md
MD(4) MD(4)
NAME
md - Multiple Device driver aka Linux Software Raid
SYNOPSIS
/dev/mdn
/dev/md/n
DESCRIPTION
The md driver provides virtual devices that are created from one or
more independent underlying devices. This array of devices often con-
tains redundancy, and hence the acronym RAID which stands for a Redun-
dant Array of Independent Devices.
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 lev-
els 4 and 5, two for RAID level 6, and all but one (N-1) for RAID
level 1.
md also supports a number of pseudo RAID (non-redundant) configura-
tions including RAID0 (striped array), LINEAR (catenated array), MUL-
TIPATH (a set of different interfaces to the same device), and FAULTY
(a layer over a single device that sythesises errors).
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 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 avail-
able 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, multipath).
UUID a 128 bit Universally Unique Identifier that identifies the
array that this device is part of.
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.
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 arrange-
ment 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.
RAID0
A RAID0 array (which has zero redundancy) is also known as a striped
array. A RAID0 array is configured at creation with a 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 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.
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.
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 con-
tains the "parity" for the whole stripe. i.e. its contents is equiva-
lent 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.
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 dis-
tributed across all devices. This allows more parallelism when writ-
ing 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.
RAID6
RAID6 is similar to RAID5, but can handle the loss of any two 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.
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 prob-
lems), the multipath driver will attempt to redirect requests to
another interface.
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 (subse-
quent 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 rele-
vant 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 sec-
tor 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 tran-
sient.
The list of faulty sectors can be flushed, and the active list of
failure modes can be cleared.
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 inconsis-
tency. 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 par-
ity block has the correct data. This process, known as "resynchronis-
ing" 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 unde-
tectably corrupted. The 2.4 md driver 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.
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 speed_limit_min and
speed_limit_max control files mentioned below.
KERNEL PARAMETERS
The md driver recognised three different kernel parameters.
raid=noautodetect
This will disable the normal detection of md arrays that hap-
pens at boot time. If a drive is partitioned with MS-DOS style
partitions, then if any of the 4 main partitions has a parti-
tion 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.
raid=partitionable
raid=part
These are available in 2.6 and later kernels only. They indi-
cate that autodetected MD arrays should be created as parti-
tionable arrays, with a different major device number to the
original non-partitionable md arrays. The device number is
listed as mdp in /proc/devices.
md=n,dev,dev,...
md=dn,dev,dev,...
This tells the md driver to assemble /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 d immediately after the = indicates that a
partitionable device (e.g. /dev/md/d0) should be created
rather than the original non-partitionable device.
md=n,l,c,i,dev...
This tells the md driver to assemble a legacy RAID0 or LINEAR
array without a superblock. n gives the md device number, l
gives the level, 0 for RAID0 or -1 for LINEAR, c gives the
chunk size as a base-2 logarithm offset by twelve, so 0 means
4K, 1 means 8K. i is ignored (legacy support).
FILES
/proc/mdstat
Contains information about the status of currently running
array.
/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.
/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.
SEE ALSO
mdadm(8), mkraid(8).
MD(4)
