Provided by: mdadm_4.4-11ubuntu1_amd64 
NAME
mdadm - manage MD devices aka Linux Software RAID
SYNOPSIS
mdadm [mode] <raiddevice> [options] <component-devices>
DESCRIPTION
RAID devices are virtual devices created from two or more real block devices. This allows multiple
devices (typically disk drives or partitions thereof) to be combined into a single device to hold (for
example) a single filesystem. Some RAID levels include redundancy and so can survive some degree of
device failure.
Linux Software RAID devices are implemented through the md (Multiple Devices) device driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4, RAID5, RAID6,
RAID10, MULTIPATH, FAULTY, and CONTAINER.
MULTIPATH is not a Software RAID mechanism, but does involve multiple devices: each device is a path to
one common physical storage device. New installations should not use md/multipath as it is not well
supported and has no ongoing development. Use the Device Mapper based multipath-tools instead. It is
deprecated and support will be removed in the future.
FAULTY is also not true RAID, and it only involves one device. It provides a layer over a true device
that can be used to inject faults. It is deprecated and support will be removed in the future.
CONTAINER is different again. A CONTAINER is a collection of devices that are managed as a set. This is
similar to the set of devices connected to a hardware RAID controller. The set of devices may contain a
number of different RAID arrays each utilising some (or all) of the blocks from a number of the devices
in the set. For example, two devices in a 5-device set might form a RAID1 using the whole devices. The
remaining three might have a RAID5 over the first half of each device, and a RAID0 over the second half.
With a CONTAINER, there is one set of metadata that describes all of the arrays in the container. So
when mdadm creates a CONTAINER device, the device just represents the metadata. Other normal arrays
(RAID1 etc) can be created inside the container.
MODES
mdadm has several major modes of operation:
Assemble
Assemble the components of a previously created array into an active array. Components can be
explicitly given or can be searched for. mdadm checks that the components do form a bona fide
array, and can, on request, fiddle superblock information so as to assemble a faulty array.
Build Build an array that doesn't have per-device metadata (superblocks). For these sorts of arrays,
mdadm cannot differentiate between initial creation and subsequent assembly of an array. It also
cannot perform any checks that appropriate components have been requested. Because of this, the
Build mode should only be used together with a complete understanding of what you are doing.
Create Create a new array with per-device metadata (superblocks). Appropriate metadata is written to
each device, and then the array comprising those devices is activated. A 'resync' process is
started to make sure that the array is consistent (e.g. both sides of a mirror contain the same
data) but the content of the device is left otherwise untouched. The array can be used as soon as
it has been created. There is no need to wait for the initial resync to finish.
Follow or Monitor
Monitor one or more md devices and act on any state changes. This is only meaningful for RAID1,
4, 5, 6, 10 or multipath arrays, as only these have interesting state. RAID0 or Linear never have
missing, spare, or failed drives, so there is nothing to monitor.
Grow Grow (or shrink) an array, or otherwise reshape it in some way. Currently supported growth
options including changing the active size of component devices and changing the number of active
devices in Linear and RAID levels 0/1/4/5/6, changing the RAID level between 0, 1, 5, and 6, and
between 0 and 10, changing the chunk size and layout for RAID 0,4,5,6,10 as well as adding or
removing a write-intent bitmap and changing the array's consistency policy.
Incremental Assembly
Add a single device to an appropriate array. If the addition of the device makes the array
runnable, the array will be started. This provides a convenient interface to a hot-plug system.
As each device is detected, mdadm has a chance to include it in some array as appropriate.
Optionally, when the --fail flag is passed in we will remove the device from any active array
instead of adding it.
If a CONTAINER is passed to mdadm in this mode, then any arrays within that container will be
assembled and started.
Manage This is for doing things to specific components of an array such as adding new spares and removing
faulty devices.
Misc This is an 'everything else' mode that supports operations on active arrays, operations on
component devices such as erasing old superblocks, and information-gathering operations.
Auto-detect
This mode does not act on a specific device or array, but rather it requests the Linux Kernel to
activate any auto-detected arrays.
OPTIONS
Options for selecting a mode are:
-A, --assemble
Assemble a pre-existing array.
-B, --build
Build a legacy array without superblocks.
-C, --create
Create a new array.
-F, --follow, --monitor
Select Monitor mode.
-G, --grow
Change the size or shape of an active array.
-I, --incremental
Add/remove a single device to/from an appropriate array, and possibly start the array.
--auto-detect
Request that the kernel starts any auto-detected arrays. This can only work if md is compiled
into the kernel — not if it is a module. Arrays can be auto-detected by the kernel if all the
components are in primary MS-DOS partitions with partition type FD, and all use v0.90 metadata.
In-kernel autodetect is not recommended for new installations. Using mdadm to detect and assemble
arrays — possibly in an initrd — is substantially more flexible and should be preferred.
If a device is given before any options, or if the first option is one of --add, --re-add, --add-spare,
--fail, --remove, or --replace, then the MANAGE mode is assumed. Anything other than these will cause
the Misc mode to be assumed.
Options that are not mode-specific are:
-h, --help
Display a general help message or, after one of the above options, a mode-specific help message.
--help-options
Display more detailed help about command-line parsing and some commonly used options.
-V, --version
Print version information for mdadm.
-v, --verbose
Be more verbose about what is happening. This can be used twice to be extra-verbose. The extra
verbosity currently only affects --detail --scan and --examine --scan.
-q, --quiet
Avoid printing purely informative messages. With this, mdadm will be silent unless there is
something really important to report.
-f, --force
Be more forceful about certain operations. See the various modes for the exact meaning of this
option in different contexts.
-c, --config=
Specify the config file or directory. If not specified, the default config file and default
conf.d directory will be used. See mdadm.conf(5) for more details.
If the config file given is partitions then nothing will be read, but mdadm will act as though the
config file contained exactly
DEVICE partitions containers
and will read /proc/partitions to find a list of devices to scan, and /proc/mdstat to find a list
of containers to examine. If the word none is given for the config file, then mdadm will act as
though the config file were empty.
If the name given is of a directory, then mdadm will collect all the files contained in the
directory with a name ending in .conf, sort them lexically, and process all of those files as
config files.
-s, --scan
Scan config file or /proc/mdstat for missing information. In general, this option gives mdadm
permission to get any missing information (like component devices, array devices, array
identities, and alert destination) from the configuration file (see previous option); one
exception is MISC mode when using --detail or --stop, in which case --scan says to get a list of
array devices from /proc/mdstat.
-e, --metadata=
Declare the style of RAID metadata (superblock) to be used. The default is 1.2 for --create, and
to guess for other operations. The default can be overridden by setting the metadata value for
the CREATE keyword in mdadm.conf.
Options are:
0, 0.90
Use the original 0.90 format superblock. This format limits arrays to 28 component devices
and limits component devices of levels 1 and greater to 2 terabytes. It is also possible
for there to be confusion about whether the superblock applies to a whole device or just
the last partition, if that partition starts on a 64K boundary.
1, 1.0, 1.1, 1.2 default
Use the new version-1 format superblock. This has fewer restrictions. It can easily be
moved between hosts with different endian-ness, and a recovery operation can be
checkpointed and restarted. The different sub-versions store the superblock at different
locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from the
start (for 1.2). "1" is equivalent to "1.2" (the commonly preferred 1.x format).
"default" is equivalent to "1.2".
ddf Use the "Industry Standard" DDF (Disk Data Format) format defined by SNIA. DDF is
deprecated and there is no active development around it. When creating a DDF array a
CONTAINER will be created, and normal arrays can be created in that container.
imsm Use the Intel(R) Matrix Storage Manager metadata format. This creates a CONTAINER which is
managed in a similar manner to DDF, and is supported by an option-rom on some platforms:
https://www.intel.com/content/www/us/en/support/products/122484
--homehost=
This will override any HOMEHOST setting in the config file and provides the identity of the host
which should be considered the home for any arrays.
When creating an array, the homehost will be recorded in the metadata. For version-1 superblocks,
it will be prefixed to the array name. For version-0.90 superblocks, part of the SHA1 hash of the
hostname will be stored in the latter half of the UUID.
When reporting information about an array, any array which is tagged for the given homehost will
be reported as such.
When using Auto-Assemble, only arrays tagged for the given homehost will be allowed to use 'local'
names (i.e. not ending in '_' followed by a digit string). See below under Auto-Assembly.
The special name "any" can be used as a wild card. If an array is created with --homehost=any
then the name "any" will be stored in the array and it can be assembled in the same way on any
host. If an array is assembled with this option, then the homehost recorded on the array will be
ignored.
--prefer=
When mdadm needs to print the name for a device it normally finds the name in /dev which refers to
the device and is the shortest. When a path component is given with --prefer mdadm will prefer a
longer name if it contains that component. For example --prefer=by-uuid will prefer a name in a
subdirectory of /dev called by-uuid.
This functionality is currently only provided by --detail and --monitor.
--home-cluster=
specifies the cluster name for the md device. The md device can be assembled only on the cluster
which matches the name specified. If this option is not provided, mdadm tries to detect the
cluster name automatically.
For create, build, or grow:
-n, --raid-devices=
Specify the number of active devices in the array. This, plus the number of spare devices (see
below) must equal the number of component-devices (including "missing" devices) that are listed on
the command line for --create. Setting a value of 1 is probably a mistake and so requires that
--force be specified first. A value of 1 will then be allowed for linear, multipath, RAID0 and
RAID1. It is never allowed for RAID4, RAID5 or RAID6.
