MD(4) Kernel Interfaces Manual MD(4)
md - Multiple Device driver aka Linux Software RAID
/dev/mdn
/dev/md/n
/dev/md/name
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).
MD METADATA
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.
ARRAYS WITHOUT METADATA
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.
ARRAYS WITH EXTERNAL METADATA
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
usually be 2) and whether their layout should be "near", "far" or
"offset" (with "offset" being available since Linux 2.6.18).
About the RAID10 Layout Examples:
The examples below visualise the chunk distribution on the underlying
devices for the respective layout.
For simplicity it is assumed that the size of the chunks equals the
size of the blocks of the underlying devices as well as those of the
RAID10 device exported by the kernel (for example /dev/md/name).
Therefore the chunks / chunk numbers map directly to the
blocks /block addresses of the exported RAID10 device.
Decimal numbers (0, 1, 2, ...) are the chunks of the RAID10 and due
to the above assumption also the blocks and block addresses of the
exported RAID10 device.
Repeated numbers mean copies of a chunk / block (obviously on
different underlying devices).
Hexadecimal numbers (0x00, 0x01, 0x02, ...) are the block addresses
of the underlying devices.
"near" Layout
When "near" replicas are chosen, the multiple copies of a
given chunk are laid out consecutively ("as close to each
other as possible") across the stripes of the array.
With an even number of devices, they will likely (unless some
misalignment is present) lay at the very same offset on the
different devices.
This is as the "classic" RAID1+0; that is two groups of
mirrored devices (in the example below the groups
Device #1 / #2 and Device #3 / #4 are each a RAID1) both in
turn forming a striped RAID0.
Example with 2 copies per chunk and an even number (4) of
devices:
┌───────────┌───────────┌───────────┌───────────┐
│ Device #1 │ Device #2 │ Device #3 │ Device #4 │
┌─────├───────────├───────────├───────────├───────────┤
│0x00 │ 0 │ 0 │ 1 │ 1 │
│0x01 │ 2 │ 2 │ 3 │ 3 │
│... │ ... │ ... │ ... │ ... │
│ : │ : │ : │ : │ : │
│... │ ... │ ... │ ... │ ... │
│0x80 │ 254 │ 254 │ 255 │ 255 │
└─────└───────────└───────────└───────────└───────────┘
\---------v---------/ \---------v---------/
RAID1 RAID1
\---------------------v---------------------/
RAID0
Example with 2 copies per chunk and an odd number (5) of
devices:
┌────────┌────────┌────────┌────────┌────────┐
│ Dev #1 │ Dev #2 │ Dev #3 │ Dev #4 │ Dev #5 │
┌─────├────────├────────├────────├────────├────────┤
│0x00 │ 0 │ 0 │ 1 │ 1 │ 2 │
│0x01 │ 2 │ 3 │ 3 │ 4 │ 4 │
│... │ ... │ ... │ ... │ ... │ ... │
│ : │ : │ : │ : │ : │ : │
│... │ ... │ ... │ ... │ ... │ ... │
│0x80 │ 317 │ 318 │ 318 │ 319 │ 319 │
└─────└────────└────────└────────└────────└────────┘
"far" Layout
When "far" replicas are chosen, the multiple copies of a given
chunk are laid out quite distant ("as far as reasonably
possible") from each other.
First a complete sequence of all data blocks (that is all the
data one sees on the exported RAID10 block device) is striped
over the devices. Then another (though "shifted") complete
sequence of all data blocks; and so on (in the case of more
than 2 copies per chunk).
The "shift" needed to prevent placing copies of the same
chunks on the same devices is actually a cyclic permutation
with offset 1 of each of the stripes within a complete
sequence of chunks.
