NAME¶
md - Multiple Device driver aka Linux Software RAID
SYNOPSIS¶
/dev/mdn
/dev/md/n
/dev/md/name
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 the devices are often disk drives, hence the acronym RAID which
stands for a Redundant Array of Independent Disks.
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 dependent on configuration for level 10.
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).
Each device in an array may have some
metadata stored in the device. This
metadata is sometimes called a
superblock. The metadata 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,
md provides support for two different
formats of metadata, 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-dependent 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 metadata format stores multibyte data in a
processor-independent format and supports up to hundreds of component devices
(version 0.90 only supports 28).
The metadata contains, among other things:
- LEVEL
- The manner in which the devices are arranged into the array
(linear, raid0, raid1, raid4, raid5, raid10, multipath).
- UUID
- a 128 bit Universally Unique Identifier that identifies the
array that contains this device.
When a version 0.90 array is being reshaped (e.g. adding extra devices to a
RAID5), the version number is temporarily set to 0.91. This ensures that if
the reshape process is stopped in the middle (e.g. by a system crash) and the
machine boots into an older kernel that does not support reshaping, then the
array will not be assembled (which would cause data corruption) but will be
left untouched until a kernel that can complete the reshape processes is used.
While it is usually best to create arrays with superblocks so that they can be
assembled reliably, there are some circumstances when an array without
superblocks is preferred. These include:
- LEGACY ARRAYS
- Early versions of the 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, md continues to support
them.
- FAULTY
- Being a largely transparent layer over a different device,
the FAULTY personality doesn't gain anything from having a
superblock.
- 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.
- RAID1
- In some configurations it might be desired to create a
raid1 configuration that does not use a superblock, and to maintain the
state of the array elsewhere. While not encouraged for general use, it
does have special-purpose uses and is supported.
From release 2.6.28, the
md driver supports arrays with externally
managed metadata. That is, the metadata is not managed by the kernel but
rather by a user-space program which is external to the kernel. This allows
support for a variety of metadata formats without cluttering the kernel with
lots of details.
md is able to communicate with the user-space program through various
sysfs attributes so that it can make appropriate changes to the metadata - for
example to mark a device as faulty. When necessary,
md will wait for
the program to acknowledge the event by writing to a sysfs attribute. The
manual page for
mdmon(8) contains more detail about this interaction.
CONTAINERS¶
Many metadata formats use a single block of metadata to describe a number of
different arrays which all use the same set of devices. In this case it is
helpful for the kernel to know about the full set of devices as a whole. This
set is known to md as a
container. A container is an
md array
with externally managed metadata and with device offset and size so that it
just covers the metadata part of the devices. The remainder of each device is
available to be incorporated into various arrays.
LINEAR¶
A linear array simply catenates the available space on each drive 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, so the
array is made bigger without disturbing the data that is on the array. This
can even 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.
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 (prior to Linux 2.6.31), 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, and 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).
Note that the read balancing done by the driver does not make the RAID1
performance profile be the same as for RAID0; a single stream of sequential
input will not be accelerated (e.g. a single dd), but multiple sequential
streams or a random workload will use more than one spindle. In theory, having
an N-disk RAID1 will allow N sequential threads to read from all disks.
Individual devices in a RAID1 can be marked as "write-mostly". These
drives are excluded from the normal read balancing and will only be read from
when there is no other option. This can be useful for devices connected over a
slow link.
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 content 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.
RAID10¶
RAID10 provides a combination of RAID1 and RAID0, and is 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', 'offset' or 'far'. (Note that the
'offset' layout is only available from 2.6.18).
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 reduced write performance.
When 'offset' replicas are chosen, the multiple copies of a given chunk are laid
out on consecutive drives and at consecutive offsets. Effectively each stripe
is duplicated and the copies are offset by one device. This should give
similar read characteristics to 'far' if a suitably large chunk size is used,
but without as much seeking for writes.
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; however, 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
an 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.
MULTIPATH¶
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.
The MULTIPATH drive is not receiving any ongoing development and should be
considered a legacy driver. The device-mapper based multipath drivers should
be preferred for new installations.
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 persistent (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.
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 at 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. 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, two
for RAID6) 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 2.6 md driver will fail to start an array in this condition
without manual intervention, though this behaviour can be overridden by a
kernel parameter.
RECOVERY¶
If the md driver detects a write 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 are spare
drives, the driver will start recreating on one of the spare drives the data
which 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 and copying originals for RAID10.