This number can only be changed using --grow for RAID1, RAID4, RAID5 and RAID6 arrays.
-x, --spare-devices=
Specify the number of spare (eXtra) devices in the initial array. Spares can also be added and
removed later. The number of component devices listed on the command line must equal the number
of RAID devices plus the number of spare devices.
-z, --size=
Amount (in Kilobytes) of space to use from each drive in RAID levels 1/4/5/6/10 and for RAID 0 on
external metadata. This must be a multiple of the chunk size, and must leave about 128Kb of space
at the end of the drive for the RAID superblock. When specified as ¸max¸ (as it often is) the
smallest drive (or partition) sets the size. In that case, a warning will follow if the drives,
as a group, have sizes that differ by more than one percent.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate Kilobytes, Megabytes, Gigabytes or
Terabytes respectively.
Sometimes a replacement drive can be a little smaller than the original drives though this should
be minimised by IDEMA standards. Such a replacement drive will be rejected by md. To guard
against this it can be useful to set the initial size slightly smaller than the smaller device
with the aim that it will still be larger than any replacement.
This option can be used with --create for determining the initial size of an array. For external
metadata, it can be used on a volume, but not on a container itself. Setting the initial size of
RAID 0 array is only valid for external metadata.
This value can be set with --grow for RAID level 1/4/5/6/10 though DDF arrays may not be able to
support this. RAID 0 array size cannot be changed. If the array was created with a size smaller
than the currently active drives, the extra space can be accessed using --grow. The size can be
given as max which means to choose the largest size that fits on all current drives.
Before reducing the size of the array (with --grow --size=) you should make sure that space isn't
needed. If the device holds a filesystem, you would need to resize the filesystem to use less
space.
After reducing the array size you should check that the data stored in the device is still
available. If the device holds a filesystem, then an 'fsck' of the filesystem is a minimum
requirement. If there are problems the array can be made bigger again with no loss with another
--grow --size= command.
-Z, --array-size=
This is only meaningful with --grow and its effect is not persistent: when the array is stopped
and restarted the default array size will be restored.
Setting the array-size causes the array to appear smaller to programs that access the data. This
is particularly needed before reshaping an array so that it will be smaller. As the reshape is
not reversible, but setting the size with --array-size is, it is required that the array size is
reduced as appropriate before the number of devices in the array is reduced.
Before reducing the size of the array you should make sure that space isn't needed. If the device
holds a filesystem, you would need to resize the filesystem to use less space.
After reducing the array size you should check that the data stored in the device is still
available. If the device holds a filesystem, then an 'fsck' of the filesystem is a minimum
requirement. If there are problems the array can be made bigger again with no loss with another
--grow --array-size= command.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate Kilobytes, Megabytes, Gigabytes or
Terabytes respectively. A value of max restores the apparent size of the array to be whatever the
real amount of available space is.
Clustered arrays do not support this parameter yet.
-c, --chunk=
Specify chunk size in kilobytes. The default when creating an array is 512KB. To ensure
compatibility with earlier versions, the default when building an array with no persistent
metadata is 64KB. This is only meaningful for RAID0, RAID4, RAID5, RAID6, and RAID10.
RAID4, RAID5, RAID6, and RAID10 require the chunk size to be a power of 2, with minimal chunk size
being 4KB.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate Kilobytes, Megabytes, Gigabytes or
Terabytes respectively.
--rounding=
Specify the rounding factor for a Linear array. The size of each component will be rounded down
to a multiple of this size. This is a synonym for --chunk but highlights the different meaning
for Linear as compared to other RAID levels. The default is 0K (i.e. no rounding).
-l, --level=
Set RAID level. When used with --create, options are: linear, raid0, 0, stripe, raid1, 1, mirror,
raid4, 4, raid5, 5, raid6, 6, raid10, 10, multipath, mp, faulty, container. Obviously some of
these are synonymous.
When a CONTAINER metadata type is requested, only the container level is permitted, and it does
not need to be explicitly given.
When used with --build, only linear, stripe, raid0, 0, raid1, multipath, mp, and faulty are valid.
Can be used with --grow to change the RAID level in some cases. See LEVEL CHANGES below.
-p, --layout=
This option configures the fine details of data layout for RAID5, RAID6, and RAID10 arrays, and
controls the failure modes for faulty. It can also be used for working around a kernel bug with
RAID0, but generally doesn't need to be used explicitly.
The layout of the RAID5 parity block can be one of left-asymmetric, left-symmetric,
right-asymmetric, right-symmetric, la, ra, ls, rs. The default is left-symmetric.
It is also possible to cause RAID5 to use a RAID4-like layout by choosing parity-first, or
parity-last.
Finally for RAID5 there are DDF-compatible layouts, ddf-zero-restart, ddf-N-restart, and
ddf-N-continue.
These same layouts are available for RAID6. There are also 4 layouts that will provide an
intermediate stage for converting between RAID5 and RAID6. These provide a layout which is
identical to the corresponding RAID5 layout on the first N-1 devices, and has the 'Q' syndrome
(the second 'parity' block used by RAID6) on the last device. These layouts are:
left-symmetric-6, right-symmetric-6, left-asymmetric-6, right-asymmetric-6, and parity-first-6.
When setting the failure mode for level faulty, the options are: write-transient, wt,
read-transient, rt, write-persistent, wp, read-persistent, rp, write-all, read-fixable, rf, clear,
flush, none.
Each failure mode can be followed by a number, which is used as a period between fault generation.
Without a number, the fault is generated once on the first relevant request. With a number, the
fault will be generated after that many requests, and will continue to be generated every time the
period elapses.
Multiple failure modes can be current simultaneously by using the --grow option to set subsequent
failure modes.
"clear" or "none" will remove any pending or periodic failure modes, and "flush" will clear any
persistent faults.
The layout options for RAID10 are one of 'n', 'o' or 'f' followed by a small number signifying the
number of copies of each datablock. The default is 'n2'. The supported options are:
'n' signals 'near' copies. Multiple copies of one data block are at similar offsets in different
devices.
'o' signals 'offset' copies. Rather than the chunks being duplicated within a stripe, whole
stripes are duplicated but are rotated by one device so duplicate blocks are on different devices.
Thus subsequent copies of a block are in the next drive, and are one chunk further down.
'f' signals 'far' copies (multiple copies have very different offsets). See md(4) for more detail
about 'near', 'offset', and 'far'.
As for the number of copies of each data block, 2 is normal, 3 can be useful. This number can be
at most equal to the number of devices in the array. It does not need to divide evenly into that
number (e.g. it is perfectly legal to have an 'n2' layout for an array with an odd number of
devices).
A bug introduced in Linux 3.14 means that RAID0 arrays with devices of differing sizes started
using a different layout. This could lead to data corruption. Since Linux 5.4 (and various
stable releases that received backports), the kernel will not accept such an array unless a layout
is explicitly set. It can be set to 'original' or 'alternate'. When creating a new array, mdadm
will select 'original' by default, so the layout does not normally need to be set. An array
created for either 'original' or 'alternate' will not be recognized by an (unpatched) kernel prior
to 5.4. To create a RAID0 array with devices of differing sizes that can be used on an older
kernel, you can set the layout to 'dangerous'. This will use whichever layout the running kernel
supports, so the data on the array may become corrupt when changing kernel from pre-3.14 to a
later kernel.
When an array is converted between RAID5 and RAID6 an intermediate RAID6 layout is used in which
the second parity block (Q) is always on the last device. To convert a RAID5 to RAID6 and leave
it in this new layout (which does not require re-striping) use --layout=preserve. This will try
to avoid any restriping.
The converse of this is --layout=normalise which will change a non-standard RAID6 layout into a
more standard arrangement.
--parity=
same as --layout (thus explaining the p of -p).
-b, --bitmap=
Specify how to store a write-intent bitmap. Following values are supported:
internal - the bitmap is stored with the metadata on the array and so is replicated on all
devices.
clustered - the array is created for a clustered environment. One bitmap is created for each node
as defined by the --nodes parameter and are stored internally.
none - create array with no bitmap or remove any present bitmap (grow mode).
--bitmap-chunk=
Set the chunk size of the bitmap. Each bit corresponds to that many Kilobytes of storage.
internal bitmap, the chunk size defaults to 64Meg, or larger if necessary to fit the bitmap into
the available space.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate Kilobytes, Megabytes, Gigabytes or
Terabytes respectively.
-W, --write-mostly
subsequent devices listed in a --build, --create, or --add command will be flagged as
'write-mostly'. This is valid for RAID1 only and means that the 'md' driver will avoid reading
from these devices if at all possible. This can be useful if mirroring over a slow link.
--write-behind=
Specify that write-behind mode should be enabled (valid for RAID1 only). If an argument is
specified, it will set the maximum number of outstanding writes allowed. The default value is
256. A write-intent bitmap is required in order to use write-behind mode, and write-behind is
only attempted on drives marked as write-mostly.
--failfast
subsequent devices listed in a --create or --add command will be flagged as 'failfast'. This is
valid for RAID1 and RAID10 only. IO requests to these devices will be encouraged to fail quickly
rather than cause long delays due to error handling. Also no attempt is made to repair a read
error on these devices.
If an array becomes degraded so that the 'failfast' device is the only usable device, the
'failfast' flag will then be ignored and extended delays will be preferred to complete failure.
