-''' 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.
+.\" 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
+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 hence the acronym RAID which stands for a Redundant
-Array of Independent Devices.
+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
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 dependant on configuration for level 10.
+RAID level 1, and dependent on configuration for level 10.
.PP
.B md
also supports a number of pseudo RAID (non-redundant) configurations
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 SUPER BLOCK
-Each device in an array may have a
-.I superblock
-which records information about the structure and state of the array.
+.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 this superblock, and
+provides support for two different formats of metadata, and
other formats can be added. Prior to this release, only one format is
supported.
-The common format - known as version 0.90 - has
+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
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-dependant
+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
+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 superblock format stores multibyte data in a
-processor-independent format and has supports up to hundreds of
+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 superblock contains, among other things:
+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).
+(LINEAR, RAID0, RAID1, RAID4, RAID5, RAID10, MULTIPATH).
.TP
UUID
a 128 bit Universally Unique Identifier that identifies the array that
-this device is part of.
+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
would cause data corruption) but will be left untouched until a kernel
that can complete the reshape processes is used.
-.SS ARRAYS WITHOUT SUPERBLOCKS
+.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 where an
-array without superblocks in preferred. This include:
+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
+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
a MULTIPATH array with no superblock makes sense.
.TP
RAID1
-In some configurations it might be desired to create a raid1
-configuration that does use a superblock, and to maintain the state of
-the array elsewhere. While not encouraged for general us, it does
+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 together to form one large virtual drive.
+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 and so the array is made bigger without disturbing the
-data that is on the array. However this cannot be done on a live
+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
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, and at least 4 kibibytes.
+.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
+drives have been assigned one chunk. This collection of chunks forms a
.BR stripe .
-Further chunks are gathered into stripes in the same way which are
+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
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
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.
+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
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
+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 contents is
+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.
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
+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
+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
.SS RAID10
-RAID10 provides a combination of RAID1 and RAID0, and sometimes known
+RAID10 provides a combination of RAID1 and RAID0, and is sometimes known
as RAID1+0. Every datablock is duplicated some number of times, and
the resulting collection of datablocks are distributed over multiple
drives.
-When configuring a RAID10 array it is necessary to specify the number
-of replicas of each data block that are required (this will normally
-be 2) and whether the replicas should be 'near', 'offset' or 'far'.
-(Note that the 'offset' layout is only available from 2.6.18).
-
-When 'near' replicas are chosen, the multiple copies of a given chunk
-are laid out consecutively across the stripes of the array, so the two
-copies of a datablock will likely be at the same offset on two
-adjacent devices.
-
-When 'far' replicas are chosen, the multiple copies of a given chunk
-are laid out quite distant from each other. The first copy of all
-data blocks will be striped across the early part of all drives in
-RAID0 fashion, and then the next copy of all blocks will be striped
-across a later section of all drives, always ensuring that all copies
-of any given block are on different drives.
-
-The 'far' arrangement can give sequential read performance equal to
-that of a RAID0 array, but at the cost of degraded write performance.
-
-When 'offset' replicas are chosen, the multiple copies of a given
-chunk are laid out on consecutive drives and at consecutive offsets.
-Effectively each stripe is duplicated and the copies are offset by one
-device. This should give similar read characteristics to 'far' if a
-suitably large chunk size is used, but without as much seeking for
-writes.
+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, those
+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 and array is configured with 2 near copies and 2 far
+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 MUTIPATH
+.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
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.
+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
+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.
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
+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
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 are least two block to be written to different
-devices, and these writes probably wont happen at exactly the same
+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.
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) when it is restarted after an unclean shutdown, it cannot
+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 over-ridden by a kernel parameter.
+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 is a spare drive, the driver will start recreating
-on one of the spare drives the data what was on that failed drive,
+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 a copying originals for RAID10.
+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
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.
+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
.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,
This bitmap is used for two optimisations.
-Firstly, after an unclear shutdown, the resync process will consult
+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 increase resync time.
+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 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 or an active array, or remove an
+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,
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 ,
.IR 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
+array (so the stripes are wider), changing the chunk size (so stripes
are deeper or shallower), or changing the arrangement of data and
-parity, possibly changing the raid level (e.g. 1 to 5 or 5 to 6).
+parity (possibly changing the RAID level, e.g. 1 to 5 or 5 to 6).
-As of Linux 2.6.17, md can reshape a raid5 array to have more
-devices. Other possibilities may follow in future kernels.
