|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
|
MD(4) Kernel Interfaces Manual MD(4)
md - Multiple Device driver aka Linux Software RAID
/dev/mdn
/dev/md/n
/dev/md/name
The md driver provides virtual devices that are created from one
or more independent underlying devices. This array of devices
often contains redundancy and the devices are often disk drives,
hence the acronym RAID which stands for a Redundant Array of
Independent Disks.
md supports RAID levels 1 (mirroring), 4 (striped array with
parity device), 5 (striped array with distributed parity
information), 6 (striped array with distributed dual redundancy
information), and 10 (striped and mirrored). If some number of
underlying devices fails while using one of these levels, the
array will continue to function; this number is one for RAID
levels 4 and 5, two for RAID level 6, and all but one (N-1) for
RAID level 1, and dependent on configuration for level 10.
md also supports a number of pseudo RAID (non-redundant)
configurations including RAID0 (striped array), LINEAR (catenated
array), MULTIPATH (a set of different interfaces to the same
device), and FAULTY (a layer over a single device into which
errors can be injected).
MD METADATA
Each device in an array may have some metadata stored in the
device. This metadata is sometimes called a superblock. The
metadata records information about the structure and state of the
array. This allows the array to be reliably re-assembled after a
shutdown.
From Linux kernel version 2.6.10, md provides support for two
different formats of metadata, and other formats can be added.
Prior to this release, only one format is supported.
The common format — known as version 0.90 — has a superblock that
is 4K long and is written into a 64K aligned block that starts at
least 64K and less than 128K from the end of the device (i.e. to
get the address of the superblock round the size of the device
down to a multiple of 64K and then subtract 64K). The available
size of each device is the amount of space before the super
block, so between 64K and 128K is lost when a device in
incorporated into an MD array. This superblock stores multi-byte
fields in a processor-dependent manner, so arrays cannot easily
be moved between computers with different processors.
The new format — known as version 1 — has a superblock that is
normally 1K long, but can be longer. It is normally stored
between 8K and 12K from the end of the device, on a 4K boundary,
though variations can be stored at the start of the device
(version 1.1) or 4K from the start of the device (version 1.2).
This metadata format stores multibyte data in a processor-
independent format and supports up to hundreds of component
devices (version 0.90 only supports 28).
The metadata contains, among other things:
LEVEL The manner in which the devices are arranged into the
array (LINEAR, RAID0, RAID1, RAID4, RAID5, RAID10,
MULTIPATH).
UUID a 128 bit Universally Unique Identifier that identifies
the array that contains this device.
When a version 0.90 array is being reshaped (e.g. adding extra
devices to a RAID5), the version number is temporarily set to
0.91. This ensures that if the reshape process is stopped in the
middle (e.g. by a system crash) and the machine boots into an
older kernel that does not support reshaping, then the array will
not be assembled (which would cause data corruption) but will be
left untouched until a kernel that can complete the reshape
processes is used.
ARRAYS WITHOUT METADATA
While it is usually best to create arrays with superblocks so
that they can be assembled reliably, there are some circumstances
when an array without superblocks is preferred. These include:
LEGACY ARRAYS
Early versions of the md driver only supported LINEAR and
RAID0 configurations and did not use a superblock (which
is less critical with these configurations). While such
arrays should be rebuilt with superblocks if possible, md
continues to support them.
FAULTY Being a largely transparent layer over a different device,
the FAULTY personality doesn't gain anything from having a
superblock.
MULTIPATH
It is often possible to detect devices which are different
paths to the same storage directly rather than having a
distinctive superblock written to the device and searched
for on all paths. In this case, a MULTIPATH array with no
superblock makes sense.
RAID1 In some configurations it might be desired to create a
RAID1 configuration that does not use a superblock, and to
maintain the state of the array elsewhere. While not
encouraged for general use, it does have special-purpose
uses and is supported.
ARRAYS WITH EXTERNAL METADATA
From release 2.6.28, the md driver supports arrays with
externally managed metadata. That is, the metadata is not
managed by the kernel but rather by a user-space program which is
external to the kernel. This allows support for a variety of
metadata formats without cluttering the kernel with lots of
details.
md is able to communicate with the user-space program through
various sysfs attributes so that it can make appropriate changes
to the metadata - for example to mark a device as faulty. When
necessary, md will wait for the program to acknowledge the event
by writing to a sysfs attribute. The manual page for mdmon(8)
contains more detail about this interaction.
CONTAINERS
Many metadata formats use a single block of metadata to describe
a number of different arrays which all use the same set of
devices. In this case it is helpful for the kernel to know about
the full set of devices as a whole. This set is known to md as a
container. A container is an md array with externally managed
metadata and with device offset and size so that it just covers
the metadata part of the devices. The remainder of each device
is available to be incorporated into various arrays.
LINEAR
A LINEAR array simply catenates the available space on each drive
to form one large virtual drive.