The offset 1 is relative to the previous complete sequence of
chunks, so in case of more than 2 copies per chunk one gets
the following offsets:
1. complete sequence of chunks: offset = 0
2. complete sequence of chunks: offset = 1
3. complete sequence of chunks: offset = 2
:
n. complete sequence of chunks: offset = n-1
Example with 2 copies per chunk and an even number (4) of
devices:
┌───────────┌───────────┌───────────┌───────────┐
│ Device #1 │ Device #2 │ Device #3 │ Device #4 │
┌─────├───────────├───────────├───────────├───────────┤
│0x00 │ 0 │ 1 │ 2 │ 3 │ \
│0x01 │ 4 │ 5 │ 6 │ 7 │ > [#]
│... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ :
│... │ ... │ ... │ ... │ ... │ :
│0x40 │ 252 │ 253 │ 254 │ 255 │ /
│0x41 │ 3 │ 0 │ 1 │ 2 │ \
│0x42 │ 7 │ 4 │ 5 │ 6 │ > [#]~
│... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ :
│... │ ... │ ... │ ... │ ... │ :
│0x80 │ 255 │ 252 │ 253 │ 254 │ /
└─────└───────────└───────────└───────────└───────────┘
Example with 2 copies per chunk and an odd number (5) of
devices:
┌────────┌────────┌────────┌────────┌────────┐
│ Dev #1 │ Dev #2 │ Dev #3 │ Dev #4 │ Dev #5 │
┌─────├────────├────────├────────├────────├────────┤
│0x00 │ 0 │ 1 │ 2 │ 3 │ 4 │ \
│0x01 │ 5 │ 6 │ 7 │ 8 │ 9 │ > [#]
│... │ ... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ : │ :
│... │ ... │ ... │ ... │ ... │ ... │ :
│0x40 │ 315 │ 316 │ 317 │ 318 │ 319 │ /
│0x41 │ 4 │ 0 │ 1 │ 2 │ 3 │ \
│0x42 │ 9 │ 5 │ 6 │ 7 │ 8 │ > [#]~
│... │ ... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ : │ :
│... │ ... │ ... │ ... │ ... │ ... │ :
│0x80 │ 319 │ 315 │ 316 │ 317 │ 318 │ /
└─────└────────└────────└────────└────────└────────┘
With [#] being the complete sequence of chunks and [#]~ the
cyclic permutation with offset 1 thereof (in the case of more
than 2 copies per chunk there would be
([#]~)~, (([#]~)~)~, ...).
The advantage of this layout is that MD can easily spread
sequential reads over the devices, making them similar to
RAID0 in terms of speed.
The cost is more seeking for writes, making them substantially
slower.
"offset" Layout
When "offset" replicas are chosen, all the copies of a given
chunk are striped consecutively ("offset by the stripe length
after each other") over the devices.
Explained in detail, <number of devices> consecutive chunks
are striped over the devices, immediately followed by a
"shifted" copy of these chunks (and by further such "shifted"
copies in the case of more than 2 copies per chunk).
This pattern repeats for all further consecutive chunks of the
exported RAID10 device (in other words: all further data
blocks).
The "shift" needed to prevent placing copies of the same
chunks on the same devices is actually a cyclic permutation
with offset 1 of each of the striped copies of <number of
devices> consecutive chunks.
The offset 1 is relative to the previous striped copy of
<number of devices> consecutive chunks, so in case of more
than 2 copies per chunk one gets the following offsets:
1. <number of devices> consecutive chunks: offset = 0
2. <number of devices> consecutive chunks: offset = 1
3. <number of devices> consecutive chunks: offset = 2
:
n. <number of devices> consecutive chunks: offset = n-1
Example with 2 copies per chunk and an even number (4) of
devices:
┌───────────┌───────────┌───────────┌───────────┐
│ Device #1 │ Device #2 │ Device #3 │ Device #4 │
┌─────├───────────├───────────├───────────├───────────┤
│0x00 │ 0 │ 1 │ 2 │ 3 │ ) AA
│0x01 │ 3 │ 0 │ 1 │ 2 │ ) AA~
│0x02 │ 4 │ 5 │ 6 │ 7 │ ) AB
│0x03 │ 7 │ 4 │ 5 │ 6 │ ) AB~
│... │ ... │ ... │ ... │ ... │ ) ...
│ : │ : │ : │ : │ : │ :
│... │ ... │ ... │ ... │ ... │ ) ...
│0x79 │ 251 │ 252 │ 253 │ 254 │ ) EX
│0x80 │ 254 │ 251 │ 252 │ 253 │ ) EX~
└─────└───────────└───────────└───────────└───────────┘
Example with 2 copies per chunk and an odd number (5) of
devices:
┌────────┌────────┌────────┌────────┌────────┐
│ Dev #1 │ Dev #2 │ Dev #3 │ Dev #4 │ Dev #5 │
┌─────├────────├────────├────────├────────├────────┤
│0x00 │ 0 │ 1 │ 2 │ 3 │ 4 │ ) AA
│0x01 │ 4 │ 0 │ 1 │ 2 │ 3 │ ) AA~
│0x02 │ 5 │ 6 │ 7 │ 8 │ 9 │ ) AB
│0x03 │ 9 │ 5 │ 6 │ 7 │ 8 │ ) AB~
│... │ ... │ ... │ ... │ ... │ ... │ ) ...
│ : │ : │ : │ : │ : │ : │ :
│... │ ... │ ... │ ... │ ... │ ... │ ) ...
