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 de-
vices. This allows multiple devices (typically disk drives or parti-
tions thereof) to be combined into a single device to hold (for example)
a single filesystem. Some RAID levels include redundancy and so can
survive some degree of device failure.
Linux Software RAID devices are implemented through the md (Multiple De-
vices) device driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1
(mirroring), RAID4, RAID5, RAID6, RAID10, MULTIPATH, FAULTY, and CON-
TAINER.
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 sup-
ported and has no ongoing development. Use the Device Mapper based mul-
tipath-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 pro-
vides 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 con-
nected 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 de-
vices in a 5-device set might form a RAID1 using the whole devices. The
remaining three might have a RAID5 over the first half of each device,
and a RAID0 over the second half.
With a CONTAINER, there is one set of metadata that describes all of the
arrays in the container. So when mdadm creates a CONTAINER device, the
device just represents the metadata. Other normal arrays (RAID1 etc)
can be created inside the container.
MODES
mdadm has several major modes of operation:
Assemble
Assemble the components of a previously created array into an ac-
tive array. Components can be explicitly given or can be
searched for. mdadm checks that the components do form a bona
fide array, and can, on request, fiddle superblock information so
as to assemble a faulty array.
Build Build an array that doesn't have per-device metadata (su-
perblocks). For these sorts of arrays, mdadm cannot differenti-
ate between initial creation and subsequent assembly of an array.
It also cannot perform any checks that appropriate components
have been requested. Because of this, the Build mode should only
be used together with a complete understanding of what you are
doing.
Create Create a new array with per-device metadata (superblocks). Ap-
propriate metadata is written to each device, and then the array
comprising those devices is activated. A 'resync' process is
started to make sure that the array is consistent (e.g. both
sides of a mirror contain the same data) but the content of the
device is left otherwise untouched. The array can be used as
soon as it has been created. There is no need to wait for the
initial resync to finish.
Follow or Monitor
Monitor one or more md devices and act on any state changes.
This is only meaningful for RAID1, 4, 5, 6, 10 or multipath ar-
rays, as only these have interesting state. RAID0 or Linear
never have missing, spare, or failed drives, so there is nothing
to monitor.
Grow Grow (or shrink) an array, or otherwise reshape it in some way.
Currently supported growth options including changing the active
size of component devices and changing the number of active de-
vices 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 consis-
tency 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 ac-
tive 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 ar-
rays.
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 pos-
sibly 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 rec-
ommended for new installations. Using mdadm to detect and assem-
ble arrays — possibly in an initrd — is substantially more flexi-
ble and should be preferred.
If a device is given before any options, or if the first option is one
of --add, --re-add, --add-spare, --fail, --remove, or --replace, then
the MANAGE mode is assumed. Anything other than these will cause the
Misc mode to be assumed.
Options that are not mode-specific are:
-h, --help
Display a general help message or, after one of the above op-
tions, a mode-specific help message.
--help-options
Display more detailed help about command-line parsing and some
commonly used options.
-V, --version
Print version information for mdadm.
-v, --verbose
Be more verbose about what is happening. This can be used twice
to be extra-verbose. The extra verbosity currently only affects
--detail --scan and --examine --scan.
-q, --quiet
Avoid printing purely informative messages. With this, mdadm
will be silent unless there is something really important to re-
port.
-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 de-
fault config file and default conf.d directory will be used. See
mdadm.conf(5) for more details.
If the config file given is partitions then nothing will be read,
but mdadm will act as though the config file contained exactly
DEVICE partitions containers
and will read /proc/partitions to find a list of devices to scan,
and /proc/mdstat to find a list of containers to examine. If the
word none is given for the config file, then mdadm will act as
though the config file were empty.
If the name given is of a directory, then mdadm will collect all
the files contained in the directory with a name ending in .conf,
sort them lexically, and process all of those files as config
files.