In kernels prior to about 2.6.15, a read error would cause the same effect as a
write error. In later kernels, a read-error will instead cause md to attempt a
recovery by overwriting the bad block. i.e. it will find the correct data from
elsewhere, write it over the block that failed, and then try to read it back
again. If either the write or the re-read fail, md will treat the error the
same way that a write error is treated, and will fail the whole device.
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.
SCRUBBING AND MISMATCHES¶
As storage devices can develop bad blocks at any time it is valuable to
regularly read all blocks on all devices in an array so as to catch such bad
blocks early. This process is called
scrubbing.
md arrays can be scrubbed by writing either
check or
repair to the
file
md/sync_action in the
sysfs directory for the device.
Requesting a scrub will cause
md to read every block on every device in
the array, and check that the data is consistent. For RAID1 and RAID10, this
means checking that the copies are identical. For RAID4, RAID5, RAID6 this
means checking that the parity block is (or blocks are) correct.
If a read error is detected during this process, the normal read-error handling
causes correct data to be found from other devices and to be written back to
the faulty device. In many case this will effectively
fix the bad
block.
If all blocks read successfully but are found to not be consistent, then this is
regarded as a
mismatch.
If
check was used, then no action is taken to handle the mismatch, it is
simply recorded. If
repair was used, then a mismatch will be repaired
in the same way that
resync repairs arrays. For RAID5/RAID6 new parity
blocks are written. For RAID1/RAID10, all but one block are overwritten with
the content of that one block.
A count of mismatches is recorded in the
sysfs file
md/mismatch_cnt. This is set to zero when a scrub starts and is
incremented whenever a sector is found that is a mismatch.
md normally
works in units much larger than a single sector and when it finds a mismatch,
it does not determine exactly how many actual sectors were affected but simply
adds the number of sectors in the IO unit that was used. So a value of 128
could simply mean that a single 64KB check found an error (128 x 512bytes =
64KB).
If an array is created by
mdadm with
--assume-clean then a
subsequent check could be expected to find some mismatches.
On a truly clean RAID5 or RAID6 array, any mismatches should indicate a hardware
problem at some level - software issues should never cause such a mismatch.
However on RAID1 and RAID10 it is possible for software issues to cause a
mismatch to be reported. This does not necessarily mean that the data on the
array is corrupted. It could simply be that the system does not care what is
stored on that part of the array - it is unused space.
The most likely cause for an unexpected mismatch on RAID1 or RAID10 occurs if a
swap partition or swap file is stored on the array.
When the swap subsystem wants to write a page of memory out, it flags the page
as 'clean' in the memory manager and requests the swap device to write it out.
It is quite possible that the memory will be changed while the write-out is
happening. In that case the 'clean' flag will be found to be clear when the
write completes and so the swap subsystem will simply forget that the swapout
had been attempted, and will possibly choose a different page to write out.
If the swap device was on RAID1 (or RAID10), then the data is sent from memory
to a device twice (or more depending on the number of devices in the array).
Thus it is possible that the memory gets changed between the times it is sent,
so different data can be written to the different devices in the array. This
will be detected by
check as a mismatch. However it does not reflect
any corruption as the block where this mismatch occurs is being treated by the
swap system as being empty, and the data will never be read from that block.
It is conceivable for a similar situation to occur on non-swap files, though it
is less likely.
Thus the
mismatch_cnt value can not be interpreted very reliably on RAID1
or RAID10, especially when the device is used for swap.
BITMAP WRITE-INTENT LOGGING¶
From Linux 2.6.13,
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 unclean 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 reduce 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.
It is possible to add an intent log to an active array, or remove an intent log
if one is present.
In 2.6.13, intent bitmaps are only supported with RAID1. Other levels with
redundancy are supported from 2.6.15.
WRITE-BEHIND¶
From Linux 2.6.14,
md supports WRITE-BEHIND on RAID1 arrays.
This allows certain devices in the array to be flagged as
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 remote 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.
RESTRIPING¶
Restriping, also known as
Reshaping, is the processes of
re-arranging the data stored in each stripe into a new layout. This might
involve changing the number of devices in the array (so the stripes are
wider), changing the chunk size (so stripes are deeper or shallower), or
changing the arrangement of data and parity (possibly changing the raid level,
e.g. 1 to 5 or 5 to 6).