The 'failfast' flag is appropriate for storage arrays which have a low probability of true
failure, but which may sometimes cause unacceptable delays due to internal maintenance functions.
--assume-clean
Tell mdadm that the array pre-existed and is known to be clean. It can be useful when trying to
recover from a major failure as you can be sure that no data will be affected unless you actually
write to the array. It can also be used when creating a RAID1 or RAID10 if you want to avoid the
initial resync, however this practice — while normally safe — is not recommended. Use this only
if you really know what you are doing.
When the devices that will be part of a new array were filled with zeros before creation the
operator knows the array is actually clean. If that is the case, such as after running badblocks,
this argument can be used to tell mdadm the facts the operator knows.
When an array is resized to a larger size with --grow --size= the new space is normally resynced
in that same way that the whole array is resynced at creation. --assume-clean can be used with
that command to avoid the automatic resync.
--write-zeroes
When creating an array, send write zeroes requests to all the block devices. This should zero the
data area on all disks such that the initial sync is not necessary and, if successful, will behave
as if --assume-clean was specified.
This is intended for use with devices that have hardware offload for zeroing, but despite this
zeroing can still take several minutes for large disks. Thus a message is printed before and
after zeroing and each disk is zeroed in parallel with the others.
This is only meaningful with --create.
--backup-file=
This is needed when --grow is used to increase the number of raid devices in a RAID5 or RAID6 if
there are no spare devices available, or to shrink, change RAID level or layout. See the GROW
MODE section below on RAID-DEVICES CHANGES. The file must be stored on a separate device, not on
the RAID array being reshaped.
--data-offset=
Arrays with 1.x metadata can leave a gap between the start of the device and the start of array
data. This gap can be used for various metadata. The start of data is known as the data-offset.
Normally an appropriate data offset is computed automatically. However it can be useful to set it
explicitly such as when re-creating an array which was originally created using a different
version of mdadm which computed a different offset.
Setting the offset explicitly over-rides the default. The value given is in Kilobytes unless a
suffix of 'K', 'M', 'G' or 'T' is used to explicitly indicate Kilobytes, Megabytes, Gigabytes or
Terabytes respectively.
--data-offset can also be used with --grow for some RAID levels (initially on RAID10). This
allows the data-offset to be changed as part of the reshape process. When the data offset is
changed, no backup file is required as the difference in offsets is used to provide the same
functionality.
When the new offset is earlier than the old offset, the number of devices in the array cannot
shrink. When it is after the old offset, the number of devices in the array cannot increase.
When creating an array, --data-offset can be specified as variable. In the case each member
device is expected to have an offset appended to the name, separated by a colon. This makes it
possible to recreate exactly an array which has varying data offsets (as can happen when different
versions of mdadm are used to add different devices).
--continue
This option is complementary to the --freeze-reshape option for assembly. It is needed when --grow
operation is interrupted and it is not restarted automatically due to --freeze-reshape usage
during array assembly. This option is used together with -G , ( --grow ) command and device for a
pending reshape to be continued. All parameters required for reshape continuation will be read
from array metadata. If initial --grow command had required --backup-file= option to be set,
continuation option will require to have exactly the same backup file given as well.
Any other parameter passed together with --continue option will be ignored.
-N, --name=
Set a name for the array. It must be POSIX PORTABLE NAME compatible and cannot be longer than 32
chars. This is effective when creating an array with a v1 metadata, or an external array.
If name is needed but not specified, it is taken from the basename of the device that is being
created. See DEVICE NAMES
-R, --run
Insist that mdadm run the array, even if some of the components appear to be active in another
array or filesystem. Normally mdadm will ask for confirmation before including such components in
an array. This option causes that question to be suppressed.
-f, --force
Insist that mdadm accept the geometry and layout specified without question. Normally mdadm will
not allow the creation of an array with only one device, and will try to create a RAID5 array with
one missing drive (as this makes the initial resync work faster). With --force, mdadm will not
try to be so clever.
-o, --readonly
Start the array read only rather than read-write as normal. No writes will be allowed to the
array, and no resync, recovery, or reshape will be started. It works with Create, Assemble, Manage
and Misc mode.
-a, --add
This option can be used in Grow mode in two cases.
If the target array is a Linear array, then --add can be used to add one or more devices to the
array. They are simply catenated on to the end of the array. Once added, the devices cannot be
removed.
If the --raid-disks option is being used to increase the number of devices in an array, then --add
can be used to add some extra devices to be included in the array. In most cases this is not
needed as the extra devices can be added as spares first, and then the number of raid disks can be
changed. However, for RAID0 it is not possible to add spares. So to increase the number of
devices in a RAID0, it is necessary to set the new number of devices, and to add the new devices,
in the same command.
--nodes
Only works when the array is created for a clustered environment. It specifies the maximum number
of nodes in the cluster that will use this device simultaneously. If not specified, this defaults
to 4.
--write-journal
Specify journal device for the RAID-4/5/6 array. The journal device should be an SSD with a
reasonable lifetime.
-k, --consistency-policy=
Specify how the array maintains consistency in the case of an unexpected shutdown. Only relevant
for RAID levels with redundancy. Currently supported options are:
resync Full resync is performed and all redundancy is regenerated when the array is started after
an unclean shutdown.
bitmap Resync assisted by a write-intent bitmap. Implicitly selected when using --bitmap.
journal
For RAID levels 4/5/6, the journal device is used to log transactions and replay after an
unclean shutdown. Implicitly selected when using --write-journal.
ppl For RAID5 only, Partial Parity Log is used to close the write hole and eliminate resync.
PPL is stored in the metadata region of RAID member drives, no additional journal drive is
needed.
Can be used with --grow to change the consistency policy of an active array in some cases. See
CONSISTENCY POLICY CHANGES below.
For assemble:
-u, --uuid=
uuid of array to assemble. Devices which don't have this uuid are excluded
-m, --super-minor=
Minor number of device that array was created for. Devices which don't have this minor number are
excluded. If you create an array as /dev/md1, then all superblocks will contain the minor number
1, even if the array is later assembled as /dev/md2.
Giving the literal word "dev" for --super-minor will cause mdadm to use the minor number of the md
device that is being assembled. e.g. when assembling /dev/md0, --super-minor=dev will look for
super blocks with a minor number of 0.
--super-minor is only relevant for v0.90 metadata, and should not normally be used. Using --uuid
is much safer.
-N, --name=
Specify the name of the array to assemble. It must be POSIX PORTABLE NAME compatible and cannot be
longer than 32 chars. This must be the name that was specified when creating the array. It must
either match the name stored in the superblock exactly, or it must match with the current homehost
prefixed to the start of the given name.
-f, --force
Assemble the array even if the metadata on some devices appears to be out-of-date. If mdadm
cannot find enough working devices to start the array, but can find some devices that are recorded
as having failed, then it will mark those devices as working so that the array can be started.
This works only for native. For external metadata it allows one to start dirty degraded RAID 4, 5,
6. An array which requires --force to be started may contain data corruption. Use it carefully.
-R, --run
Attempt to start the array even if fewer drives were given than were present last time the array
was active. Normally if not all the expected drives are found and --scan is not used, then the
array will be assembled but not started. With --run an attempt will be made to start it anyway.
--no-degraded
This is the reverse of --run in that it inhibits the startup of array unless all expected drives
are present. This is only needed with --scan, and can be used if the physical connections to
devices are not as reliable as you would like.
--backup-file=
If --backup-file was used while reshaping an array (e.g. changing number of devices or chunk size)
and the system crashed during the critical section, then the same --backup-file must be presented
to --assemble to allow possibly corrupted data to be restored, and the reshape to be completed.
--invalid-backup
If the file needed for the above option is not available for any reason an empty file can be given
together with this option to indicate that the backup file is invalid. In this case the data that
was being rearranged at the time of the crash could be irrecoverably lost, but the rest of the
array may still be recoverable. This option should only be used as a last resort if there is no
way to recover the backup file.
-U, --update=
Update the superblock on each device while assembling the array. The argument given to this flag
can be one of summaries, uuid, name, nodes, homehost, home-cluster, resync, byteorder, devicesize,
no-bitmap, bbl, no-bbl, ppl, no-ppl, layout-original, layout-alternate, layout-unspecified,
metadata, or super-minor.
The super-minor option will update the preferred minor field on each superblock to match the minor
number of the array being assembled. This can be useful if --examine reports a different
"Preferred Minor" to --detail. In some cases this update will be performed automatically by the
kernel driver. In particular, the update happens automatically at the first write to an array
with redundancy (RAID level 1 or greater).
The uuid option will change the uuid of the array. If a UUID is given with the --uuid option that
UUID will be used as a new UUID and will NOT be used to help identify the devices in the array.
If no --uuid is given, a random UUID is chosen.
The name option will change the name of the array as stored in the superblock. This is only
supported for version-1 superblocks.
The nodes option will change the nodes of the array as stored in the bitmap superblock. This
option only works for a clustered environment.
The homehost option will change the homehost as recorded in the superblock. For version-0
superblocks, this is the same as updating the UUID. For version-1 superblocks, this involves
updating the name.
The home-cluster option will change the cluster name as recorded in the superblock and bitmap.
This option only works for a clustered environment.
The resync option will cause the array to be marked dirty meaning that any redundancy in the array
(e.g. parity for RAID5, copies for RAID1) may be incorrect. This will cause the RAID system to
perform a "resync" pass to make sure that all redundant information is correct.
