MD(4) Kernel Interfaces Manual MD(4)
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 con-
tains redundancy and the devices are often disk drives, hence the acro-
nym RAID which stands for a Redundant Array of Independent Disks.
md supports RAID levels 1 (mirroring), 4 (striped array with parity de-
vice), 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 re-
lease, 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 compo-
nent 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 ar-
ray 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 en-
sures 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 crit-
ical 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 su-
perblock.
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 gen-
eral 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 man-
aged 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 meta-
data - 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 at-
tribute. 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 num-
ber 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 re-
mainder of each device is available to be incorporated into various ar-
rays.
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 arrange-
ment is that the array may be reconfigured at a later time with an ex-
tra 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 de-
vice, 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 small-
est device has been exhausted, the RAID0 driver starts collecting
chunks into smaller stripes that only span the drives which still have
remaining space.
A bug was introduced in linux 3.14 which changed the layout of blocks
in a RAID0 beyond the region that is striped over all devices. This
bug does not affect an array with all devices the same size, but can
affect other RAID0 arrays.
Linux 5.4 (and some stable kernels to which the change was backported)
will not normally assemble such an array as it cannot know which layout
to use. There is a module parameter "raid0.default_layout" which can
be set to "1" to force the kernel to use the pre-3.14 layout or to "2"
to force it to use the 3.14-and-later layout. when creating a new
RAID0 array, mdadm will record the chosen layout in the metadata in a
way that allows newer kernels to assemble the array without needing a
module parameter.
To assemble an old array on a new kernel without using the module pa-
rameter, use either the --update=layout-original option or the --up-
date=layout-alternate option.
Once you have updated the layout you will not be able to mount the ar-
ray on an older kernel. If you need to revert to an older kernel, the
layout information can be erased with the --update=layout-unspecificed
option. If you use this option to --assemble while running a newer
kernel, the array will NOT assemble, but the metadata will be update so
that it can be assembled on an older kernel.
No that setting the layout to "unspecified" removes protections against
this bug, and you must be sure that the kernel you use matches the lay-
out of the array.
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 re-
quests 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 se-
quential 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 de-
vices 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. Un-
like 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 content 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 par-
ity 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.
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 multi-
ple 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 ex-
ported 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 dif-
ferent 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 De-
vice #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 de-
vices:
┌───────────┌───────────┌───────────┌───────────┐
│ 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 de-
vices:
┌────────┌────────┌────────┌────────┌────────┐
│ 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 possi-
ble") 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 se-
quence 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 off-
set 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 de-
vices:
┌───────────┌───────────┌───────────┌───────────┐
│ 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 de-
vices:
┌────────┌────────┌────────┌────────┌────────┐
│ 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 se-
quential 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 off-
set 1 of each of the striped copies of <number of devices> con-
secutive 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 de-
vices:
┌───────────┌───────────┌───────────┌───────────┐
│ 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 de-
vices:
┌────────┌────────┌────────┌────────┌────────┐
│ 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 de-
vices> 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 ev-
ery 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 differ-
ent 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 de-
vices, 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 in-
terface.
The MULTIPATH drive is not receiving any ongoing development and should
be considered a legacy driver. The device-mapper based multipath driv-
ers 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 (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 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 tran-
sient.
The list of faulty sectors can be flushed, and the active list of fail-
ure 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 de-
vices, 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" be-
fore 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 recalculat-
ing the parity for each stripe and making sure that the parity block
has the correct data. For RAID10 it involves copying one of the repli-
cas of each block onto all the others. This process, known as "resyn-
chronising" 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 de-
vices. 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 calcu-
lations 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 ef-
fect 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 re-
paired 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 oc-
curs 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 possi-
bly 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 mis-
match 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 bit-
map 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 al-
low 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 re-
covered 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. At-
tempting to write to a known bad block will be ignored if any write er-
rors have been reported by the device. If there have been no write er-
rors 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 differ-
ent 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.
RAID WRITE HOLE
Due to non-atomicity nature of RAID write operations, interruption of
write operations (system crash, etc.) to RAID456 array can lead to in-
consistent parity and data loss (so called RAID-5 write hole). To plug
the write hole md supports two mechanisms described below.
DIRTY STRIPE JOURNAL
From Linux 4.4, 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.