As of Linux 2.6.35, md can reshape a RAID4, RAID5, or RAID6 array to have a
different number of devices (more or fewer) and to have a different layout or
chunk size. It can also convert between these different RAID levels. It can
also convert between RAID0 and RAID10, and between RAID0 and RAID4 or RAID5.
Other possibilities may follow in future kernels.
During any stripe process there is a 'critical section' during which live data
is being overwritten on disk. For the operation of increasing the number of
drives in a raid5, this critical section covers the first few stripes (the
number being the product of the old and new number of devices). After this
critical section is passed, data is only written to areas of the array which
no longer hold live data — the live data has already been located away.
For a reshape which reduces the number of devices, the 'critical section' is at
the end of the reshape process.
md is not able to ensure data preservation if there is a crash (e.g. power
failure) during the critical section. If md is asked to start an array which
failed during a critical section of restriping, it will fail to start the
array.
To deal with this possibility, a user-space program must
- •
- Disable writes to that section of the array (using the
sysfs interface),
- •
- take a copy of the data somewhere (i.e. make a
backup),
- •
- allow the process to continue and invalidate the backup and
restore write access once the critical section is passed, and
- •
- provide for restoring the critical data before restarting
the array after a system crash.
mdadm versions from 2.4 do this for growing a RAID5 array.
For operations that do not change the size of the array, like simply increasing
chunk size, or converting RAID5 to RAID6 with one extra device, the entire
process is the critical section. In this case, the restripe will need to
progress in stages, as a section is suspended, backed up, restriped, and
released.
SYSFS INTERFACE¶
Each block device appears as a directory in
sysfs (which is usually
mounted at
/sys). For MD devices, this directory will contain a
subdirectory called
md which contains various files for providing
access to information about the array.
This interface is documented more fully in the file
Documentation/md.txt
which is distributed with the kernel sources. That file should be consulted
for full documentation. The following are just a selection of attribute files
that are available.
- md/sync_speed_min
- This value, if set, overrides the system-wide setting in
/proc/sys/dev/raid/speed_limit_min for this array only. Writing the
value system to this file will cause the system-wide setting to
have effect.
- md/sync_speed_max
- This is the partner of md/sync_speed_min and
overrides /proc/sys/dev/raid/speed_limit_max described below.
- md/sync_action
- This can be used to monitor and control the resync/recovery
process of MD. In particular, writing "check" here will cause
the array to read all data block and check that they are consistent (e.g.
parity is correct, or all mirror replicas are the same). Any discrepancies
found are NOT corrected.
A count of problems found will be stored in md/mismatch_count.
Alternately, "repair" can be written which will cause the same
check to be performed, but any errors will be corrected.
Finally, "idle" can be written to stop the check/repair process.
- md/stripe_cache_size
- This is only available on RAID5 and RAID6. It records the
size (in pages per device) of the stripe cache which is used for
synchronising all write operations to the array and all read operations if
the array is degraded. The default is 256. Valid values are 17 to 32768.
Increasing this number can increase performance in some situations, at
some cost in system memory. Note, setting this value too high can result
in an "out of memory" condition for the system.
memory_consumed = system_page_size * nr_disks * stripe_cache_size
- md/preread_bypass_threshold
- This is only available on RAID5 and RAID6. This variable
sets the number of times MD will service a full-stripe-write before
servicing a stripe that requires some "prereading". For fairness
this defaults to 1. Valid values are 0 to stripe_cache_size. Setting this
to 0 maximizes sequential-write throughput at the cost of fairness to
threads doing small or random writes.
KERNEL PARAMETERS¶
The md driver recognised several 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 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 parameter disables this behaviour.
- raid=partitionable
- 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 mdp in
/proc/devices.
- md_mod.start_ro=1
- /sys/module/md_mod/parameters/start_ro
- This tells md to start all arrays in read-only mode. This
is a soft read-only that will automatically switch to read-write on the
first write request. However until that write request, nothing is written
to any device by md, and in particular, no resync or recovery operation is
started.
- md_mod.start_dirty_degraded=1
- /sys/module/md_mod/parameters/start_dirty_degraded
- As mentioned above, md will not normally start a RAID4,
RAID5, or RAID6 that is both dirty and degraded as this situation can
imply hidden data loss. This can be awkward if the root filesystem is
affected. Using this module parameter allows such arrays to be started at
boot time. It should be understood that there is a real (though small)
risk of data corruption in this situation.
- 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
disks will shuffle more data for a given speed). The default is 1000.
- /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 200,000.
SEE ALSO¶
mdadm(8),
mkraid(8).