The byteorder option allows arrays to be moved between machines with different byte-order, such as
from a big-endian machine like a Sparc or some MIPS machines, to a little-endian x86_64 machine.
When assembling such an array for the first time after a move, giving --update=byteorder will
cause mdadm to expect superblocks to have their byteorder reversed, and will correct that order
before assembling the array. This is only valid with original (Version 0.90) superblocks.
The summaries option will correct the summaries in the superblock. That is the counts of total,
working, active, failed, and spare devices.
The devicesize option will rarely be of use. It applies to version 1.1 and 1.2 metadata only
(where the metadata is at the start of the device) and is only useful when the component device
has changed size (typically become larger). The version 1 metadata records the amount of the
device that can be used to store data, so if a device in a version 1.1 or 1.2 array becomes
larger, the metadata will still be visible, but the extra space will not. In this case it might
be useful to assemble the array with --update=devicesize. This will cause mdadm to determine the
maximum usable amount of space on each device and update the relevant field in the metadata.
The metadata option only works on v0.90 metadata arrays and will convert them to v1.0 metadata.
The array must not be dirty (i.e. it must not need a sync) and it must not have a write-intent
bitmap.
The old metadata will remain on the devices, but will appear older than the new metadata and so
will usually be ignored. The old metadata (or indeed the new metadata) can be removed by giving
the appropriate --metadata= option to --zero-superblock.
The no-bitmap option can be used when an array has an internal bitmap which is corrupt in some way
so that assembling the array normally fails. It will cause any internal bitmap to be ignored.
The bbl option will reserve space in each device for a bad block list. This will be 4K in size
and positioned near the end of any free space between the superblock and the data.
The no-bbl option will cause any reservation of space for a bad block list to be removed. If the
bad block list contains entries, this will fail, as removing the list could cause data corruption.
The ppl option will enable PPL for a RAID5 array and reserve space for PPL on each device. There
must be enough free space between the data and superblock and a write-intent bitmap or journal
must not be used.
The no-ppl option will disable PPL in the superblock.
The layout-original and layout-alternate options are for RAID0 arrays with non-uniform devices
size that were in use before Linux 5.4. If the array was being used with Linux 3.13 or earlier,
then to assemble the array on a new kernel, --update=layout-original must be given. If the array
was created and used with a kernel from Linux 3.14 to Linux 5.3, then --update=layout-alternate
must be given. This only needs to be given once. Subsequent assembly of the array will happen
normally. For more information, see md(4).
The layout-unspecified option reverts the effect of layout-orignal or layout-alternate and allows
the array to be again used on a kernel prior to Linux 5.3. This option should be used with great
caution.
--freeze-reshape
This option is intended to be used in start-up scripts during the initrd boot phase. When the
array under reshape is assembled during the initrd phase, this option stops the reshape after the
reshape-critical section has been restored. This happens before the file system pivot operation
and avoids loss of filesystem context. Losing file system context would cause reshape to be
broken.
Reshape can be continued later using the --continue option for the grow command.
For Manage mode:
-t, --test
Unless a more serious error occurred, mdadm will exit with a status of 2 if no changes were made
to the array and 0 if at least one change was made. This can be useful when an indirect specifier
such as missing, detached or faulty is used in requesting an operation on the array. --test will
report failure if these specifiers didn't find any match.
-a, --add
hot-add listed devices. If a device appears to have recently been part of the array (possibly it
failed or was removed) the device is re-added as described in the next point. If that fails or
the device was never part of the array, the device is added as a hot-spare. If the array is
degraded, it will immediately start to rebuild data onto that spare.
Note that this and the following options are only meaningful on array with redundancy. They don't
apply to RAID0 or Linear.
--re-add
re-add a device that was previously removed from an array. If the metadata on the device reports
that it is a member of the array, and the slot that it used is still vacant, then the device will
be added back to the array in the same position. This will normally cause the data for that
device to be recovered. However, based on the event count on the device, the recovery may only
require sections that are flagged by a write-intent bitmap to be recovered or may not require any
recovery at all.
When used on an array that has no metadata (i.e. it was built with --build) it will be assumed
that bitmap-based recovery is enough to make the device fully consistent with the array.
--re-add can also be accompanied by --update=devicesize, --update=bbl, or --update=no-bbl. See
descriptions of these options when used in Assemble mode for an explanation of their use.
If the device name given is missing then mdadm will try to find any device that looks like it
should be part of the array but isn't and will try to re-add all such devices.
If the device name given is faulty then mdadm will find all devices in the array that are marked
faulty, remove them and attempt to immediately re-add them. This can be useful if you are certain
that the reason for failure has been resolved.
--add-spare
Add a device as a spare. This is similar to --add except that it does not attempt --re-add first.
The device will be added as a spare even if it looks like it could be a recent member of the
array.
-r, --remove
remove listed devices. They must not be active. i.e. they should be failed or spare devices.
As well as the name of a device file (e.g. /dev/sda1) the words failed, detached and names like
set-A can be given to --remove. The first causes all failed devices to be removed. The second
causes any device which is no longer connected to the system (i.e an 'open' returns ENXIO) to be
removed. The third will remove a set as described below under --fail.
-f, --fail
Mark listed devices as faulty. As well as the name of a device file, the word detached or a set
name like set-A can be given. The former will cause any device that has been detached from the
system to be marked as failed. It can then be removed.
For RAID10 arrays where the number of copies evenly divides the number of devices, the devices can
be conceptually divided into sets where each set contains a single complete copy of the data on
the array. Sometimes a RAID10 array will be configured so that these sets are on separate
controllers. In this case, all the devices in one set can be failed by giving a name like set-A
or set-B to --fail. The appropriate set names are reported by --detail.
--set-faulty
same as --fail.
--replace
Mark listed devices as requiring replacement. As soon as a spare is available, it will be rebuilt
and will replace the marked device. This is similar to marking a device as faulty, but the device
remains in service during the recovery process to increase resilience against multiple failures.
When the replacement process finishes, the replaced device will be marked as faulty.
--with This can follow a list of --replace devices. The devices listed after --with will preferentially
be used to replace the devices listed after --replace. These devices must already be spare
devices in the array.
--write-mostly
Subsequent devices that are added or re-added will have the 'write-mostly' flag set. This is only
valid for RAID1 and means that the 'md' driver will avoid reading from these devices if possible.
--readwrite
Subsequent devices that are added or re-added will have the 'write-mostly' flag cleared.
--cluster-confirm
Confirm the existence of the device. This is issued in response to an --add request by a node in a
cluster. When a node adds a device it sends a message to all nodes in the cluster to look for a
device with a UUID. This translates to a udev notification with the UUID of the device to be added
and the slot number. The receiving node must acknowledge this message with --cluster-confirm.
Valid arguments are <slot>:<devicename> in case the device is found or <slot>:missing in case the
device is not found.
--add-journal
Add a journal to an existing array, or recreate journal for a RAID-4/5/6 array that lost a journal
device. To avoid interrupting ongoing write operations, --add-journal only works for array in
Read-Only state.
--failfast
Subsequent devices that are added or re-added will have the 'failfast' flag set. This is only
valid for RAID1 and RAID10 and means that the 'md' driver will avoid long timeouts on error
handling where possible.
--nofailfast
Subsequent devices that are re-added will be re-added without the 'failfast' flag set.
Each of these options requires that the first device listed is the array to be acted upon, and the
remainder are component devices to be added, removed, marked as faulty, etc. Several different
operations can be specified for different devices, e.g.
mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1
Each operation applies to all devices listed until the next operation.
If an array is using a write-intent bitmap, then devices which have been removed can be re-added in a way
that avoids a full reconstruction but instead just updates the blocks that have changed since the device
was removed. For arrays with persistent metadata (superblocks) this is done automatically. For arrays
created with --build mdadm needs to be told that this device we removed recently with --re-add.
Devices can only be removed from an array if they are not in active use, i.e. that must be spares or
failed devices. To remove an active device, it must first be marked as faulty.
For Misc mode:
-Q, --query
Examine a device to see (1) if it is an md device and (2) if it is a component of an md array.
Information about what is discovered is presented.
-D, --detail
Print details of one or more md devices.
--detail-platform
Print details of the platform's RAID capabilities (firmware / hardware topology) for a given
metadata format. If used without an argument, mdadm will scan all controllers looking for their
capabilities. Otherwise, mdadm will only look at the controller specified by the argument in the
form of an absolute filepath or a link, e.g. /sys/devices/pci0000:00/0000:00:1f.2.
-Y, --export
When used with --detail, --detail-platform, --examine, or --incremental output will be formatted
as key=value pairs for easy import into the environment.
With --incremental The value MD_STARTED indicates whether an array was started (yes) or not, which
may include a reason (unsafe, nothing, no). Also the value MD_FOREIGN indicates if the array is
expected on this host (no), or seems to be from elsewhere (yes).
-E, --examine
Print contents of the metadata stored on the named device(s). Note the contrast between --examine
and --detail. --examine applies to devices which are components of an array, while --detail
applies to a whole array which is currently active.
-X, --examine-bitmap
Report information about a bitmap. The argument is an array component. Note that running this on
an array device (e.g. /dev/md0) does not report the bitmap for that array.
--examine-badblocks
List the bad-blocks recorded for the device, if a bad-blocks list has been configured. Currently
only 1.x and IMSM metadata support bad-blocks lists.