One advantage of this arrangement over the more common RAID0
arrangement is that the array may be reconfigured at a later time
with an extra drive, so the array is made bigger without
disturbing the data that is on the array. This can even be done
on a live array.
If a chunksize is given with a LINEAR array, the usable space on
each device is rounded down to a multiple of this chunksize.
RAID0
A RAID0 array (which has zero redundancy) is also known as a
striped array. A RAID0 array is configured at creation with a
Chunk Size which must be a power of two (prior to Linux 2.6.31),
and at least 4 kibibytes.
The RAID0 driver assigns the first chunk of the array to the
first device, the second chunk to the second device, and so on
until all drives have been assigned one chunk. This collection
of chunks forms a stripe. Further chunks are gathered into
stripes in the same way, and are assigned to the remaining space
in the drives.
If devices in the array are not all the same size, then once the
smallest device has been exhausted, the RAID0 driver starts
collecting chunks into smaller stripes that only span the drives
which still have remaining space.
A bug was introduced in linux 3.14 which changed the layout of
blocks in a RAID0 beyond the region that is striped over all
devices. This bug does not affect an array with all devices the
same size, but can affect other RAID0 arrays.
Linux 5.4 (and some stable kernels to which the change was
backported) will not normally assemble such an array as it cannot
know which layout to use. There is a module parameter
"raid0.default_layout" which can be set to "1" to force the
kernel to use the pre-3.14 layout or to "2" to force it to use
the 3.14-and-later layout. when creating a new RAID0 array,
mdadm will record the chosen layout in the metadata in a way that
allows newer kernels to assemble the array without needing a
module parameter.
To assemble an old array on a new kernel without using the module
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 kernel. If you need to revert to an older
kernel, the layout information can be erased with the
--update=layout-unspecificed option. If you use this option to
--assemble while running a newer kernel, the array will NOT
assemble, but the metadata will be update so that it can be
assembled on an older kernel.
No that setting the layout to "unspecified" removes protections
against this bug, and you must be sure that the kernel you use
matches the 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 attempts to
distribute read requests across all devices to maximise
performance.
All devices in a RAID1 array should be the same size. If they
are not, then only the amount of space available on the smallest
device is used (any extra space on other devices is wasted).
Note that the read balancing done by the driver does not make the
RAID1 performance profile be the same as for RAID0; a single
stream of sequential input will not be accelerated (e.g. a single
dd), but multiple sequential streams or a random workload will
use more than one spindle. In theory, having an N-disk RAID1 will
allow N sequential threads to read from all disks.
Individual devices in a RAID1 can be marked as "write-mostly".
These drives are excluded from the normal read balancing and will
only be read from when there is no other option. This can be
useful for devices connected over a slow link.
RAID4
A RAID4 array is like a RAID0 array with an extra device for
storing parity. This device is the last of the active devices in
the array. Unlike RAID0, RAID4 also requires that all stripes
span all drives, so extra space on devices that are larger than
the smallest is wasted.
When any block in a RAID4 array is modified, the parity block for
that stripe (i.e. the block in the parity device at the same
device offset as the stripe) is also modified so that the parity
block always contains the "parity" for the whole stripe. I.e.
its content is equivalent to the result of performing an
exclusive-or operation between all the data blocks in the stripe.
This allows the array to continue to function if one device
fails. The data that was on that device can be calculated as
needed from the parity block and the other data blocks.
RAID5
RAID5 is very similar to RAID4. The difference is that the
parity blocks for each stripe, instead of being on a single
device, are distributed across all devices. This allows more
parallelism when writing, as two different block updates will
quite possibly affect parity blocks on different devices so there
is less contention.
This also allows more parallelism when reading, as read requests
are distributed over all the devices in the array instead of all
but one.
RAID6
RAID6 is similar to RAID5, but can handle the loss of any two
devices without data loss. Accordingly, it requires N+2 drives
to store N drives worth of data.
The performance for RAID6 is slightly lower but comparable to
RAID5 in normal mode and single disk failure mode. It is very
slow in dual disk failure mode, however.
RAID10
RAID10 provides a combination of RAID1 and RAID0, and is
sometimes known as RAID1+0. Every datablock is duplicated some
number of times, and the resulting collection of datablocks are
distributed over multiple drives.
When configuring a RAID10 array, it is necessary to specify the
number of replicas of each data block that are required (this
will usually be 2) and whether their layout should be "near",
"far" or "offset" (with "offset" being available since
Linux 2.6.18).
About the RAID10 Layout Examples:
The examples below visualise the chunk distribution on the
underlying devices for the respective layout.
For simplicity it is assumed that the size of the chunks equals
the size of the blocks of the underlying devices as well as those
of the RAID10 device exported by the kernel (for example
/dev/md/name).