-s, --scan
Scan config file or /proc/mdstat for missing information. In
general, this option gives mdadm permission to get any missing
information (like component devices, array devices, array identi-
ties, and alert destination) from the configuration file (see
previous option); one exception is MISC mode when using --detail
or --stop, in which case --scan says to get a list of array de-
vices 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 lim-
its arrays to 28 component devices and limits component
devices of levels 1 and greater to 2 terabytes. It is
also possible for there to be confusion about whether the
superblock applies to a whole device or just the last par-
tition, 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 CON-
TAINER will be created, and normal arrays can be created
in that container.
imsm Use the Intel(R) Matrix Storage Manager metadata format.
This creates a CONTAINER which is managed in a similar
manner to DDF, and is supported by an option-rom on some
platforms:
https://www.intel.com/content/www/us/en/support/prod-
ucts/122484
--homehost=
This will override any HOMEHOST setting in the config file and
provides the identity of the host which should be considered the
home for any arrays.
When creating an array, the homehost will be recorded in the
metadata. For version-1 superblocks, it will be prefixed to the
array name. For version-0.90 superblocks, part of the SHA1 hash
of the hostname will be stored in the latter half of the UUID.
When reporting information about an array, any array which is
tagged for the given homehost will be reported as such.
When using Auto-Assemble, only arrays tagged for the given home-
host will be allowed to use 'local' names (i.e. not ending in '_'
followed by a digit string). See below under Auto-Assembly.
The special name "any" can be used as a wild card. If an array
is created with --homehost=any then the name "any" will be stored
in the array and it can be assembled in the same way on any host.
If an array is assembled with this option, then the homehost
recorded on the array will be ignored.
--prefer=
When mdadm needs to print the name for a device it normally finds
the name in /dev which refers to the device and is the shortest.
When a path component is given with --prefer mdadm will prefer a
longer name if it contains that component. For example --pre-
fer=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 speci-
fied. 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 proba-
bly a mistake and so requires that --force be specified first. A
value of 1 will then be allowed for linear, multipath, RAID0 and
RAID1. It is never allowed for RAID4, RAID5 or RAID6.
This number can only be changed using --grow for RAID1, RAID4,
RAID5 and RAID6 arrays.
-x, --spare-devices=
Specify the number of spare (eXtra) devices in the initial array.
Spares can also be added and removed later. The number of compo-
nent 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 spec-
ified as ¸max¸ (as it often is) the smallest drive (or partition)
sets the size. In that case, a warning will follow if the dri-
ves, 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 Kilo-
bytes, 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 stan-
dards. 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 vol-
ume, but not on a container itself. Setting the initial size of
RAID 0 array is only valid for external metadata.
This value can be set with --grow for RAID level 1/4/5/6/10
though DDF arrays may not be able to support this. RAID 0 array
size cannot be changed. If the array was created with a size
smaller than the currently active drives, the extra space can be
accessed using --grow. The size can be given as max which means
to choose the largest size that fits on all current drives.
Before reducing the size of the array (with --grow --size=) you
should make sure that space isn't needed. If the device holds a
filesystem, you would need to resize the filesystem to use less
space.
After reducing the array size you should check that the data
stored in the device is still available. If the device holds a
filesystem, then an 'fsck' of the filesystem is a minimum re-
quirement. 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 persis-
tent: when the array is stopped and restarted the default array
size will be restored.
Setting the array-size causes the array to appear smaller to pro-
grams that access the data. This is particularly needed before
reshaping an array so that it will be smaller. As the reshape is
not reversible, but setting the size with --array-size is, it is
required that the array size is reduced as appropriate before the
number of devices in the array is reduced.
Before reducing the size of the array you should make sure that
space isn't needed. If the device holds a filesystem, you would
need to resize the filesystem to use less space.
After reducing the array size you should check that the data
stored in the device is still available. If the device holds a
filesystem, then an 'fsck' of the filesystem is a minimum re-
quirement. 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 Kilo-
bytes, 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 Kilo-
bytes, Megabytes, Gigabytes or Terabytes respectively.
