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md.4
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.\" Copyright Neil Brown and others.
.\" This program is free software; you can redistribute it and/or modify
.\" it under the terms of the GNU General Public License as published by
.\" the Free Software Foundation; either version 2 of the License, or
.\" (at your option) any later version.
.\" See file COPYING in distribution for details.
.if n .pl 1000v
.TH MD 4
.SH NAME
md \- Multiple Device driver aka Linux Software RAID
.SH SYNOPSIS
.BI /dev/md n
.br
.BI /dev/md/ n
.br
.BR /dev/md/ name
.SH DESCRIPTION
The
.B md
driver provides virtual devices that are created from one or more
independent underlying devices. This array of devices often contains
redundancy and the devices are often disk drives, hence the acronym RAID
which stands for a Redundant Array of Independent Disks.
.PP
.B md
supports RAID levels
1 (mirroring),
4 (striped array with parity device),
5 (striped array with distributed parity information),
6 (striped array with distributed dual redundancy information), and
10 (striped and mirrored).
If some number of underlying devices fails while using one of these
levels, the array will continue to function; this number is one for
RAID levels 4 and 5, two for RAID level 6, and all but one (N-1) for
RAID level 1, and dependent on configuration for level 10.
.PP
.B 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).
.SS MD METADATA
Each device in an array may have some
.I metadata
stored in the device. This metadata is sometimes called a
.BR 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,
.B md
provides support for two different formats of metadata, and
other formats can be added. Prior to this release, only one format is
supported.
The common format \(em known as version 0.90 \(em 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 \(em known as version 1 \(em 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:
.TP
LEVEL
The manner in which the devices are arranged into the array
(LINEAR, RAID0, RAID1, RAID4, RAID5, RAID10, MULTIPATH).
.TP
UUID
a 128 bit Universally Unique Identifier that identifies the array that
contains this device.
.PP
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.
.SS 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:
.TP
LEGACY ARRAYS
Early versions of the
.B 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,
.B md
continues to support them.
.TP
FAULTY
Being a largely transparent layer over a different device, the FAULTY
personality doesn't gain anything from having a superblock.
.TP
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.
.TP
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.
.SS ARRAYS WITH EXTERNAL METADATA
From release 2.6.28, the
.I 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.
.PP
.I 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,
.I md
will wait for the program to acknowledge the event by writing to a
sysfs attribute.
The manual page for
.IR mdmon (8)
contains more detail about this interaction.
.SS CONTAINERS
Many metadata formats use a single block of metadata to describe a
number of different arrays which all use the same set of devices.
In this case it is helpful for the kernel to know about the full set
of devices as a whole. This set is known to md as a
.IR container .
A container is an
.I 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.
.SS LINEAR
A LINEAR array simply catenates the available space on each
drive to form one large virtual drive.
One advantage of this arrangement over the more common RAID0
arrangement is that the array may be reconfigured at a later time with
an extra drive, so the array is made bigger without disturbing the
data that is on the array. This can even be done on a live
array.
If a chunksize is given with a LINEAR array, the usable space on each
device is rounded down to a multiple of this chunksize.
.SS RAID0
A RAID0 array (which has zero redundancy) is also known as a
striped array.
A RAID0 array is configured at creation with a
.B "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
.BR 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,
.I 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 parameter,
use either the
.B "--update=layout-original"
option or the
.B "--update=layout-alternate"
option.
.SS RAID1
A RAID1 array is also known as a mirrored set (though mirrors tend to
provide reflected images, which RAID1 does not) or a plex.
Once initialised, each device in a RAID1 array contains exactly the
same data. Changes are written to all devices in parallel. Data is
read from any one device. The driver attempts to distribute read
requests across all devices to maximise performance.
All devices in a RAID1 array should be the same size. If they are
not, then only the amount of space available on the smallest device is
used (any extra space on other devices is wasted).
Note that the read balancing done by the driver does not make the RAID1
performance profile be the same as for RAID0; a single stream of
sequential input will not be accelerated (e.g. a single dd), but
multiple sequential streams or a random workload will use more than one
spindle. In theory, having an N-disk RAID1 will allow N sequential
threads to read from all disks.
Individual devices in a RAID1 can be marked as "write-mostly".
These drives are excluded from the normal read balancing and will only
be read from when there is no other option. This can be useful for
devices connected over a slow link.
.SS 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.
.SS 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.
.SS RAID6
RAID6 is similar to RAID5, but can handle the loss of any \fItwo\fP
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.
.SS RAID10
RAID10 provides a combination of RAID1 and RAID0, and is sometimes known
as RAID1+0. Every datablock is duplicated some number of times, and
the resulting collection of datablocks are distributed over multiple
drives.