Therefore the chunks / chunk numbers map directly to the
blocks /block addresses of the exported RAID10 device.
Decimal numbers (0, 1, 2, ...) are the chunks of the RAID10 and
due to the above assumption also the blocks and block addresses
of the exported RAID10 device.
Repeated numbers mean copies of a chunk / block (obviously on
different underlying devices).
Hexadecimal numbers (0x00, 0x01, 0x02, ...) are the block
addresses of the underlying devices.
"near" Layout
When "near" replicas are chosen, the multiple copies of a
given chunk are laid out consecutively ("as close to each
other as possible") across the stripes of the array.
With an even number of devices, they will likely (unless
some misalignment is present) lay at the very same offset
on the different devices.
This is as the "classic" RAID1+0; that is two groups of
mirrored devices (in the example below the groups
Device #1 / #2 and Device #3 / #4 are each a RAID1) both
in turn forming a striped RAID0.
Example with 2 copies per chunk and an even number (4) of
devices:
┌───────────┌───────────┌───────────┌───────────┐
│ Device #1 │ Device #2 │ Device #3 │ Device #4 │
┌──────├───────────├───────────├───────────├───────────┤
│ 0x00 │ 0 │ 0 │ 1 │ 1 │
│ 0x01 │ 2 │ 2 │ 3 │ 3 │
│ ... │ ... │ ... │ ... │ ... │
│ : │ : │ : │ : │ : │
│ ... │ ... │ ... │ ... │ ... │
│ 0x80 │ 254 │ 254 │ 255 │ 255 │
└──────└───────────└───────────└───────────└───────────┘
\---------v---------/ \---------v---------/
RAID1 RAID1
\---------------------v---------------------/
RAID0
Example with 2 copies per chunk and an odd number (5) of
devices:
┌────────┌────────┌────────┌────────┌────────┐
│ Dev #1 │ Dev #2 │ Dev #3 │ Dev #4 │ Dev #5 │
┌──────├────────├────────├────────├────────├────────┤
│ 0x00 │ 0 │ 0 │ 1 │ 1 │ 2 │
│ 0x01 │ 2 │ 3 │ 3 │ 4 │ 4 │
│ ... │ ... │ ... │ ... │ ... │ ... │
│ : │ : │ : │ : │ : │ : │
│ ... │ ... │ ... │ ... │ ... │ ... │
│ 0x80 │ 317 │ 318 │ 318 │ 319 │ 319 │
└──────└────────└────────└────────└────────└────────┘
"far" Layout
When "far" replicas are chosen, the multiple copies of a
given chunk are laid out quite distant ("as far as
reasonably possible") from each other.
First a complete sequence of all data blocks (that is all
the data one sees on the exported RAID10 block device) is
striped over the devices. Then another (though "shifted")
complete sequence of all data blocks; and so on (in the
case of more than 2 copies per chunk).
The "shift" needed to prevent placing copies of the same
chunks on the same devices is actually a cyclic
permutation with offset 1 of each of the stripes within a
complete sequence of chunks.
The offset 1 is relative to the previous complete sequence
of chunks, so in case of more than 2 copies per chunk one
gets the following offsets:
1. complete sequence of chunks: offset = 0
2. complete sequence of chunks: offset = 1
3. complete sequence of chunks: offset = 2
:
n. complete sequence of chunks: offset = n-1
Example with 2 copies per chunk and an even number (4) of
devices:
┌───────────┌───────────┌───────────┌───────────┐
│ Device #1 │ Device #2 │ Device #3 │ Device #4 │
┌──────├───────────├───────────├───────────├───────────┤
│ 0x00 │ 0 │ 1 │ 2 │ 3 │ \
│ 0x01 │ 4 │ 5 │ 6 │ 7 │ > [#]
│ ... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ :
│ ... │ ... │ ... │ ... │ ... │ :
│ 0x40 │ 252 │ 253 │ 254 │ 255 │ /
│ 0x41 │ 3 │ 0 │ 1 │ 2 │ \
│ 0x42 │ 7 │ 4 │ 5 │ 6 │ > [#]~
│ ... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ :
│ ... │ ... │ ... │ ... │ ... │ :
│ 0x80 │ 255 │ 252 │ 253 │ 254 │ /
└──────└───────────└───────────└───────────└───────────┘
Example with 2 copies per chunk and an odd number (5) of
devices:
┌────────┌────────┌────────┌────────┌────────┐
│ Dev #1 │ Dev #2 │ Dev #3 │ Dev #4 │ Dev #5 │
┌──────├────────├────────├────────├────────├────────┤
│ 0x00 │ 0 │ 1 │ 2 │ 3 │ 4 │ \
│ 0x01 │ 5 │ 6 │ 7 │ 8 │ 9 │ > [#]
│ ... │ ... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ : │ :
│ ... │ ... │ ... │ ... │ ... │ ... │ :
│ 0x40 │ 315 │ 316 │ 317 │ 318 │ 319 │ /
│ 0x41 │ 4 │ 0 │ 1 │ 2 │ 3 │ \
│ 0x42 │ 9 │ 5 │ 6 │ 7 │ 8 │ > [#]~
│ ... │ ... │ ... │ ... │ ... │ ... │ :
│ : │ : │ : │ : │ : │ : │ :
│ ... │ ... │ ... │ ... │ ... │ ... │ :
│ 0x80 │ 319 │ 315 │ 316 │ 317 │ 318 │ /
└──────└────────└────────└────────└────────└────────┘
With [#] being the complete sequence of chunks and
[#]~ the cyclic permutation with offset 1 thereof (in the
case of more than 2 copies per chunk there would be
([#]~)~, (([#]~)~)~, ...).