--rounding=
Specify the rounding factor for a Linear array. The size of each
component will be rounded down to a multiple of this size. This
is a synonym for --chunk but highlights the different meaning for
Linear as compared to other RAID levels. The default is 0K (i.e.
no rounding).
-l, --level=
Set RAID level. When used with --create, options are: linear,
raid0, 0, stripe, raid1, 1, mirror, raid4, 4, raid5, 5, raid6, 6,
raid10, 10, multipath, mp, faulty, container. Obviously some of
these are synonymous.
When a CONTAINER metadata type is requested, only the container
level is permitted, and it does not need to be explicitly given.
When used with --build, only linear, stripe, raid0, 0, raid1,
multipath, mp, and faulty are valid.
Can be used with --grow to change the RAID level in some cases.
See LEVEL CHANGES below.
-p, --layout=
This option configures the fine details of data layout for RAID5,
RAID6, and RAID10 arrays, and controls the failure modes for
faulty. It can also be used for working around a kernel bug with
RAID0, but generally doesn't need to be used explicitly.
The layout of the RAID5 parity block can be one of left-asymmet-
ric, left-symmetric, right-asymmetric, right-symmetric, la, ra,
ls, rs. The default is left-symmetric.
It is also possible to cause RAID5 to use a RAID4-like layout by
choosing parity-first, or parity-last.
Finally for RAID5 there are DDF-compatible layouts,
ddf-zero-restart, ddf-N-restart, and ddf-N-continue.
These same layouts are available for RAID6. There are also 4
layouts that will provide an intermediate stage for converting
between RAID5 and RAID6. These provide a layout which is identi-
cal to the corresponding RAID5 layout on the first N-1 devices,
and has the 'Q' syndrome (the second 'parity' block used by
RAID6) on the last device. These layouts are: left-symmetric-6,
right-symmetric-6, left-asymmetric-6, right-asymmetric-6, and
parity-first-6.
When setting the failure mode for level faulty, the options are:
write-transient, wt, read-transient, rt, write-persistent, wp,
read-persistent, rp, write-all, read-fixable, rf, clear, flush,
none.
Each failure mode can be followed by a number, which is used as a
period between fault generation. Without a number, the fault is
generated once on the first relevant request. With a number, the
fault will be generated after that many requests, and will con-
tinue 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 data-
block. 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 dupli-
cated within a stripe, whole stripes are duplicated but are ro-
tated 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 de-
vices of differing sizes started using a different layout. This
could lead to data corruption. Since Linux 5.4 (and various sta-
ble 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 nor-
mally need to be set. An array created for either 'original' or
'alternate' will not be recognized by an (unpatched) kernel prior
to 5.4. To create a RAID0 array with devices of differing sizes
that can be used on an older kernel, you can set the layout to
'dangerous'. This will use whichever layout the running kernel
supports, so the data on the array may become corrupt when chang-
ing kernel from pre-3.14 to a later kernel.
When an array is converted between RAID5 and RAID6 an intermedi-
ate 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 parame-
ter 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 Kilo-
bytes, Megabytes, Gigabytes or Terabytes respectively.
-W, --write-mostly
subsequent devices listed in a --build, --create, or --add com-
mand 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 num-
ber 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 at-
tempt 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 actu-
ally 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 min-
utes for large disks. Thus a message is printed before and after
zeroing and each disk is zeroed in parallel with the others.
This is only meaningful with --create.
--backup-file=
This is needed when --grow is used to increase the number of raid
devices in a RAID5 or RAID6 if there are no spare devices avail-
able, 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 re-
shaped.
--data-offset=
Arrays with 1.x metadata can leave a gap between the start of the
device and the start of array data. This gap can be used for
various metadata. The start of data is known as the data-offset.
Normally an appropriate data offset is computed automatically.
However it can be useful to set it explicitly such as when re-
creating an array which was originally created using a different
version of mdadm which computed a different offset.