When configuring a RAID10 array, it is necessary to specify the number
of replicas of each data block that are required (this will usually
be\ 2) and whether their layout should be "near", "far" or "offset"
(with "offset" being available since Linux\ 2.6.18).
.B About the RAID10 Layout Examples:
.br
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 \fB/dev/md/\fPname).
.br
Therefore the chunks\ /\ chunk numbers map directly to the blocks\ /\
block addresses of the exported RAID10 device.
Decimal numbers (0,\ 1, 2,\ ...) are the chunks of the RAID10 and due
to the above assumption also the blocks and block addresses of the
exported RAID10 device.
.br
Repeated numbers mean copies of a chunk\ /\ block (obviously on
different underlying devices).
.br
Hexadecimal numbers (0x00,\ 0x01, 0x02,\ ...) are the block addresses
of the underlying devices.
.TP
\fB "near" Layout\fP
When "near" replicas are chosen, the multiple copies of a given chunk are laid
out consecutively ("as close to each other as possible") across the stripes of
the array.
With an even number of devices, they will likely (unless some misalignment is
present) lay at the very same offset on the different devices.
.br
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.
.ne 10
.B Example with 2\ copies per chunk and an even number\ (4) of devices:
.TS
tab(;);
C - - - -
C | C | C | C | C |
| - | - | - | - | - |
| C | C | C | C | C |
| C | C | C | C | C |
| C | C | C | C | C |
| C | C | C | C | C |
| C | C | C | C | C |
| C | C | C | C | C |
| - | - | - | - | - |
C C S C S
C C S C S
C C S S S
C C S S S.
;
;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
.TE
.ne 10
.B Example with 2\ copies per chunk and an odd number\ (5) of devices:
.TS
tab(;);
C - - - - -
C | C | C | C | C | C |
| - | - | - | - | - | - |
| C | C | C | C | C | C |
| C | C | C | C | C | C |
| C | C | C | C | C | C |
| C | C | C | C | C | C |
| C | C | C | C | C | C |
| C | C | C | C | C | C |
| - | - | - | - | - | - |
C.
;
;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
;
.TE
.TP
\fB "far" Layout\fP
When "far" replicas are chosen, the multiple copies of a given chunk
are laid out quite distant ("as far as reasonably possible") from each
other.
First a complete sequence of all data blocks (that is all the data one
sees on the exported RAID10 block device) is striped over the
devices. Then another (though "shifted") complete sequence of all data
blocks; and so on (in the case of more than 2\ copies per chunk).
The "shift" needed to prevent placing copies of the same chunks on the
same devices is actually a cyclic permutation with offset\ 1 of each
of the stripes within a complete sequence of chunks.
.br
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:
.br
1.\ complete sequence of chunks: offset\ =\ \ 0
.br
2.\ complete sequence of chunks: offset\ =\ \ 1
.br
3.\ complete sequence of chunks: offset\ =\ \ 2
.br
:
.br
n.\ complete sequence of chunks: offset\ =\ n-1
.ne 10
.B Example with 2\ copies per chunk and an even number\ (4) of devices:
.TS
tab(;);
C - - - -
C | C | C | C | C |
| - | - | - | - | - |
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| - | - | - | - | - |
C.
;
;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;/
;
.TE
.ne 10
.B Example with 2\ copies per chunk and an odd number\ (5) of devices:
.TS
tab(;);
C - - - - -
C | C | C | C | C | C |
| - | - | - | - | - | - |
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| - | - | - | - | - | - |
C.
;
;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;/
;
.TE
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.
.br
The cost is more seeking for writes, making them substantially slower.
.TP
\fB"offset" Layout\fP
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).
.br
This pattern repeats for all further consecutive chunks of the
exported RAID10 device (in other words: all further data blocks).
The "shift" needed to prevent placing copies of the same chunks on the
same devices is actually a cyclic permutation with offset\ 1 of each
of the striped copies of <number of devices> consecutive chunks.
.br
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:
.br
1.\ <number of devices> consecutive chunks: offset\ =\ \ 0
.br
2.\ <number of devices> consecutive chunks: offset\ =\ \ 1
.br
3.\ <number of devices> consecutive chunks: offset\ =\ \ 2
.br
:
.br
n.\ <number of devices> consecutive chunks: offset\ =\ n-1
.ne 10
.B Example with 2\ copies per chunk and an even number\ (4) of devices:
.TS
tab(;);
C - - - -
C | C | C | C | C |
| - | - | - | - | - |
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| C | C | C | C | C | L
| - | - | - | - | - |
C.
;
;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~
;
.TE
.ne 10
.B Example with 2\ copies per chunk and an odd number\ (5) of devices:
.TS
tab(;);
C - - - - -
C | C | C | C | C | C |
| - | - | - | - | - | - |
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| C | C | C | C | C | C | L
| - | - | - | - | - | - |
C.