PARTIAL PARITY LOG
From Linux 4.12 md supports Partial Parity Log (PPL) for RAID5
arrays only. Partial parity for a write operation is the XOR of
stripe data chunks not modified by the write. PPL is stored in
the metadata region of RAID member drives, no additional journal
drive is needed. After crashes, if one of the not modified data
disks of the stripe is missing, this updated parity can be used
to recover its data.
This mechanism is documented more fully in the file Documenta-
tion/md/raid5-ppl.rst
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 usu-
ally 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 de-
vice(s). md cannot control the timeout that the underlying devices use
to determine failure. Any changes desired to that timeout must be set
explicitly 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 set-
ting FAILFAST involves a trade-off. The gain is that the chance of un-
acceptable delays is substantially reduced. The cost is that the un-
likely event of data-loss on one device is slightly more likely to re-
sult 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 spe-
cific 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-in-
tent 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 chang-
ing 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 dif-
ferent layout or chunk size. It can also convert between these differ-
ent RAID levels. It can also convert between RAID0 and RAID10, and be-
tween RAID0 and RAID4 or RAID5. Other possibilities may follow in fu-
ture kernels.
During any stripe process there is a 'critical section' during which
live data is being overwritten on disk. For the operation of increas-
ing 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 num-
ber 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 sec-
tion' 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 inter-
face),
• 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 de-
vice, 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 subdi-
rectory called md which contains various files for providing access to
information about the array.
This interface is documented more fully in the file Documentation/ad-
min-guide/md.rst 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 consis-
tent (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 op-
erations 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 mem-
ory" 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 fair-
ness 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.
md/bitmap/backlog
The value stored in the file only has any effect on RAID1 when
write-mostly devices are active, and write requests to those de-
vices are proceed in the background.
This variable sets a limit on the number of concurrent back-
ground writes, the valid values are 0 to 16383, 0 means that
write-behind is not allowed, while any other number means it can
happen. If there are more write requests than the number, new
writes will by synchronous.
md/bitmap/can_clear
This is for externally managed bitmaps, where the kernel writes
the bitmap itself, but metadata describing the bitmap is managed
by mdmon or similar.
When the array is degraded, bits mustn't be cleared. When the
array becomes optimal again, bit can be cleared, but first the
metadata needs to record the current event count. So md sets
this to 'false' and notifies mdmon, then mdmon updates the meta-
data and writes 'true'.
There is no code in mdmon to actually do this, so maybe it
doesn't even work.
md/bitmap/chunksize
The bitmap chunksize can only be changed when no bitmap is ac-
tive, and the value should be power of 2 and at least 512.
md/bitmap/location
This indicates where the write-intent bitmap for the array is
stored. It can be "none" or "file" or a signed offset from the
array metadata - measured in sectors. You cannot set a file by
writing here - that can only be done with the SET_BITMAP_FILE
ioctl.
Write 'none' to 'bitmap/location' will clear bitmap, and the
previous location value must be write to it to restore bitmap.
md/bitmap/max_backlog_used
This keeps track of the maximum number of concurrent write-be-
hind requests for an md array, writing any value to this file
will clear it.
md/bitmap/metadata
This can be 'internal' or 'clustered' or 'external'. 'internal'
is set by default, which means the metadata for bitmap is stored
in the first 256 bytes of the bitmap space. 'clustered' means
separate bitmap metadata are used for each cluster node. 'exter-
nal' means that bitmap metadata is managed externally to the
kernel.
md/bitmap/space
This shows the space (in sectors) which is available at md/bit-
map/location, and allows the kernel to know when it is safe to
resize the bitmap to match a resized array. It should big enough
to contain the total bytes in the bitmap.
For 1.0 metadata, assume we can use up to the superblock if be-
fore, else to 4K beyond superblock. For other metadata versions,
assume no change is possible.
md/bitmap/time_base
This shows the time (in seconds) between disk flushes, and is
used to looking for bits in the bitmap to be cleared.
The default value is 5 seconds, and it should be an unsigned
long value.
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 par-
titions, 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 indi-
cate that autodetected MD arrays should be created as partition-
able arrays, with a different major device number to the origi-
nal 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 ar-
ray.
/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),
MD(4)
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