The advantage of this layout is that MD can easily spread
sequential reads over the devices, making them similar to
RAID0 in terms of speed.
The cost is more seeking for writes, making them
substantially slower.
"offset" Layout
When "offset" replicas are chosen, all the copies of a
given chunk are striped consecutively ("offset by the
stripe length after each other") over the devices.
Explained in detail, <number of devices> consecutive
chunks are striped over the devices, immediately followed
by a "shifted" copy of these chunks (and by further such
"shifted" copies in the case of more than 2 copies per
chunk).
This pattern repeats for all further consecutive chunks of
the exported RAID10 device (in other words: all further
data blocks).
The "shift" needed to prevent placing copies of the same
chunks on the same devices is actually a cyclic
permutation with offset 1 of each of the striped copies of
<number of devices> consecutive chunks.
The offset 1 is relative to the previous striped copy of
<number of devices> consecutive chunks, so in case of more
than 2 copies per chunk one gets the following offsets:
1. <number of devices> consecutive chunks: offset = 0
2. <number of devices> consecutive chunks: offset = 1
3. <number of devices> consecutive chunks: offset = 2
:
n. <number of devices> consecutive chunks: offset = n-1
Example with 2 copies per chunk and an even number (4) of
devices:
┌───────────┌───────────┌───────────┌───────────┐
│ Device #1 │ Device #2 │ Device #3 │ Device #4 │
┌──────├───────────├───────────├───────────├───────────┤
│ 0x00 │ 0 │ 1 │ 2 │ 3 │ ) AA
│ 0x01 │ 3 │ 0 │ 1 │ 2 │ ) AA~
│ 0x02 │ 4 │ 5 │ 6 │ 7 │ ) AB
│ 0x03 │ 7 │ 4 │ 5 │ 6 │ ) AB~
│ ... │ ... │ ... │ ... │ ... │ ) ...
│ : │ : │ : │ : │ : │ :
│ ... │ ... │ ... │ ... │ ... │ ) ...
│ 0x79 │ 251 │ 252 │ 253 │ 254 │ ) EX
│ 0x80 │ 254 │ 251 │ 252 │ 253 │ ) EX~
└──────└───────────└───────────└───────────└───────────┘
Example with 2 copies per chunk and an odd number (5) of
devices:
┌────────┌────────┌────────┌────────┌────────┐
│ Dev #1 │ Dev #2 │ Dev #3 │ Dev #4 │ Dev #5 │
┌──────├────────├────────├────────├────────├────────┤
│ 0x00 │ 0 │ 1 │ 2 │ 3 │ 4 │ ) AA
│ 0x01 │ 4 │ 0 │ 1 │ 2 │ 3 │ ) AA~
│ 0x02 │ 5 │ 6 │ 7 │ 8 │ 9 │ ) AB
│ 0x03 │ 9 │ 5 │ 6 │ 7 │ 8 │ ) AB~
│ ... │ ... │ ... │ ... │ ... │ ... │ ) ...
│ : │ : │ : │ : │ : │ : │ :
│ ... │ ... │ ... │ ... │ ... │ ... │ ) ...
│ 0x79 │ 314 │ 315 │ 316 │ 317 │ 318 │ ) EX
│ 0x80 │ 318 │ 314 │ 315 │ 316 │ 317 │ ) EX~
└──────└────────└────────└────────└────────└────────┘
With AA, AB, ..., AZ, BA, ... being the sets of <number of
devices> consecutive chunks and AA~, AB~, ...,
AZ~, BA~, ... the cyclic permutations with offset 1
thereof (in the case of more than 2 copies per chunk there
would be (AA~)~, ... as well as ((AA~)~)~, ... and so
on).
This should give similar read characteristics to "far" if
a suitably large chunk size is used, but without as much
seeking for writes.
It should be noted that the number of devices in a RAID10 array
need not be a multiple of the number of replica of each data
block; however, there must be at least as many devices as
replicas.
If, for example, an array is created with 5 devices and 2
replicas, then space equivalent to 2.5 of the devices will be
available, and every block will be stored on two different
devices.