--dump=directory
--restore=directory
Save metadata from lists devices, or restore metadata to listed devices.
-R, --run
start a partially assembled array. If --assemble did not find enough devices to fully start the
array, it might leaving it partially assembled. If you wish, you can then use --run to start the
array in degraded mode.
-S, --stop
deactivate array, releasing all resources.
-o, --readonly
mark array as readonly.
-w, --readwrite
mark array as readwrite.
--zero-superblock
If the device contains a valid md superblock, the block is overwritten with zeros. With --force
the block where the superblock would be is overwritten even if it doesn't appear to be valid.
Note: Be careful when calling --zero-superblock with clustered raid. Make sure the array isn't
used or assembled in another cluster node before executing it.
--kill-subarray=
If the device is a container and the argument to --kill-subarray specifies an inactive subarray in
the container, then the subarray is deleted. Deleting all subarrays will leave an 'empty-
container' or spare superblock on the drives. See --zero-superblock for completely removing a
superblock. Note that some formats depend on the subarray index for generating a UUID, this
command will fail if it would change the UUID of an active subarray.
--update-subarray=
If the device is a container and the argument to --update-subarray specifies a subarray in the
container, then attempt to update the given superblock field in the subarray. See below in MISC
MODE for details.
-t, --test
When used with --detail, the exit status of mdadm is set to reflect the status of the device. See
below in MISC MODE for details.
-W, --wait
For each md device given, wait for any resync, recovery, or reshape activity to finish before
returning. mdadm will return with success if it actually waited for every device listed,
otherwise it will return failure.
--wait-clean
For each md device given, or each device in /proc/mdstat if --scan is given, arrange for the array
to be marked clean as soon as possible. mdadm will return with success if the array uses external
metadata and we successfully waited. For native arrays, this returns immediately as the kernel
handles dirty-clean transitions at shutdown. No action is taken if safe-mode handling is
disabled.
--action=
Set the "sync_action" for all md devices given to one of idle, frozen, check, repair. Setting to
idle will abort any currently running action though some actions will automatically restart.
Setting to frozen will abort any current action and ensure no other action starts automatically.
Details of check and repair can be found in md(4) under SCRUBBING AND MISMATCHES.
--udev-rules=
it generates the udev rules to the file that handles hot-plug bare devices. Given the POLICYs
defined under /etc/mdadm.conf (or/etc/mdadm/mdadm.conf)
See mdadm.conf(5) for more details and usage examples about POLICY.
For Incremental Assembly mode:
--rebuild-map, -r
Rebuild the map file (/run/mdadm/map) that mdadm uses to help track which arrays are currently
being assembled.
--run, -R
Run any array assembled as soon as a minimal number of devices is available, rather than waiting
until all expected devices are present.
--scan, -s
Only meaningful with -R this will scan the map file for arrays that are being incrementally
assembled and will try to start any that are not already started.
--fail, -f
This allows the hot-plug system to remove devices that have fully disappeared from the kernel. It
will first fail and then remove the device from any array it belongs to. The device name given
should be a kernel device name such as "sda", not a name in /dev.
--path=
Only used with --fail. The 'path' given will be recorded so that if a new device appears at the
same location it can be automatically added to the same array. This allows the failed device to
be automatically replaced by a new device without metadata if it appears at specified path. This
option is normally only set by an udev script.
For Monitor mode:
-m, --mail
Give an mail address to send alerts to. Can be configured in mdadm.conf as MAILADDR.
-p, --program, --alert
Give a program to be run whenever an event is detected. Can be configured in mdadm.conf as
PROGRAM.
-y, --syslog
Cause all events to be reported through 'syslog'. The messages have facility of 'daemon' and
varying priorities.
-d, --delay
Give a delay in seconds. The default is 60 seconds. mdadm polls the md arrays and then waits this
many seconds before polling again if no event happened. Can be configured in mdadm.conf as
MONITORDELAY.
-r, --increment
Give a percentage increment. mdadm will generate RebuildNN events with the given percentage
increment.
-f, --daemonise
Tell mdadm to run as a background daemon if it decides to monitor anything. This causes it to
fork and run in the child, and to disconnect from the terminal. The process id of the child is
written to stdout. This is useful with --scan which will only continue monitoring if a mail
address or alert program is found in the config file.
-i, --pid-file
When mdadm is running in daemon mode, write the pid of the daemon process to the specified file,
instead of printing it on standard output.
-1, --oneshot
Check arrays only once. This will generate NewArray events and more significantly DegradedArray
and SparesMissing events. Running
mdadm --monitor --scan -1
from a cron script will ensure regular notification of any degraded arrays.
-t, --test
Generate a TestMessage alert for every array found at startup. This alert gets mailed and passed
to the alert program. This can be used for testing that alert message do get through
successfully.
--no-sharing
This inhibits the functionality for moving spares between arrays. Only one monitoring process
started with --scan but without this flag is allowed, otherwise the two could interfere with each
other.
ASSEMBLE MODE
Usage: mdadm --assemble md-device options-and-component-devices...
Usage: mdadm --assemble --scan md-devices-and-options...
Usage: mdadm --assemble --scan options...
This usage assembles one or more RAID arrays from pre-existing components. For each array, mdadm needs
to know the md device, the identity of the array, and the number of component devices. These can be
found in a number of ways.
In the first usage example (without the --scan) the first device given is the md device. In the second
usage example, all devices listed are treated as md devices and assembly is attempted. In the third
(where no devices are listed) all md devices that are listed in the configuration file are assembled. If
no arrays are described by the configuration file, then any arrays that can be found on unused devices
will be assembled.
If precisely one device is listed, but --scan is not given, then mdadm acts as though --scan was given
and identity information is extracted from the configuration file.
The identity can be given with the --uuid option, the --name option, or the --super-minor option, will be
taken from the md-device record in the config file, or will be taken from the super block of the first
component-device listed on the command line.
Devices can be given on the --assemble command line or in the config file. Only devices which have an md
superblock which contains the right identity will be considered for any array.
The config file is only used if explicitly named with --config or requested with (a possibly implicit)
--scan. In the latter case, the default config file is used. See mdadm.conf(5) for more details.
If --scan is not given, then the config file will only be used to find the identity of md arrays.
Normally the array will be started after it is assembled. However if --scan is not given and not all
expected drives were listed, then the array is not started (to guard against usage errors). To insist
that the array be started in this case (as may work for RAID1, 4, 5, 6, or 10), give the --run flag.
If udev is active, mdadm does not create any entries in /dev but leaves that to udev. It does record
information in /run/mdadm/map which will allow udev to choose the correct name.
If mdadm detects that udev is not configured, it will create the devices in /dev itself.
Auto-Assembly
When --assemble is used with --scan and no devices are listed, mdadm will first attempt to assemble all
the arrays listed in the config file.
If no arrays are listed in the config (other than those marked <ignore>) it will look through the
available devices for possible arrays and will try to assemble anything that it finds. Arrays which are
tagged as belonging to the given homehost will be assembled and started normally. Arrays which do not
obviously belong to this host are given names that are expected not to conflict with anything local, and
are started "read-auto" so that nothing is written to any device until the array is written to. i.e.
automatic resync etc is delayed.
If mdadm finds a consistent set of devices that look like they should comprise an array, and if the
superblock is tagged as belonging to the given home host, it will automatically choose a device name and
try to assemble the array. If the array uses version-0.90 metadata, then the minor number as recorded in
the superblock is used to create a name in /dev/md/ so for example /dev/md/3. If the array uses
version-1 metadata, then the name from the superblock is used to similarly create a name in /dev/md/ (the
name will have any 'host' prefix stripped first).
This behaviour can be modified by the AUTO line in the mdadm.conf configuration file. This line can
indicate that specific metadata type should, or should not, be automatically assembled. If an array is
found which is not listed in mdadm.conf and has a metadata format that is denied by the AUTO line, then
it will not be assembled. The AUTO line can also request that all arrays identified as being for this
homehost should be assembled regardless of their metadata type. See mdadm.conf(5) for further details.
Note: Auto-assembly cannot be used for assembling and activating some arrays which are undergoing
reshape. In particular as the backup-file cannot be given, any reshape which requires a backup file to
continue cannot be started by auto-assembly. An array which is growing to more devices and has passed
the critical section can be assembled using auto-assembly.
BUILD MODE
Usage: mdadm --build md-device --chunk=X --level=Y --raid-devices=Z devices
This usage is similar to --create. The difference is that it creates an array without a superblock.
With these arrays there is no difference between initially creating the array and subsequently assembling
the array, except that hopefully there is useful data there in the second case.
The level may raid0, linear, raid1, raid10, multipath, or faulty, or one of their synonyms. All devices
must be listed and the array will be started once complete. It will often be appropriate to use
--assume-clean with levels raid1 or raid10.
CREATE MODE
Usage: mdadm --create md-device --chunk=X --level=Y --raid-devices=Z devices
This usage will initialize a new md array, associate some devices with it, and activate the array.
md-device is a new device. This could be standard name or chosen name. For details see: DEVICE NAMES
The named device will normally not exist when mdadm --create is run, but will be created by udev once the
array becomes active.
The max length md-device name is limited to 32 characters. Different metadata types have more strict
limitation (like IMSM where only 16 characters are allowed). For that reason, long name could be
truncated or rejected, it depends on metadata policy.