+As of Linux 2.6.35, md can reshape a RAID4, RAID5, or RAID6 array to
+have a different number of devices (more or fewer) and to have a
+different layout or chunk size. It can also convert between these
+different RAID levels. It can also convert between RAID0 and RAID10,
+and between RAID0 and RAID4 or RAID5.
+Other possibilities may follow in future kernels.
During any stripe process there is a 'critical section' during which
-live data is being over-written on disk. For the operation of
-increasing the number of drives in a raid5, this critical section
+live data is being overwritten on disk. For the operation of
+increasing the number of drives in a RAID5, this critical section
covers the first few stripes (the number being the product of the old
and new number of devices). After this critical section is passed,
data is only written to areas of the array which no longer hold live
-data - the live data has already been located away.
+data \(em 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
.B sysfs
interface),
.IP \(bu 4
-Take a copy of the data somewhere (i.e. make a backup)
+take a copy of the data somewhere (i.e. make a backup),
.IP \(bu 4
-Allow the process to continue and invalidate the backup and restore
+allow the process to continue and invalidate the backup and restore
write access once the critical section is passed, and
.IP \(bu 4
-Provide for restoring the critical data before restarting the array
+provide for restoring the critical data before restarting the array
after a system crash.
.PP
.B mdadm
-version 2.4 and later will do this for growing a RAID5 array.
+versions from 2.4 do this for growing a RAID5 array.
For operations that do not change the size of the array, like simply
increasing chunk size, or converting RAID5 to RAID6 with one extra
-device, the entire process is the critical section. In this case the
-restripe will need to progress in stages as a section is suspended,
-backed up,
-restriped, and released. This is not yet implemented.
+device, the entire process is the critical section. In this case, the
+restripe will need to progress in stages, as a section is suspended,
+backed up, restriped, and released.
.SS SYSFS INTERFACE
-All block devices appear as a directory in
+Each block device appears as a directory in
.I sysfs
-(usually mounted at
+(which is usually mounted at
.BR /sys ).
For MD devices, this directory will contain a subdirectory called
.B md
.B /proc/sys/dev/raid/speed_limit_min
for this array only.
Writing the value
-.B system
-to this file cause the system-wide setting to have effect.
+.B "system"
+to this file will cause the system-wide setting to have effect.
.TP
.B md/sync_speed_max
This is the partner of
.B md/sync_speed_min
and overrides
-.B /proc/sys/dev/raid/spool_limit_max
+.B /proc/sys/dev/raid/speed_limit_max
described below.
.TP
.B 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 read and write operations to the array. The default is 128.
+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.
+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
+
+.TP
+.B 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.
+
+.TP
+.B 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.
+
+.TP
+.B 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 metadata and writes 'true'.
+
+There is no code in mdmon to actually do this, so maybe it doesn't even
+work.
+
+.TP
+.B 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.
+
+.TP
+.B 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.
+
+.TP
+.B 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.
+
+.TP
+.B 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.
+
+.TP
+.B md/bitmap/space
+This shows the space (in sectors) which is available at md/bitmap/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 before, else
+to 4K beyond superblock. For other metadata versions, assume no change is
+possible.
+
+.TP
+.B 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.
.SS KERNEL PARAMETERS
.TP
.B md_mod.start_ro=1
+.TP
+.B /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
.TP
.B md_mod.start_dirty_degraded=1
+.TP
+.B /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 the module parameter allows such arrays to be started
+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.
.I n
gives the md device number,
.I l
-gives the level, 0 for RAID0 or -1 for LINEAR,
+gives the level, 0 for RAID0 or \-1 for LINEAR,
.I c
gives the chunk size as a base-2 logarithm offset by twelve, so 0
means 4K, 1 means 8K.
Contains information about the status of currently running array.
.TP
.B /proc/sys/dev/raid/speed_limit_min
-A readable and writable file that reflects the current goal rebuild
+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 disc will shuffle
-more data for a given speed). The default is 100.
+per-array rate (which means that an array with more disks will shuffle
+more data for a given speed). The default is 1000.
.TP
.B /proc/sys/dev/raid/speed_limit_max
-A readable and writable file that reflects the current goal rebuild
+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 100,000.
+The default is 200,000.
.SH SEE ALSO
.BR mdadm (8),
-.BR mkraid (8).
projects / thirdparty / mdadm.git / blobdiff