MD(4) - phpMan

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 un-
derlying devices. This array of devices often contains redundancy and the devices are of-
ten 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 redun-
dancy 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 meta-
data, and other formats can be added. Prior to this release, only one format is sup-
ported.

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 pro-
cessors.

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 stor-
age 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 inter-
action.

CONTAINERS
Many metadata formats use a single block of metadata to describe a number of different ar-
rays 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 vir-
tual 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 with-
out 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.

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 ker-
nels to assemble the array without needing a module parameter.

To assemble an old array on a new kernel without using the module parameter, use either
the --update=layout-original option or the --update=layout-alternate option.

Once you have updated the layout you will not be able to mount the array on an older ker-
nel. 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 run-
ning 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 layout 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 at-
tempts 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 pro-
file 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. Ev-
ery datablock is duplicated some number of times, and the resulting collection of dat-
ablocks 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 re-
spective 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 assump-
tion also the blocks and block addresses of the exported RAID10 device.
Repeated numbers mean copies of a chunk / block (obviously on different underlying de-
vices).
Hexadecimal numbers (0x00, 0x01, 0x02, ...) are the block addresses of the underlying de-
vices.

"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 ar-
ray.

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 com-
plete 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 con-
secutively ("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 de-
vice (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> con-
secutive 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 in-
stallations.

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 possi-
bility 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 ex-
actly 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 in-
consistency. 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 condi-
tion without manual intervention, though this behaviour can be overridden by a kernel pa-
rameter.

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 op-
eration on the remaining devices. If there are spare drives, the driver will start recre-
ating on one of the spare drives the data which was on that failed drive, either by copy-
ing 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 er-
ror. In later kernels, a read-error will instead cause md to attempt a recovery by over-
writing 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 ac-
cess to the array will not be unduly affected. When no other activity is happening, the
recovery process proceeds at full speed. The actual speed targets for the two different
situations can be controlled by the speed_limit_min and speed_limit_max control files men-
tioned below.

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 num-
ber 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 ex-
pected 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 par-
tition 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 differ-
ent 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. How-
ever 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, espe-
cially 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 pe-
riod 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 dramati-
cally 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 re-
cover 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 enor-
mous 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 at-
tempt 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.

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 inconsistent 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-jour-
nal 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 opera-
tions, 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. Par-
tial 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 addi-
tional 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 Documentation/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 com-
plete 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 under-
lying 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 explicitly on the underlying device, sepa-
rately 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 us-
able 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 ar-
ray 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 shal-
lower), 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 num-
ber 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 writ-
ten 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

o Disable writes to that section of the array (using the sysfs interface),

o take a copy of the data somewhere (i.e. make a backup),

o allow the process to continue and invalidate the backup and restore write access once
the critical section is passed, and

o 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 criti-
cal 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/admin-guide/md.rst which
is distributed with the kernel sources. That file should be consulted for full documenta-
tion. 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 par-
ticular, 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 per-
formed, 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 de-
vice) 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.

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 devices are proceed in the background.

This variable sets a limit on the number of concurrent background 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 active, 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_BIT-
MAP_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-behind 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.
'external' means that bitmap metadata is managed externally to the kernel.

md/bitmap/space
This shows the space (in sectors) which is available at md/bitmap/location, and al-
lows the kernel to know when it is safe to resize the bitmap to match a resized ar-
ray. 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 before, else to 4K be-
yond 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 partitions, then if any of the 4 main par-
titions has a partition type of 0xFD, then that partition will normally be in-
spected 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 de-
vice.

md=n,l,c,i,dev...
This tells the md driver to assemble a legacy RAID0 or LINEAR array without a su-
perblock. 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 ar-
ray 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.