As devices are added, they are checked to see if they contain RAID superblocks or filesystems. They are
also checked to see if the variance in device size exceeds 1%.
If any discrepancy is found, the array will not automatically be run, though the presence of a --run can
override this caution.
To create a "degraded" array in which some devices are missing, simply give the word "missing" in place
of a device name. This will cause mdadm to leave the corresponding slot in the array empty. For a RAID4
or RAID5 array at most one slot can be "missing"; for a RAID6 array at most two slots. For a RAID1
array, only one real device needs to be given. All of the others can be "missing".
When creating a RAID5 array, mdadm will automatically create a degraded array with an extra spare drive.
This is because building the spare into a degraded array is in general faster than resyncing the parity
on a non-degraded, but not clean, array. This feature can be overridden with the --force option.
When creating a partition based array, using mdadm with version-1.x metadata, the partition type should
be set to 0xDA (non fs-data). This type of selection allows for greater precision since using any other
[RAID auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create problems in the event of array
recovery through a live cdrom.
A new array will normally get a randomly assigned 128bit UUID which is very likely to be unique. If you
have a specific need, you can choose a UUID for the array by giving the --uuid= option. Be warned that
creating two arrays with the same UUID is a recipe for disaster. Also, using --uuid= when creating a
v0.90 array will silently override any --homehost= setting.
Space for a bitmap will be reserved so that one can be added later with --grow --bitmap=internal.
If the metadata type supports it (currently only 1.x and IMSM metadata), space will be allocated to store
a bad block list. This allows a modest number of bad blocks to be recorded, allowing the drive to remain
in service while only partially functional.
When creating an array within a CONTAINER mdadm can be given either the list of devices to use, or simply
the name of the container. The former case gives control over which devices in the container will be
used for the array. The latter case allows mdadm to automatically choose which devices to use based on
how much spare space is available.
The General Management options that are valid with --create are:
--run insist on running the array even if some devices look like they might be in use.
--readonly
start the array in readonly mode.
MANAGE MODE
Usage: mdadm device options... devices...
This usage will allow individual devices in an array to be failed, removed or added. It is possible to
perform multiple operations with on command. For example:
mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
will firstly mark /dev/hda1 as faulty in /dev/md0 and will then remove it from the array and finally add
it back in as a spare. However, only one md array can be affected by a single command.
When a device is added to an active array, mdadm checks to see if it has metadata on it which suggests
that it was recently a member of the array. If it does, it tries to "re-add" the device. If there have
been no changes since the device was removed, or if the array has a write-intent bitmap which has
recorded whatever changes there were, then the device will immediately become a full member of the array
and those differences recorded in the bitmap will be resolved.
MISC MODE
Usage: mdadm options ... devices ...
MISC mode includes a number of distinct operations that operate on distinct devices. The operations are:
--query
The device is examined to see if it is (1) an active md array, or (2) a component of an md array.
The information discovered is reported.
--detail
The device should be an active md device. mdadm will display a detailed description of the array.
--brief or --scan will cause the output to be less detailed and the format to be suitable for
inclusion in mdadm.conf. The exit status of mdadm will normally be 0 unless mdadm failed to get
useful information about the device(s); however, if the --test option is given, then the exit
status will be:
0 The array is functioning normally.
1 The array has at least one failed device.
2 The array has multiple failed devices such that it is unusable.
4 There was an error while trying to get information about the device.
--detail-platform
Print detail of the platform's RAID capabilities (firmware / hardware topology). If the metadata
is specified with -e or --metadata= then the return status will be:
0 metadata successfully enumerated its platform components on this system
1 metadata is platform independent
2 metadata failed to find its platform components on this system
--update-subarray=
If the device is a container and the argument to --update-subarray specifies a subarray in the
container, then attempt to update the given superblock field in the subarray. Similar to updating
an array in "assemble" mode, the field to update is selected by -U or --update= option. The
supported options are name, ppl, no-ppl, bitmap and no-bitmap.
The name option updates the subarray name in the metadata. It must be POSIX PORTABLE NAME
compatible and cannot be longer than 32 chars. If successes, new value will be respected after
next assembly.
The ppl and no-ppl options enable and disable PPL in the metadata. Currently supported only for
IMSM subarrays.
The bitmap and no-bitmap options enable and disable write-intent bitmap in the metadata. Currently
supported only for IMSM subarrays.
--examine
The device should be a component of an md array. mdadm will read the md superblock of the device
and display the contents. If --brief or --scan is given, then multiple devices that are
components of the one array are grouped together and reported in a single entry suitable for
inclusion in mdadm.conf.
Having --scan without listing any devices will cause all devices listed in the config file to be
examined.
--dump=directory
If the device contains RAID metadata, a file will be created in the directory and the metadata
will be written to it. The file will be the same size as the device and will have the metadata
written at the same location as it exists in the device. However, the file will be "sparse" so
that only those blocks containing metadata will be allocated. The total space used will be small.
The filename used in the directory will be the base name of the device. Further, if any links
appear in /dev/disk/by-id which point to the device, then hard links to the file will be created
in directory based on these by-id names.
Multiple devices can be listed and their metadata will all be stored in the one directory.
--restore=directory
This is the reverse of --dump. mdadm will locate a file in the directory that has a name
appropriate for the given device and will restore metadata from it. Names that match
/dev/disk/by-id names are preferred, however if two of those refer to different files, mdadm will
not choose between them but will abort the operation.
If a file name is given instead of a directory then mdadm will restore from that file to a single
device, always provided the size of the file matches that of the device, and the file contains
valid metadata.
--stop The devices should be active md arrays which will be deactivated, as long as they are not
currently in use.
--run This will fully activate a partially assembled md array.
--readonly
This will mark an active array as read-only, providing that it is not currently being used.
--readwrite
This will change a readonly array back to being read/write.
--scan For all operations except --examine, --scan will cause the operation to be applied to all arrays
listed in /proc/mdstat. For --examine, --scan causes all devices listed in the config file to be
examined.
-b, --brief
Be less verbose. This is used with --detail and --examine. Using --brief with --verbose gives an
intermediate level of verbosity.
MONITOR MODE
Usage: mdadm --monitor options... devices...
Monitor option can work in two modes:
• system wide mode, follow all md devices based on /proc/mdstat,
• follow only specified MD devices in command line.
--scan - indicates system wide mode. Option causes the monitor to track all md devices that appear in
/proc/mdstat. If it is not set, then at least one device must be specified.
Monitor usage causes mdadm to periodically poll a number of md arrays and to report on any events
noticed.
In both modes, monitor will work as long as there is an active array with redundancy and it is defined to
follow (for --scan every array is followed).
As well as reporting events, mdadm may move a spare drive from one array to another if they are in the
same spare-group or domain and if the destination array has a failed drive but no spares.
The result of monitoring the arrays is the generation of events. These events are passed to a separate
program (if specified) and may be mailed to a given E-mail address.
When passing events to a program, the program is run once for each event, and is given 2 or 3 command-
line arguments: the first is the name of the event (see below), the second is the name of the md device
which is affected, and the third is the name of a related device if relevant (such as a component device
that has failed).
If --scan is given, then a program or an e-mail address must be specified on the command line or in the
config file. If neither are available, then mdadm will not monitor anything. For devices given directly
in command line, without program or email specified, each event is reported to stdout.
Note: On systems where mdadm monitoring is managed through systemd, the mdmonitor.service should be
present. This service is designed to be the primary solution for array monitoring. It is configured to
operate in system-wide mode. It is initiated by udev when start criteria are met, e.g. mdadm.conf exists
and necessary configuration parameters are set. It is kept alive as long as a redundant RAID array is
active; it stops otherwise. User should customize MAILADDR in mdadm.conf to receive mail notifications.
MONITORDELAY, MAILFROM and PROGRAM are optional. See mdadm.conf(5) for detailed description of these
options. Use systemctl status mdmonitor.service to verify status or determine if additional
configuration is needed.
The different events are:
DeviceDisappeared
An md array which previously was configured appears to no longer be configured. (syslog
priority: Critical)
If mdadm was told to monitor an array which is RAID0 or Linear, then it will report
DeviceDisappeared with the extra information Wrong-Level. This is because RAID0 and Linear do
not support the device-failed, hot-spare and resync operations which are monitored.
RebuildStarted
An md array started reconstruction (e.g. recovery, resync, reshape, check, repair). (syslog
priority: Warning)
RebuildNN
Where NN is a two-digit number (eg. 05, 48). This indicates that the rebuild has reached that
percentage of the total. The events are generated at a fixed increment from 0. The increment
size may be specified with a command-line option (the default is 20). (syslog priority:
Warning)
RebuildFinished
An md array that was rebuilding, isn't any more, either because it finished normally or was
aborted. (syslog priority: Warning)
Fail An active component device of an array has been marked as faulty. (syslog priority: Critical)
FailSpare
A spare component device which was being rebuilt to replace a faulty device has failed.
