md - phpMan

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 contains redundancy, and hence the
acronym RAID which stands for a Redundant 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 a 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.

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).

MD SUPER BLOCK
With the exception of Legacy Arrays described below, each device that is incorporated into
an MD array has 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.

LEGACY ARRAYS
Early versions of the md driver only supported Linear and Raid0 configurations and so did
not use an MD superblock (as there is no 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 md driver still supports legacy linear and raid0 md
arrays that do not have a superblock.

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.

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 contains the "parity" for the whole stripe. i.e. its con-
tents 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.

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.

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 logical different devices, often fibre chan-
nel 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.

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 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. 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 does not alert the operator
to this condition. The 2.5 md driver will fail to start an array in this condition with-
out 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 men-
tioned below.

KERNEL PARAMETERS
The md driver recognised three different kernel parameters.

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 par-
titions 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.

md=n,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.

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.