Mdadm/man
MDADM(8) System Manager's Manual MDADM(8)
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 lev‐
els 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 con‐
tainer.
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 su‐
perblock 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 differ‐
entiate between initial creation and subsequent assembly of an array. It also cannot perform any checks that appro‐
priate 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 con‐
sistent (e.g. both sides of a mirror contain the same data) but the content of the device is left otherwise un‐
touched. 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 mul‐
tipath 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 chang‐
ing 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 pre‐
ferred.
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 impor‐
tant 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 con‐
tained 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 config‐
uration 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 compo‐
nent 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 simi‐
lar 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 con‐
sidered the home for any arrays.
When creating an array, the homehost will be recorded in the metadata. For version-1 superblocks, it will be pre‐
fixed 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 com‐
ponent. 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 al‐
lowed 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 de‐
vice 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 de‐
fault 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 filesys‐
tem, 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 ex‐
plicitly 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-symmet‐
ric, 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-per‐
sistent, 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 lay‐
out. 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 cor‐
rupt 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 par‐
allel 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 af‐
ter 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 in‐
terrupted 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= op‐
tion 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 ef‐
fective 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 ques‐
tion to be suppressed.
-f, --force
Insist that mdadm accept the geometry and layout specified without question. Normally mdadm will not allow the cre‐
ation 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 sim‐
ply 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 shut‐
down.
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 assem‐
bled 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 be‐
ing 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 work‐
ing 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. Nor‐
mally 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 sum‐
maries, 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 ar‐
ray 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 automati‐
cally 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 re‐
versed, and will correct that order before assembling the array. This is only valid with original (Version 0.90) su‐
perblocks.
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 us‐
able 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 ig‐
nored. 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 con‐
tains 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 ker‐
nel, --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 re‐
stored. This happens before the file system pivot operation and avoids loss of filesystem context. Losing file sys‐
tem 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 re‐
moved) the device is re-added as described in the next point. If that fails or the device was never part of the ar‐
ray, 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 re‐
covery 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 re‐
covery 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 acknowl‐
edge 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 persis‐
tent 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 re‐
move 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/de‐
vices/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 rea‐
son (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 cur‐
rently 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 dri‐
ves. See --zero-superblock for completely removing a superblock. Note that some formats depend on the subarray in‐
dex 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 at‐
tempt 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 be‐
fore 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 pro‐
gram. 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 de‐
vice, 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 de‐
vices 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 informa‐
tion 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 lat‐
ter 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 pos‐
sible 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 ac‐
tive.
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 cau‐
tion.
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= set‐
ting.
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 func‐
tional.
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 op‐
erations 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 re‐
moved, or if the array has a write-intent bitmap which has recorded whatever changes there were, then the device will imme‐
diately 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 at‐
tempt 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 to‐
gether 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 pro‐
vided 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/md‐
stat. 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 speci‐
fied) 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 nei‐
ther 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 re‐
ceive 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 pri‐
ority: 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: Criti‐
cal)
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 SpareAc‐
tive 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 con‐
tainer 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 re‐
duce 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 va‐
cant. 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 de‐
vices 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 re‐
shaped, 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 re‐
quired 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 sec‐
tion" 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-stan‐
dard 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 ar‐
ray. 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 filesys‐
tem, 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{DE‐
VLINKS}.
+ 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 meta‐
data 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 ar‐
ray 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 (particu‐
larly mdmon) rather than forking and running them in the background. This can be suppressed by setting MDADM_NO_SYS‐
TEMCTL=1.
IMSM_NO_PLATFORM
A key value of IMSM metadata is that it allows interoperability with boot ROMs on Intel platforms, and with other ma‐
jor 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 be‐
ing 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 environ‐
ment.
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 de‐
vice.
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 ar‐
rays 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 in‐
formation 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 su‐
perblock 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 de‐
vices. 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/md‐
stat.
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 deter‐
mine 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 multi‐
plying 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)