(syslog priority: Critical)
SpareActive
A spare component device which was being rebuilt to replace a faulty device has been
successfully rebuilt and has been made active. (syslog priority: Info)
NewArray
A new md array has been detected in the /proc/mdstat file. (syslog priority: Info)
DegradedArray
A newly noticed array appears to be degraded. This message is not generated when mdadm
notices a drive failure which causes degradation, but only when mdadm notices that an array is
degraded when it first sees the array. (syslog priority: Critical)
MoveSpare
A spare drive has been moved from one array in a spare-group or domain to another to allow a
failed drive to be replaced. (syslog priority: Info)
SparesMissing
If mdadm has been told, via the config file, that an array should have a certain number of
spare devices, and mdadm detects that it has fewer than this number when it first sees the
array, it will report a SparesMissing message. (syslog priority: Warning)
TestMessage
An array was found at startup, and the --test flag was given. (syslog priority: Info)
Only Fail, FailSpare, DegradedArray, SparesMissing and TestMessage cause Email to be sent. All events
cause the program to be run. The program is run with two or three arguments: the event name, the array
device and possibly a second device.
Each event has an associated array device (e.g. /dev/md1) and possibly a second device. For Fail,
FailSpare, and SpareActive the second device is the relevant component device. For MoveSpare the second
device is the array that the spare was moved from.
For mdadm to move spares from one array to another, the different arrays need to be labeled with the same
spare-group or the spares must be allowed to migrate through matching POLICY domains in the configuration
file. The spare-group name can be any string; it is only necessary that different spare groups use
different names.
When mdadm detects that an array in a spare group has fewer active devices than necessary for the
complete array, and has no spare devices, it will look for another array in the same spare group that has
a full complement of working drives and a spare. It will then attempt to remove the spare from the
second array and add it to the first. If the removal succeeds but the adding fails, then it is added
back to the original array.
If the spare group for a degraded array is not defined, mdadm will look at the rules of spare migration
specified by POLICY lines in mdadm.conf and then follow similar steps as above if a matching spare is
found.
GROW MODE
The GROW mode is used for changing the size or shape of an active array.
The following changes are supported:
• change the "size" attribute for RAID1, RAID4, RAID5 and RAID6.
• increase or decrease the "raid-devices" attribute of RAID0, RAID1, RAID4, RAID5, and RAID6.
• change the chunk-size and layout of RAID0, RAID4, RAID5, RAID6 and RAID10.
• convert between RAID1 and RAID5, between RAID5 and RAID6, between RAID0, RAID4, and RAID5, and
between RAID0 and RAID10 (in the near-2 mode).
• add a write-intent bitmap to any array which supports these bitmaps, or remove a write-intent bitmap
from such an array.
• change the array's consistency policy.
Using GROW on containers is currently supported only for Intel's IMSM container format. The number of
devices in a container can be increased - which affects all arrays in the container - or an array in a
container can be converted between levels where those levels are supported by the container, and the
conversion is on of those listed above.
Notes:
• Intel's native checkpointing doesn't use --backup-file option and it is transparent for assembly
feature.
• Roaming between Windows(R) and Linux systems for IMSM metadata is not supported during grow process.
• When growing a raid0 device, the new component disk size (or external backup size) should be larger
than LCM(old, new) * chunk-size * 2, where LCM() is the least common multiple of the old and new
count of component disks, and "* 2" comes from the fact that mdadm refuses to use more than half of a
spare device for backup space.
SIZE CHANGES
Normally when an array is built the "size" is taken from the smallest of the drives. If all the small
drives in an arrays are, over time, removed and replaced with larger drives, then you could have an array
of large drives with only a small amount used. In this situation, changing the "size" with "GROW" mode
will allow the extra space to start being used. If the size is increased in this way, a "resync" process
will start to make sure the new parts of the array are synchronised.
Note that when an array changes size, any filesystem that may be stored in the array will not
automatically grow or shrink to use or vacate the space. The filesystem will need to be explicitly told
to use the extra space after growing, or to reduce its size prior to shrinking the array.
Also, the size of an array cannot be changed while it has an active bitmap. If an array has a bitmap, it
must be removed before the size can be changed. Once the change is complete a new bitmap can be created.
RAID-DEVICES CHANGES
A RAID1 array can work with any number of devices from 1 upwards (though 1 is not very useful). There
may be times which you want to increase or decrease the number of active devices. Note that this is
different to hot-add or hot-remove which changes the number of inactive devices.
When reducing the number of devices in a RAID1 array, the slots which are to be removed from the array
must already be vacant. That is, the devices which were in those slots must be failed and removed.
When the number of devices is increased, any hot spares that are present will be activated immediately.
Changing the number of active devices in a RAID5 or RAID6 is much more effort. Every block in the array
will need to be read and written back to a new location. Linux Kernel is able to increase or decrease the
number of devices in a RAID5 and RAID6 safely, including restarting an interrupted "reshape".
The Linux Kernel is able to convert a RAID0 into a RAID4 or RAID5. mdadm uses this functionality and the
ability to add devices to a RAID4 to allow devices to be added to a RAID0. When requested to do this,
mdadm will convert the RAID0 to a RAID4, add the necessary disks and make the reshape happen, and then
convert the RAID4 back to RAID0.
When decreasing the number of devices, the size of the array will also decrease. If there was data in
the array, it could get destroyed and this is not reversible, so you should firstly shrink the filesystem
on the array to fit within the new size. To help prevent accidents, mdadm requires that the size of the
array be decreased first with mdadm --grow --array-size. This is a reversible change which simply makes
the end of the array inaccessible. The integrity of any data can then be checked before the non-
reversible reduction in the number of devices is request.
When relocating the first few stripes on a RAID5 or RAID6, it is not possible to keep the data on disk
completely consistent and crash-proof. To provide the required safety, mdadm disables writes to the
array while this "critical section" is reshaped, and takes a backup of the data that is in that section.
For grows, this backup may be stored in any spare devices that the array has, however it can also be
stored in a separate file specified with the --backup-file option, and is required to be specified for
shrinks, RAID level changes and layout changes. If this option is used, and the system does crash during
the critical period, the same file must be passed to --assemble to restore the backup and reassemble the
array. When shrinking rather than growing the array, the reshape is done from the end towards the
beginning, so the "critical section" is at the end of the reshape.
LEVEL CHANGES
Changing the RAID level of any array happens instantaneously. However in the RAID5 to RAID6 case this
requires a non-standard layout of the RAID6 data, and in the RAID6 to RAID5 case that non-standard layout
is required before the change can be accomplished. So while the level change is instant, the
accompanying layout change can take quite a long time. A --backup-file is required. If the array is not
simultaneously being grown or shrunk, so that the array size will remain the same - for example,
reshaping a 3-drive RAID5 into a 4-drive RAID6 - the backup file will be used not just for a "critical
section" but throughout the reshape operation, as described below under LAYOUT CHANGES.
CHUNK-SIZE AND LAYOUT CHANGES
Changing the chunk-size or layout without also changing the number of devices as the same time will
involve re-writing all blocks in-place. To ensure against data loss in the case of a crash, a --backup-
file must be provided for these changes. Small sections of the array will be copied to the backup file
while they are being rearranged. This means that all the data is copied twice, once to the backup and
once to the new layout on the array, so this type of reshape will go very slowly.
If the reshape is interrupted for any reason, this backup file must be made available to mdadm --assemble
so the array can be reassembled. Consequently, the file cannot be stored on the device being reshaped.
BITMAP CHANGES
A write-intent bitmap can be added to, or removed from, an active array.
CONSISTENCY POLICY CHANGES
The consistency policy of an active array can be changed by using the --consistency-policy option in Grow
mode. Currently this works only for the ppl and resync policies and allows one to enable or disable the
RAID5 Partial Parity Log (PPL).
INCREMENTAL MODE
Usage: mdadm --incremental [--run] [--quiet] component-device [optional-aliases-for-device]
Usage: mdadm --incremental --fail component-device
Usage: mdadm --incremental --rebuild-map
Usage: mdadm --incremental --run --scan
This mode is designed to be used in conjunction with a device discovery system. As devices are found in
a system, they can be passed to mdadm --incremental to be conditionally added to an appropriate array.
Conversely, it can also be used with the --fail flag to do just the opposite and find whatever array a
particular device is part of and remove the device from that array.
If the device passed is a CONTAINER device created by a previous call to mdadm, then rather than trying
to add that device to an array, all the arrays described by the metadata of the container will be
started.
mdadm performs a number of tests to determine if the device is part of an array, and which array it
should be part of. If an appropriate array is found, or can be created, mdadm adds the device to the
array and conditionally starts the array.
Note that mdadm will normally only add devices to an array which were previously working (active or
spare) parts of that array. The support for automatic inclusion of a new drive as a spare in some array
requires a configuration through POLICY in config file.
The tests that mdadm makes are as follow:
+ Is the device permitted by mdadm.conf? That is, is it listed in a DEVICES line in that file. If
DEVICES is absent then the default it to allow any device. Similarly if DEVICES contains the
special word partitions then any device is allowed. Otherwise the device name given to mdadm, or
one of the aliases given, or an alias found in the filesystem, must match one of the names or
patterns in a DEVICES line.
This is the only context where the aliases are used. They are usually provided by a udev rules
mentioning $env{DEVLINKS}.
+ Does the device have a valid md superblock? If a specific metadata version is requested with
--metadata or -e then only that style of metadata is accepted, otherwise mdadm finds any known
version of metadata. If no md metadata is found, the device may be still added to an array as a
spare if POLICY allows.
mdadm keeps a list of arrays that it has partially assembled in /run/mdadm/map. If no array exists which
matches the metadata on the new device, mdadm must choose a device name and unit number. It does this
based on any name given in mdadm.conf or any name information stored in the metadata. If this name
suggests a unit number, that number will be used, otherwise a free unit number will be chosen. Normally
mdadm will prefer to create a partitionable array, however if the CREATE line in mdadm.conf suggests that
a non-partitionable array is preferred, that will be honoured.