│0x79 │ 314 │ 315 │ 316 │ 317 │ 318 │ ) EX
│0x80 │ 318 │ 314 │ 315 │ 316 │ 317 │ ) EX~
└─────└────────└────────└────────└────────└────────┘
With AA, AB, ..., AZ, BA, ... being the sets of <number of
devices> consecutive chunks and AA~, AB~, ..., AZ~, BA~, ...
the cyclic permutations with offset 1 thereof (in the case of
more than 2 copies per chunk there would be (AA~)~, ... as
well as ((AA~)~)~, ... and so on).
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.
BAD BLOCK LIST
From Linux 3.5 each device in an md array can store a list of known-
bad-blocks. This list is 4K in size and usually positioned at the
end of the space between the superblock and the data.
When a block cannot be read and cannot be repaired by writing data
recovered from other devices, the address of the block is stored in
the bad block list. Similarly if an attempt to write a block fails,
the address will be recorded as a bad block. If attempting to record
the bad block fails, the whole device will be marked faulty.
Attempting to read from a known bad block will cause a read error.
Attempting to write to a known bad block will be ignored if any write
errors have been reported by the device. If there have been no write
errors then the data will be written to the known bad block and if
that succeeds, the address will be removed from the list.
This allows an array to fail more gracefully - a few blocks on
different devices can be faulty without taking the whole array out of
action.
The list is particularly useful when recovering to a spare. If a few
blocks cannot be read from the other devices, the bulk of the
recovery can complete and those few bad blocks will be recorded in
the bad block list.
RAID456 WRITE JOURNAL
Due to non-atomicity nature of RAID write operations, interruption of
write operations (system crash, etc.) to RAID456 array can lead to
inconsistent parity and data loss (so called RAID-5 write hole).
To plug the write hole, from Linux 4.4 (to be confirmed), md supports
write ahead journal for RAID456. When the array is created, an
additional journal device can be added to the array through write-
journal option. The RAID write journal works similar to file system
journals. Before writing to the data disks, md persists data AND
parity of the stripe to the journal device. After crashes, md
searches the journal device for incomplete write operations, and
replay them to the data disks.
When the journal device fails, the RAID array is forced to run in
read-only mode.
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.
FAILFAST
From Linux 4.10, md supports FAILFAST for RAID1 and RAID10 arrays.
This is a flag that can be set on individual drives, though it is
usually set on all drives, or no drives.
When md sends an I/O request to a drive that is marked as FAILFAST,
and when the array could survive the loss of that drive without
losing data, md will request that the underlying device does not
perform any retries. This means that a failure will be reported to
md promptly, and it can mark the device as faulty and continue using
the other device(s). md cannot control the timeout that the
underlying devices use to determine failure. Any changes desired to
that timeout must be set explictly on the underlying device,
separately from using mdadm.
If a FAILFAST request does fail, and if it is still safe to mark the
device as faulty without data loss, that will be done and the array
will continue functioning on a reduced number of devices. If it is
not possible to safely mark the device as faulty, md will retry the
request without disabling retries in the underlying device. In any
case, md will not attempt to repair read errors on a device marked as
FAILFAST by writing out the correct. It will just mark the device as
faulty.
FAILFAST is appropriate for storage arrays that have a low
probability of true failure, but will sometimes introduce
unacceptable delays to I/O requests while performing internal
maintenance. The value of setting FAILFAST involves a trade-off.
The gain is that the chance of unacceptable delays is substantially
reduced. The cost is that the unlikely event of data-loss on one
device is slightly more likely to result in data-loss for the array.
When a device in an array using FAILFAST is marked as faulty, it will
usually become usable again in a short while. mdadm makes no attempt
to detect that possibility. Some separate mechanism, tuned to the
specific details of the expected failure modes, needs to be created
to monitor devices to see when they return to full functionality, and
to then re-add them to the array. In order of this "re-add"
functionality to be effective, an array using FAILFAST should always
have a write-intent bitmap.
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).
/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.
mdadm(8),
This page is part of the mdadm (Tool for managing md arrays in Linux)
project. Information about the project can be found at
⟨http://neil.brown.name/blog/mdadm⟩. If you have a bug report for
this manual page, send it to linux-raid@vger.kernl.org. This page
was obtained from the project's upstream Git repository
⟨git://neil.brown.name/mdadm⟩ on 2017-07-05. If you discover any ren‐
dering problems in this HTML version of the page, or you believe
there is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
MD(4)
Pages that refer to this page: mdadm.conf(5), mdadm(8), mdassemble(8), mdmon(8), raid6check(8), xfs_growfs(8)