Finally, it is possible to have an array with both "near" and
"far" copies. If an array is configured with 2 near copies and 2
far copies, then there will be a total of 4 copies of each block,
each on a different drive. This is an artifact of the
implementation and is unlikely to be of real value.
MULTIPATH
MULTIPATH is not really a RAID at all as there is only one real
device in a MULTIPATH md array. However there are multiple
access points (paths) to this device, and one of these paths
might fail, so there are some similarities.
A MULTIPATH array is composed of a number of logically different
devices, often fibre channel interfaces, that all refer the the
same real device. If one of these interfaces fails (e.g. due to
cable problems), the MULTIPATH driver will attempt to redirect
requests to another interface.
The MULTIPATH drive is not receiving any ongoing development and
should be considered a legacy driver. The device-mapper based
multipath drivers should be preferred for new installations.
FAULTY
The FAULTY md module is provided for testing purposes. A FAULTY
array has exactly one component device and is normally assembled
without a superblock, so the md array created provides direct
access to all of the data in the component device.
The FAULTY module may be requested to simulate faults to allow
testing of other md levels or of filesystems. Faults can be
chosen to trigger on read requests or write requests, and can be
transient (a subsequent read/write at the address will probably
succeed) or persistent (subsequent read/write of the same address
will fail). Further, read faults can be "fixable" meaning that
they persist until a write request at the same address.
Fault types can be requested with a period. In this case, the
fault will recur repeatedly after the given number of requests of
the relevant type. For example if persistent read faults have a
period of 100, then every 100th read request would generate a
fault, and the faulty sector would be recorded so that subsequent
reads on that sector would also fail.
There is a limit to the number of faulty sectors that are
remembered. Faults generated after this limit is exhausted are
treated as transient.
The list of faulty sectors can be flushed, and the active list of
failure modes can be cleared.
UNCLEAN SHUTDOWN
When changes are made to a RAID1, RAID4, RAID5, RAID6, or RAID10
array there is a possibility of inconsistency for short periods
of time as each update requires at least two block to be written
to different devices, and these writes probably won't happen at
exactly the same time. Thus if a system with one of these arrays
is shutdown in the middle of a write operation (e.g. due to power
failure), the array may not be consistent.
To handle this situation, the md driver marks an array as "dirty"
before writing any data to it, and marks it as "clean" when the
array is being disabled, e.g. at shutdown. If the md driver
finds an array to be dirty at startup, it proceeds to correct any
possibly inconsistency. For RAID1, this involves copying the
contents of the first drive onto all other drives. For RAID4,
RAID5 and RAID6 this involves recalculating the parity for each
stripe and making sure that the parity block has the correct
data. For RAID10 it involves copying one of the replicas of each
block onto all the others. This process, known as
"resynchronising" or "resync" is performed in the background.
The array can still be used, though possibly with reduced
performance.
If a RAID4, RAID5 or RAID6 array is degraded (missing at least
one drive, two for RAID6) when it is restarted after an unclean
shutdown, it cannot recalculate parity, and so it is possible
that data might be undetectably corrupted. The 2.4 md driver
does not alert the operator to this condition. The 2.6 md driver
will fail to start an array in this condition without manual
intervention, though this behaviour can be overridden by a kernel
parameter.
RECOVERY
If the md driver detects a write error on a device in a RAID1,
RAID4, RAID5, RAID6, or RAID10 array, it immediately disables
that device (marking it as faulty) and continues operation on the
remaining devices. If there are spare drives, the driver will
start recreating on one of the spare drives the data which was on
that failed drive, either by copying a working drive in a RAID1
configuration, or by doing calculations with the parity block on
RAID4, RAID5 or RAID6, or by finding and copying originals for
RAID10.
In kernels prior to about 2.6.15, a read error would cause the
same effect as a write error. In later kernels, a read-error
will instead cause md to attempt a recovery by overwriting the
bad block. i.e. it will find the correct data from elsewhere,
write it over the block that failed, and then try to read it back
again. If either the write or the re-read fail, md will treat
the error the same way that a write error is treated, and will
fail the whole device.
While this recovery process is happening, the md driver will
monitor accesses to the array and will slow down the rate of
recovery if other activity is happening, so that normal access to
the array will not be unduly affected. When no other activity is
happening, the recovery process proceeds at full speed. The
actual speed targets for the two different situations can be
controlled by the speed_limit_min and speed_limit_max control
files mentioned below.
SCRUBBING AND MISMATCHES
As storage devices can develop bad blocks at any time it is
valuable to regularly read all blocks on all devices in an array
so as to catch such bad blocks early. This process is called
scrubbing.
md arrays can be scrubbed by writing either check or repair to
the file md/sync_action in the sysfs directory for the device.
