
driver only supported Linear and Raid0 configurations and did not use
a superblock (which is less critical with these configurations).
While such arrays should be rebuilt with superblocks if possible,
.B md
continues to support them.
.TP
FAULTY
Being a largely transparent layer over a different device, the FAULTY
personality doesn't gain anything from having a superblock.
.TP
MULTIPATH
It is often possible to detect devices which are different paths to
the same storage directly rather than having a distinctive superblock
written to the device and searched for on all paths.  In this case,
a MULTIPATH array with no superblock makes sense.
.TP
RAID1
In some configurations it might be desired to create a raid1
configuration that does use a superblock, and to maintain the state of
the array elsewhere.  While not encouraged, this is supported.

.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.

If a chunksize is given with a LINEAR array, the usable space on each
device is rounded down to a multiple of this chunksize.

.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 RAID10

RAID10 provides a combination of RAID1 and RAID0, and sometimes known
as RAID1+0.  Every datablock is duplicated some number of times, and
the resulting collection of datablocks are distributed over multiple
drives.

When configuring a RAID10 array it is necessary to specify the number
of replicas of each data block that are required (this will normally
be 2) and whether the replicas should be 'near' or 'far'.

When 'near' replicas are chosen, the multiple copies of a given chunk
are laid out consecutively across the stripes of the array, so the two
copies of a datablock will likely be at the same offset on two
adjacent devices.

When 'far' replicas are chosen, the multiple copies of a given chunk
are laid out quite distant from each other.  The first copy of all
data blocks will be striped across the early part of all drives in
RAID0 fashion, and then the next copy of all blocks will be striped
across a later section of all drives, always ensuring that all copies
of any given block are on different drives.

The 'far' arrangement can give sequential read performance equal to
that of a RAID0 array, but at the cost of degraded write performance.

It should be noted that the number of devices in a RAID10 array need
not be a multiple of the number of replica of each data block, those
there must be at least as many devices as replicas.

If, for example, an array is created with 5 devices and 2 replicas,
then space equivalent to 2.5 of the devices will be available, and
every block will be stored on two different devices.

Finally, it is possible to have an array with both 'near' and 'far'
copies.  If and array is configured with 2 near copies and 2 far
copies, then there will be a total of 4 copies of each block, each on
a different drive.  This is an artifact of the implementation and is
unlikely to be of real value.

.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, RAID6, or RAID10 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.  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.  For RAID10 it involves copying
one of the replicas of each block onto all the others.  This process,
known as "resynchronising" or "resync" is performed in the background.
The array can still be used, though possibly with reduced performance.

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, RAID6, or RAID10 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, or
by finding a copying originals for RAID10.

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 BITMAP WRITE-INTENT LOGGING

From Linux 2.6.13,
.I md
supports a bitmap based write-intent log.  If configured, the bitmap
is used to record which blocks of the array may be out of sync.
Before any write request is honoured, md will make sure that the
corresponding bit in the log is set.  After a period of time with no
writes to an area of the array, the corresponding bit will be cleared.

This bitmap is used for two optimisations.

Firstly, after an unclear shutdown, the resync process will consult
the bitmap and only resync those blocks that correspond to bits in the
bitmap that are set.  This can dramatically increase resync time.

Secondly, when a drive fails and is removed from the array, md stops
clearing bits in the intent log.  If that same drive is re-added to
the array, md will notice and will only recover the sections of the
drive that are covered by bits in the intent log that are set.  This
can allow a device to be temporarily removed and reinserted without
causing an enormous recovery cost.

The intent log can be stored in a file on a separate device, or it can
be stored near the superblocks of an array which has superblocks.

Subsequent versions of Linux will support hot-adding of bitmaps to
existing arrays.

In 2.6.13, intent bitmaps are only supported with RAID1.  Other levels
will follow.

.SS WRITE-BEHIND

From Linux 2.6.14,
.I md
will support WRITE-BEHIND on RAID1 arrays.

This allows certain devices in the array to be flagged as
.IR write-mostly .
MD will only read from such devices if there is no
other option.

If a write-intent bitmap is also provided, write requests to
write-mostly devices will be treated as write-behind requests and md
will not wait for writes to those requests to complete before
reporting the write as complete to the filesystem.

This allows for a RAID1 with WRITE-BEHIND to be used to mirror data
over a slow link to a remove computer (providing the link isn't too
slow).  The extra latency of the remote link will not slow down normal
operations, but the remote system will still have a reasonably
up-to-date copy of all data.

.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).