If the array is not found in the config file and its metadata does not identify it as belonging to the
"homehost", then mdadm will choose a name for the array which is certain not to conflict with any array
which does belong to this host. It does this be adding an underscore and a small number to the name
preferred by the metadata.
Once an appropriate array is found or created and the device is added, mdadm must decide if the array is
ready to be started. It will normally compare the number of available (non-spare) devices to the number
of devices that the metadata suggests need to be active. If there are at least that many, the array will
be started. This means that if any devices are missing the array will not be restarted.
As an alternative, --run may be passed to mdadm in which case the array will be run as soon as there are
enough devices present for the data to be accessible. For a RAID1, that means one device will start the
array. For a clean RAID5, the array will be started as soon as all but one drive is present.
Note that neither of these approaches is really ideal. If it can be known that all device discovery has
completed, then
mdadm -IRs
can be run which will try to start all arrays that are being incrementally assembled. They are started
in "read-auto" mode in which they are read-only until the first write request. This means that no
metadata updates are made and no attempt at resync or recovery happens. Further devices that are found
before the first write can still be added safely.
ENVIRONMENT
This section describes environment variables that affect how mdadm operates.
MDADM_NO_MDMON
Setting this value to 1 will prevent mdadm from automatically launching mdmon. This variable is
intended primarily for debugging mdadm/mdmon.
MDADM_NO_UDEV
Normally, mdadm does not create any device nodes in /dev, but leaves that task to udev. If udev
appears not to be configured, or if this environment variable is set to '1', the mdadm will create
and devices that are needed.
MDADM_NO_SYSTEMCTL
If mdadm detects that systemd is in use it will normally request systemd to start various
background tasks (particularly mdmon) rather than forking and running them in the background.
This can be suppressed by setting MDADM_NO_SYSTEMCTL=1.
IMSM_NO_PLATFORM
A key value of IMSM metadata is that it allows interoperability with boot ROMs on Intel platforms,
and with other major operating systems. Consequently, mdadm will only allow an IMSM array to be
created or modified if detects that it is running on an Intel platform which supports IMSM, and
supports the particular configuration of IMSM that is being requested (some functionality requires
newer OROM support).
These checks can be suppressed by setting IMSM_NO_PLATFORM=1 in the environment. This can be
useful for testing or for disaster recovery. You should be aware that interoperability may be
compromised by setting this value.
These change can also be suppressed by adding mdadm.imsm.test=1 to the kernel command line. This
makes it easy to test IMSM code in a virtual machine that doesn't have IMSM virtual hardware.
MDADM_GROW_ALLOW_OLD
If an array is stopped while it is performing a reshape and that reshape was making use of a
backup file, then when the array is re-assembled mdadm will sometimes complain that the backup
file is too old. If this happens and you are certain it is the right backup file, you can over-
ride this check by setting MDADM_GROW_ALLOW_OLD=1 in the environment.
MDADM_CONF_AUTO
Any string given in this variable is added to the start of the AUTO line in the config file, or
treated as the whole AUTO line if none is given. It can be used to disable certain metadata types
when mdadm is called from a boot script. For example
export MDADM_CONF_AUTO='-ddf -imsm'
will make sure that mdadm does not automatically assemble any DDF or IMSM arrays that are found.
This can be useful on systems configured to manage such arrays with dmraid.
EXAMPLES
mdadm --query /dev/name-of-device
This will find out if a given device is a RAID array, or is part of one, and will provide brief
information about the device.
mdadm --assemble --scan
This will assemble and start all arrays listed in the standard config file. This command will typically
go in a system startup file.
mdadm --stop --scan
This will shut down all arrays that can be shut down (i.e. are not currently in use). This will
typically go in a system shutdown script.
mdadm --follow --scan --delay=120
If (and only if) there is an Email address or program given in the standard config file, then monitor the
status of all arrays listed in that file by polling them ever 2 minutes.
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1
Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and /dev/hdc1.
echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf
mdadm --detail --scan >> mdadm.conf
This will create a prototype config file that describes currently active arrays that are known to be made
from partitions of IDE or SCSI drives. This file should be reviewed before being used as it may contain
unwanted detail.
echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf
mdadm --examine --scan --config=mdadm.conf >> mdadm.conf
This will find arrays which could be assembled from existing IDE and SCSI whole drives (not partitions),
and store the information in the format of a config file. This file is very likely to contain unwanted
detail, particularly the devices= entries. It should be reviewed and edited before being used as an
actual config file.
mdadm --examine --brief --scan --config=partitions
mdadm -Ebsc partitions
Create a list of devices by reading /proc/partitions, scan these for RAID superblocks, and printout a
brief listing of all that were found.
mdadm -Ac partitions -m 0 /dev/md0
Scan all partitions and devices listed in /proc/partitions and assemble /dev/md0 out of all such devices
with a RAID superblock with a minor number of 0.
mdadm --monitor --scan --daemonise > /run/mdadm/mon.pid
If config file contains a mail address or alert program, run mdadm in the background in monitor mode
monitoring all md devices. Also write pid of mdadm daemon to /run/mdadm/mon.pid.
mdadm -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as appropriate.
mdadm --incremental --rebuild-map --run --scan
Rebuild the array map from any current arrays, and then start any that can be started.
mdadm /dev/md4 --fail detached --remove detached
Any devices which are components of /dev/md4 will be marked as faulty and then remove from the array.
mdadm --grow /dev/md4 --level=6 --backup-file=/root/backup-md4
The array /dev/md4 which is currently a RAID5 array will be converted to RAID6. There should normally
already be a spare drive attached to the array as a RAID6 needs one more drive than a matching RAID5.
mdadm --create /dev/md/ddf --metadata=ddf --raid-disks 6 /dev/sd[a-f]
Create a DDF array over 6 devices.
mdadm --create /dev/md/home -n3 -l5 -z 30000000 /dev/md/ddf
Create a RAID5 array over any 3 devices in the given DDF set. Use only 30 gigabytes of each device.
mdadm -A /dev/md/ddf1 /dev/sd[a-f]
Assemble a pre-exist ddf array.
mdadm -I /dev/md/ddf1
Assemble all arrays contained in the ddf array, assigning names as appropriate.
mdadm --create --help
Provide help about the Create mode.
mdadm --config --help
Provide help about the format of the config file.
mdadm --help
Provide general help.
FILES
/proc/mdstat If you're using the /proc filesystem, /proc/mdstat lists all active md devices with information about them. mdadm uses this to find arrays when --scan is given in Misc mode, and to monitor array reconstruction on Monitor mode. /etc/mdadm.conf (or /etc/mdadm/mdadm.conf) Default config file. See mdadm.conf(5) for more details. /etc/mdadm.conf.d (or /etc/mdadm/mdadm.conf.d) Default directory containing configuration files. See mdadm.conf(5) for more details. /run/mdadm/map When --incremental mode is used, this file gets a list of arrays currently being created.
POSIX PORTABLE NAME
A valid name can only consist of characters "A-Za-z0-9.-_". The name cannot start with a leading "-" and
cannot exceed 255 chars.
DEVICE NAMES
mdadm understand two sorts of names for array devices.
The first is the so-called 'standard' format name, which matches the names used by the kernel and which
appear in /proc/mdstat.
The second sort can be freely chosen, but must reside in /dev/md/. When giving a device name to mdadm to
create or assemble an array, either full path name such as /dev/md0 or /dev/md/home can be given, or just
the suffix of the second sort of name, such as home can be given.
In every style, raw name must be compatible with POSIX PORTABLE NAME and has to be no longer than 32
chars.
When mdadm chooses device names during auto-assembly or incremental assembly, it will sometimes add a
small sequence number to the end of the name to avoid conflicted between multiple arrays that have the
same name. If mdadm can reasonably determine that the array really is meant for this host, either by a
hostname in the metadata, or by the presence of the array in mdadm.conf, then it will leave off the
suffix if possible. Also if the homehost is specified as <ignore> mdadm will only use a suffix if a
different array of the same name already exists or is listed in the config file.
The names for arrays are of the form:
/dev/mdNN
where NN is a number.
Names can be non-numeric following the form:
/dev/md_XXX
where XXX is any string. These names are supported by mdadm since version 3.3 provided they are enabled
in mdadm.conf.
UNDERSTANDING OUTPUT
EXAMINE
checkpoint
Checkpoint value is reported when array is performing some action including resync, recovery or
reshape. Checkpoints allow resuming action from certain point if it was interrupted.
Checkpoint is reported as combination of two values: current migration unit and number of blocks
per unit. By multiplying those values and dividing by array size checkpoint progress percentage
can be obtained in relation to current progress reported in /proc/mdstat. Checkpoint is also
related to (and sometimes based on) sysfs entry sync_completed but depending on action units may
differ. Even if units are the same, it should not be expected that checkpoint and sync_completed
will be exact match nor updated simultaneously.
NOTE
mdadm was previously known as mdctl.
SEE ALSO
For further information on mdadm usage, MD and the various levels of RAID, see:
https://raid.wiki.kernel.org/
(based upon Jakob Østergaard's Software-RAID.HOWTO)
The latest version of mdadm should always be available from
https://www.kernel.org/pub/linux/utils/raid/mdadm/
Related man pages:
mdmon(8), mdadm.conf(5), md(4).
v4.4 MDADM(8)