Requesting a scrub will cause md to read every block on every
device in the array, and check that the data is consistent. For
RAID1 and RAID10, this means checking that the copies are
identical. For RAID4, RAID5, RAID6 this means checking that the
parity block is (or blocks are) correct.
If a read error is detected during this process, the normal read-
error handling causes correct data to be found from other devices
and to be written back to the faulty device. In many case this
will effectively fix the bad block.
If all blocks read successfully but are found to not be
consistent, then this is regarded as a mismatch.
If check was used, then no action is taken to handle the
mismatch, it is simply recorded. If repair was used, then a
mismatch will be repaired in the same way that resync repairs
arrays. For RAID5/RAID6 new parity blocks are written. For
RAID1/RAID10, all but one block are overwritten with the content
of that one block.
A count of mismatches is recorded in the sysfs file
md/mismatch_cnt. This is set to zero when a scrub starts and is
incremented whenever a sector is found that is a mismatch. md
normally works in units much larger than a single sector and when
it finds a mismatch, it does not determine exactly how many
actual sectors were affected but simply adds the number of
sectors in the IO unit that was used. So a value of 128 could
simply mean that a single 64KB check found an error (128 x
512bytes = 64KB).
If an array is created by mdadm with --assume-clean then a
subsequent check could be expected to find some mismatches.
On a truly clean RAID5 or RAID6 array, any mismatches should
indicate a hardware problem at some level - software issues
should never cause such a mismatch.
However on RAID1 and RAID10 it is possible for software issues to
cause a mismatch to be reported. This does not necessarily mean
that the data on the array is corrupted. It could simply be that
the system does not care what is stored on that part of the array
- it is unused space.
The most likely cause for an unexpected mismatch on RAID1 or
RAID10 occurs if a swap partition or swap file is stored on the
array.
When the swap subsystem wants to write a page of memory out, it
flags the page as 'clean' in the memory manager and requests the
swap device to write it out. It is quite possible that the
memory will be changed while the write-out is happening. In that
case the 'clean' flag will be found to be clear when the write
completes and so the swap subsystem will simply forget that the
swapout had been attempted, and will possibly choose a different
page to write out.
If the swap device was on RAID1 (or RAID10), then the data is
sent from memory to a device twice (or more depending on the
number of devices in the array). Thus it is possible that the
memory gets changed between the times it is sent, so different
data can be written to the different devices in the array. This
will be detected by check as a mismatch. However it does not
reflect any corruption as the block where this mismatch occurs is
being treated by the swap system as being empty, and the data
will never be read from that block.
It is conceivable for a similar situation to occur on non-swap
files, though it is less likely.
Thus the mismatch_cnt value can not be interpreted very reliably
on RAID1 or RAID10, especially when the device is used for swap.
BITMAP WRITE-INTENT LOGGING
From Linux 2.6.13, md supports a bitmap based write-intent log.
If configured, the bitmap is used to record which blocks of the
array may be out of sync. Before any write request is honoured,
md will make sure that the corresponding bit in the log is set.
After a period of time with no writes to an area of the array,
the corresponding bit will be cleared.
This bitmap is used for two optimisations.
Firstly, after an unclean shutdown, the resync process will
consult the bitmap and only resync those blocks that correspond
to bits in the bitmap that are set. This can dramatically reduce
resync time.
Secondly, when a drive fails and is removed from the array, md
stops clearing bits in the intent log. If that same drive is re-
added to the array, md will notice and will only recover the
sections of the drive that are covered by bits in the intent log
that are set. This can allow a device to be temporarily removed
and reinserted without causing an enormous recovery cost.
The intent log can be stored in a file on a separate device, or
it can be stored near the superblocks of an array which has
superblocks.
It is possible to add an intent log to an active array, or remove
an intent log if one is present.
In 2.6.13, intent bitmaps are only supported with RAID1. Other
levels with redundancy are supported from 2.6.15.
BAD BLOCK LIST
From Linux 3.5 each device in an md array can store a list of
known-bad-blocks. This list is 4K in size and usually positioned
at the end of the space between the superblock and the data.
When a block cannot be read and cannot be repaired by writing
data recovered from other devices, the address of the block is
stored in the bad block list. Similarly if an attempt to write a
block fails, the address will be recorded as a bad block. If
attempting to record the bad block fails, the whole device will
be marked faulty.
Attempting to read from a known bad block will cause a read
error. Attempting to write to a known bad block will be ignored
if any write errors have been reported by the device. If there
have been no write errors then the data will be written to the
known bad block and if that succeeds, the address will be removed
from the list.
This allows an array to fail more gracefully - a few blocks on
different devices can be faulty without taking the whole array
out of action.
The list is particularly useful when recovering to a spare. If a
few blocks cannot be read from the other devices, the bulk of the
recovery can complete and those few bad blocks will be recorded
in the bad block list.