;
;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~
;
.TE
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.
.PP
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.
.SS MULTIPATH
MULTIPATH is not really a RAID at all as there is only one real device
in a MULTIPATH md array. However there are multiple access points
(paths) to this device, and one of these paths might fail, so there
are some similarities.
A MULTIPATH array is composed of a number of logically different
devices, often fibre channel interfaces, that all refer the the same
real device. If one of these interfaces fails (e.g. due to cable
problems), the MULTIPATH driver will attempt to redirect requests to
another interface.
The MULTIPATH drive is not receiving any ongoing development and
should be considered a legacy driver. The device-mapper based
multipath drivers should be preferred for new installations.
.SS 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.
.SS UNCLEAN SHUTDOWN
When changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10 array
there is a possibility of inconsistency for short periods of time as
each update requires at least two block to be written to different
devices, and these writes probably won't happen at exactly the same
time. Thus if a system with one of these arrays is shutdown in the
middle of a write operation (e.g. due to power failure), the array may
not be consistent.
To handle this situation, the md driver marks an array as "dirty"
before writing any data to it, and marks it as "clean" when the array
is being disabled, e.g. at shutdown. If the md driver finds an array
to be dirty at startup, it proceeds to correct any possibly
inconsistency. For RAID1, this involves copying the contents of the
first drive onto all other drives. For RAID4, RAID5 and RAID6 this
involves recalculating the parity for each stripe and making sure that
the parity block has the correct data. For RAID10 it involves copying
one of the replicas of each block onto all the others. This process,
known as "resynchronising" or "resync" is performed in the background.
The array can still be used, though possibly with reduced performance.
If a RAID4, RAID5 or RAID6 array is degraded (missing at least one
drive, two for RAID6) when it is restarted after an unclean shutdown, it cannot
recalculate parity, and so it is possible that data might be
undetectably corrupted. The 2.4 md driver
.B 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.
.SS RECOVERY
If the md driver detects a write error on a device in a RAID1, RAID4,
RAID5, RAID6, or RAID10 array, it immediately disables that device
(marking it as faulty) and continues operation on the remaining
devices. If there are spare drives, the driver will start recreating
on one of the spare drives the data which was on that failed drive,
either by copying a working drive in a RAID1 configuration, or by
doing calculations with the parity block on RAID4, RAID5 or RAID6, or
by finding and copying originals for RAID10.
In kernels prior to about 2.6.15, a read error would cause the same
effect as a write error. In later kernels, a read-error will instead
cause md to attempt a recovery by overwriting the bad block. i.e. it
will find the correct data from elsewhere, write it over the block
that failed, and then try to read it back again. If either the write
or the re-read fail, md will treat the error the same way that a write
error is treated, and will fail the whole device.
While this recovery process is happening, the md driver will monitor
accesses to the array and will slow down the rate of recovery if other
activity is happening, so that normal access to the array will not be
unduly affected. When no other activity is happening, the recovery
process proceeds at full speed. The actual speed targets for the two
different situations can be controlled by the
.B speed_limit_min
and
.B speed_limit_max
control files mentioned below.
.SS 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
.IR scrubbing .
md arrays can be scrubbed by writing either
.I check
or
.I repair
to the file
.I md/sync_action
in the
.I sysfs
directory for the device.
Requesting a scrub will cause
.I 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
.I fix
the bad block.
If all blocks read successfully but are found to not be consistent,
then this is regarded as a
.IR mismatch .
If
.I check
was used, then no action is taken to handle the mismatch, it is simply
recorded.
If
.I repair
was used, then a mismatch will be repaired in the same way that
.I 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
.I sysfs
file
.IR md/mismatch_cnt .
This is set to zero when a
scrub starts and is incremented whenever a sector is
found that is a mismatch.
.I 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
.I mdadm
with
.I \-\-assume\-clean
then a subsequent check could be expected to find some mismatches.
On a truly clean RAID5 or RAID6 array, any mismatches should indicate
a hardware problem at some level - software issues should never cause
such a mismatch.
However on RAID1 and RAID10 it is possible for software issues to
cause a mismatch to be reported. This does not necessarily mean that
the data on the array is corrupted. It could simply be that the
system does not care what is stored on that part of the array - it is
unused space.
The most likely cause for an unexpected mismatch on RAID1 or RAID10
occurs if a swap partition or swap file is stored on the array.
When the swap subsystem wants to write a page of memory out, it flags
the page as 'clean' in the memory manager and requests the swap device
to write it out. It is quite possible that the memory will be
changed while the write-out is happening. In that case the 'clean'
flag will be found to be clear when the write completes and so the
swap subsystem will simply forget that the swapout had been attempted,
and will possibly choose a different page to write out.