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-journal
option. The RAID write journal works similar to file
system journals. Before writing to the data disks, md
persists data AND parity of the stripe to the journal
device. After crashes, md searches the journal device for
incomplete write operations, and replay them to the data
disks.
When the journal device fails, the RAID array is forced to
run in read-only mode.
PARTIAL PARITY LOG
From Linux 4.12 md supports Partial Parity Log (PPL) for
RAID5 arrays only. Partial parity for a write operation
is the XOR of stripe data chunks not modified by the
write. PPL is stored in the metadata region of RAID member
drives, no additional journal drive is needed. After
crashes, if one of the not modified data disks of the
stripe is missing, this updated parity can be used to
recover its data.
This mechanism is documented more fully in the file
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 complete before
reporting the write as complete to the filesystem.
This allows for a RAID1 with WRITE-BEHIND to be used to mirror
data over a slow link to a remote computer (providing the link
isn't too slow). The extra latency of the remote link will not
slow down normal operations, but the remote system will still
have a reasonably up-to-date copy of all data.
FAILFAST
From Linux 4.10, md supports FAILFAST for RAID1 and RAID10
arrays. This is a flag that can be set on individual drives,
though it is usually set on all drives, or no drives.
When md sends an I/O request to a drive that is marked as
FAILFAST, and when the array could survive the loss of that drive
without losing data, md will request that the underlying device
does not perform any retries. This means that a failure will be
reported to md promptly, and it can mark the device as faulty and
continue using the other device(s). md cannot control the
timeout that the underlying devices use to determine failure.
Any changes desired to that timeout must be set explictly on the
underlying device, separately from using mdadm.
If a FAILFAST request does fail, and if it is still safe to mark
the device as faulty without data loss, that will be done and the
array will continue functioning on a reduced number of devices.
If it is not possible to safely mark the device as faulty, md
will retry the request without disabling retries in the
underlying device. In any case, md will not attempt to repair
read errors on a device marked as FAILFAST by writing out the
correct. It will just mark the device as faulty.
FAILFAST is appropriate for storage arrays that have a low
probability of true failure, but will sometimes introduce
unacceptable delays to I/O requests while performing internal
maintenance. The value of setting FAILFAST involves a trade-off.
The gain is that the chance of unacceptable delays is
substantially reduced. The cost is that the unlikely event of
data-loss on one device is slightly more likely to result in
data-loss for the array.
When a device in an array using FAILFAST is marked as faulty, it
will usually become usable again in a short while. mdadm makes
no attempt to detect that possibility. Some separate mechanism,
tuned to the specific details of the expected failure modes,
needs to be created to monitor devices to see when they return to
full functionality, and to then re-add them to the array. In
order of this "re-add" functionality to be effective, an array
using FAILFAST should always have a write-intent bitmap.
RESTRIPING
Restriping, also known as Reshaping, is the processes of re-
arranging the data stored in each stripe into a new layout. This
might involve changing the number of devices in the array (so the
stripes are wider), changing the chunk size (so stripes are
deeper or shallower), or changing the arrangement of data and
parity (possibly changing the RAID level, e.g. 1 to 5 or 5 to 6).
As of Linux 2.6.35, md can reshape a RAID4, RAID5, or RAID6 array
to have a different number of devices (more or fewer) and to have
a different layout or chunk size. It can also convert between
these different RAID levels. It can also convert between RAID0
and RAID10, and between RAID0 and RAID4 or RAID5. Other
possibilities may follow in future kernels.
During any stripe process there is a 'critical section' during
which live data is being overwritten on disk. For the operation
of increasing the number of drives in a RAID5, this critical
section covers the first few stripes (the number being the
product of the old and new number of devices). After this
critical section is passed, data is only written to areas of the
array which no longer hold live data — the live data has already
been located away.
For a reshape which reduces the number of devices, the 'critical
section' is at the end of the reshape process.
md is not able to ensure data preservation if there is a crash
(e.g. power failure) during the critical section. If md is asked
to start an array which failed during a critical section of
restriping, it will fail to start the array.
To deal with this possibility, a user-space program must
• Disable writes to that section of the array (using the sysfs
interface),
• take a copy of the data somewhere (i.e. make a backup),
• allow the process to continue and invalidate the backup and
restore write access once the critical section is passed, and
• provide for restoring the critical data before restarting the
array after a system crash.
mdadm versions from 2.4 do this for growing a RAID5 array.
For operations that do not change the size of the array, like
simply increasing chunk size, or converting RAID5 to RAID6 with
one extra device, the entire process is the critical section. In
this case, the restripe will need to progress in stages, as a
section is suspended, backed up, restriped, and released.
SYSFS INTERFACE
Each block device appears as a directory in sysfs (which is
usually mounted at /sys). For MD devices, this directory will
contain a subdirectory called md which contains various files for
providing access to information about the array.
This interface is documented more fully in the file
Documentation/admin-guide/md.rst which is distributed with the
kernel sources. That file should be consulted for full
documentation. The following are just a selection of attribute
files that are available.
md/sync_speed_min
This value, if set, overrides the system-wide setting in
/proc/sys/dev/raid/speed_limit_min for this array only.
Writing the value system to this file will cause the
system-wide setting to have effect.
md/sync_speed_max
This is the partner of md/sync_speed_min and overrides
/proc/sys/dev/raid/speed_limit_max described below.
md/sync_action
This can be used to monitor and control the
resync/recovery process of MD. In particular, writing
"check" here will cause the array to read all data block
and check that they are consistent (e.g. parity is
correct, or all mirror replicas are the same). Any
discrepancies found are NOT corrected.
A count of problems found will be stored in
md/mismatch_count.
Alternately, "repair" can be written which will cause the
same check to be performed, but any errors will be
corrected.
Finally, "idle" can be written to stop the check/repair
process.
md/stripe_cache_size
This is only available on RAID5 and RAID6. It records the
size (in pages per device) of the stripe cache which is
used for synchronising all write operations to the array
and all read operations if the array is degraded. The
default is 256. Valid values are 17 to 32768. Increasing
this number can increase performance in some situations,
at some cost in system memory. Note, setting this value
too high can result in an "out of memory" condition for
the system.
memory_consumed = system_page_size * nr_disks *
stripe_cache_size
md/preread_bypass_threshold
This is only available on RAID5 and RAID6. This variable
sets the number of times MD will service a full-stripe-
write before servicing a stripe that requires some
"prereading". For fairness this defaults to 1. Valid
values are 0 to stripe_cache_size. Setting this to 0
maximizes sequential-write throughput at the cost of
fairness to threads doing small or random writes.
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 metadata 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_BITMAP_FILE ioctl.
Write 'none' to 'bitmap/location' will clear bitmap, and
the previous location value must be write to it to restore
bitmap.
md/bitmap/max_backlog_used
This keeps track of the maximum number of concurrent
write-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 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.
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 partitions
has a partition type of 0xFD, then that partition will
normally be inspected to see if it is part of an MD array,
and if any full arrays are found, they are started. This
kernel parameter disables this behaviour.
raid=partitionable
raid=part
These are available in 2.6 and later kernels only. They
indicate that autodetected MD arrays should be created as
partitionable arrays, with a different major device number
to the original non-partitionable md arrays. The device
number is listed as mdp in /proc/devices.
md_mod.start_ro=1
/sys/module/md_mod/parameters/start_ro
This tells md to start all arrays in read-only mode. This
is a soft read-only that will automatically switch to
read-write on the first write request. However until that
write request, nothing is written to any device by md, and
in particular, no resync or recovery operation is started.
md_mod.start_dirty_degraded=1
/sys/module/md_mod/parameters/start_dirty_degraded
As mentioned above, md will not normally start a RAID4,
RAID5, or RAID6 that is both dirty and degraded as this
situation can imply hidden data loss. This can be awkward
if the root filesystem is affected. Using this module
parameter allows such arrays to be started at boot time.
It should be understood that there is a real (though
small) risk of data corruption in this situation.
md=n,dev,dev,...
md=dn,dev,dev,...
This tells the md driver to assemble /dev/md n from the
listed devices. It is only necessary to start the device
holding the root filesystem this way. Other arrays are
best started once the system is booted.
In 2.6 kernels, the d immediately after the = indicates
that a partitionable device (e.g. /dev/md/d0) should be
created rather than the original non-partitionable device.
md=n,l,c,i,dev...
This tells the md driver to assemble a legacy RAID0 or
LINEAR array without a superblock. n gives the md device
number, l gives the level, 0 for RAID0 or -1 for LINEAR, c
gives the chunk size as a base-2 logarithm offset by
twelve, so 0 means 4K, 1 means 8K. i is ignored (legacy
support).
/proc/mdstat
Contains information about the status of currently running
array.
/proc/sys/dev/raid/speed_limit_min
A readable and writable file that reflects the current
"goal" rebuild speed for times when non-rebuild activity
is current on an array. The speed is in Kibibytes per
second, and is a per-device rate, not a per-array rate
(which means that an array with more disks will shuffle
more data for a given speed). The default is 1000.
/proc/sys/dev/raid/speed_limit_max
A readable and writable file that reflects the current
"goal" rebuild speed for times when no non-rebuild
activity is current on an array. The default is 200,000.
mdadm(8),
This page is part of the mdadm (Tool for managing md arrays in
Linux) project. Information about the project can be found at
⟨http://neil.brown.name/blog/mdadm⟩. If you have a bug report for
this manual page, send it to [email protected]. This
page was obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-13.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
MD(4)
Pages that refer to this page: mdadm.conf(5), mdadm(8), mdmon(8), raid6check(8), xfs_growfs(8), xfs_info(8)
|
HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|