If the swap device was on RAID1 (or RAID10), then the data is sent
from memory to a device twice (or more depending on the number of
devices in the array). Thus it is possible that the memory gets changed
between the times it is sent, so different data can be written to
the different devices in the array. This will be detected by
.I check
as a mismatch. However it does not reflect any corruption as the
block where this mismatch occurs is being treated by the swap system as
being empty, and the data will never be read from that block.
It is conceivable for a similar situation to occur on non-swap files,
though it is less likely.
Thus the
.I mismatch_cnt
value can not be interpreted very reliably on RAID1 or RAID10,
especially when the device is used for swap.
.SS BITMAP WRITE-INTENT LOGGING
From Linux 2.6.13,
.I md
supports a bitmap based write-intent log. If configured, the bitmap
is used to record which blocks of the array may be out of sync.
Before any write request is honoured, md will make sure that the
corresponding bit in the log is set. After a period of time with no
writes to an area of the array, the corresponding bit will be cleared.
This bitmap is used for two optimisations.
Firstly, after an unclean shutdown, the resync process will consult
the bitmap and only resync those blocks that correspond to bits in the
bitmap that are set. This can dramatically reduce resync time.
Secondly, when a drive fails and is removed from the array, md stops
clearing bits in the intent log. If that same drive is re-added to
the array, md will notice and will only recover the sections of the
drive that are covered by bits in the intent log that are set. This
can allow a device to be temporarily removed and reinserted without
causing an enormous recovery cost.
The intent log can be stored in a file on a separate device, or it can
be stored near the superblocks of an array which has superblocks.
It is possible to add an intent log to an active array, or remove an
intent log if one is present.
In 2.6.13, intent bitmaps are only supported with RAID1. Other levels
with redundancy are supported from 2.6.15.
.SS BAD BLOCK LIST
From Linux 3.5 each device in an
.I md
array can store a list of known-bad-blocks. This list is 4K in size
and usually positioned at the end of the space between the superblock
and the data.
When a block cannot be read and cannot be repaired by writing data
recovered from other devices, the address of the block is stored in
the bad block list. Similarly if an attempt to write a block fails,
the address will be recorded as a bad block. If attempting to record
the bad block fails, the whole device will be marked faulty.
Attempting to read from a known bad block will cause a read error.
Attempting to write to a known bad block will be ignored if any write
errors have been reported by the device. If there have been no write
errors then the data will be written to the known bad block and if
that succeeds, the address will be removed from the list.
This allows an array to fail more gracefully - a few blocks on
different devices can be faulty without taking the whole array out of
action.
The list is particularly useful when recovering to a spare. If a few blocks
cannot be read from the other devices, the bulk of the recovery can
complete and those few bad blocks will be recorded in the bad block list.
.SS RAID456 WRITE JOURNAL
Due to non-atomicity nature of RAID write operations, interruption of
write operations (system crash, etc.) to RAID456 array can lead to
inconsistent parity and data loss (so called RAID-5 write hole).
To plug the write hole, from Linux 4.4 (to be confirmed),
.I md
supports write ahead journal for RAID456. When the array is created,
an additional journal device can be added to the array through
.IR 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.
.SS WRITE-BEHIND
From Linux 2.6.14,
.I md
supports WRITE-BEHIND on RAID1 arrays.
This allows certain devices in the array to be flagged as
.IR 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.
.SS FAILFAST
From Linux 4.10,
.I
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
.I 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,
.I md
will request that the underlying device does not perform any retries.
This means that a failure will be reported to
.I md
promptly, and it can mark the device as faulty and continue using the
other device(s).
.I md
cannot control the timeout that the underlying devices use to
determine failure. Any changes desired to that timeout must be set
explictly on the underlying device, separately from using
.IR 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,
.I md
will retry the request without disabling retries in the underlying
device. In any case,
.I md
will not attempt to repair read errors on a device marked as FAILFAST
by writing out the correct. It will just mark the device as faulty.
FAILFAST is appropriate for storage arrays that have a low probability
of true failure, but will sometimes introduce unacceptable delays to
I/O requests while performing internal maintenance. The value of
setting FAILFAST involves a trade-off. The gain is that the chance of
unacceptable delays is substantially reduced. The cost is that the
unlikely event of data-loss on one device is slightly more likely to
result in data-loss for the array.
When a device in an array using FAILFAST is marked as faulty, it will
usually become usable again in a short while.
.I mdadm
makes no attempt to detect that possibility. Some separate
mechanism, tuned to the specific details of the expected failure modes,
needs to be created to monitor devices to see when they return to full
functionality, and to then re-add them to the array. In order of
this "re-add" functionality to be effective, an array using FAILFAST
should always have a write-intent bitmap.
.SS RESTRIPING
.IR Restriping ,
also known as
.IR Reshaping ,
is the processes of re-arranging the data stored in each stripe into a