Setting the offset explicitly over-rides the default. The value
given is in Kilobytes unless a suffix of 'K', 'M', 'G' or 'T' is
used to explicitly indicate Kilobytes, Megabytes, Gigabytes or
Terabytes respectively.
--data-offset can also be used with --grow for some RAID levels
(initially on RAID10). This allows the data-offset to be changed
as part of the reshape process. When the data offset is changed,
no backup file is required as the difference in offsets is used
to provide the same functionality.
When the new offset is earlier than the old offset, the number of
devices in the array cannot shrink. When it is after the old
offset, the number of devices in the array cannot increase.
When creating an array, --data-offset can be specified as vari-
able. In the case each member device is expected to have an off-
set appended to the name, separated by a colon. This makes it
possible to recreate exactly an array which has varying data off-
sets (as can happen when different versions of mdadm are used to
add different devices).
--continue
This option is complementary to the --freeze-reshape option for
assembly. It is needed when --grow operation is interrupted and
it is not restarted automatically due to --freeze-reshape usage
during array assembly. This option is used together with -G , (
--grow ) command and device for a pending reshape to be contin-
ued. All parameters required for reshape continuation will be
read from array metadata. If initial --grow command had required
--backup-file= option to be set, continuation option will require
to have exactly the same backup file given as well.
Any other parameter passed together with --continue option will
be ignored.
-N, --name=
Set a name for the array. It must be POSIX PORTABLE NAME compati-
ble and cannot be longer than 32 chars. This is effective when
creating an array with a v1 metadata, or an external array.
If name is needed but not specified, it is taken from the base-
name of the device that is being created. See DEVICE NAMES
-R, --run
Insist that mdadm run the array, even if some of the components
appear to be active in another array or filesystem. Normally
mdadm will ask for confirmation before including such components
in an array. This option causes that question to be suppressed.
-f, --force
Insist that mdadm accept the geometry and layout specified with-
out question. Normally mdadm will not allow the creation of an
array with only one device, and will try to create a RAID5 array
with one missing drive (as this makes the initial resync work
faster). With --force, mdadm will not try to be so clever.
-o, --readonly
Start the array read only rather than read-write as normal. No
writes will be allowed to the array, and no resync, recovery, or
reshape will be started. It works with Create, Assemble, Manage
and Misc mode.
-a, --add
This option can be used in Grow mode in two cases.
If the target array is a Linear array, then --add can be used to
add one or more devices to the array. They are simply catenated
on to the end of the array. Once added, the devices cannot be
removed.
If the --raid-disks option is being used to increase the number
of devices in an array, then --add can be used to add some extra
devices to be included in the array. In most cases this is not
needed as the extra devices can be added as spares first, and
then the number of raid disks can be changed. However, for RAID0
it is not possible to add spares. So to increase the number of
devices in a RAID0, it is necessary to set the new number of de-
vices, 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 de-
vice should be an SSD with a reasonable lifetime.
-k, --consistency-policy=
Specify how the array maintains consistency in the case of an un-
expected shutdown. Only relevant for RAID levels with redun-
dancy. Currently supported options are:
resync Full resync is performed and all redundancy is regenerated
when the array is started after an unclean shutdown.
bitmap Resync assisted by a write-intent bitmap. Implicitly se-
lected when using --bitmap.
journal
For RAID levels 4/5/6, the journal device is used to log
transactions and replay after an unclean shutdown. Implic-
itly 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 jour-
nal 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 ar-
ray as /dev/md1, then all superblocks will contain the minor num-
ber 1, even if the array is later assembled as /dev/md2.
Giving the literal word "dev" for --super-minor will cause mdadm
to use the minor number of the md device that is being assembled.
e.g. when assembling /dev/md0, --super-minor=dev will look for
super blocks with a minor number of 0.
--super-minor is only relevant for v0.90 metadata, and should not
normally be used. Using --uuid is much safer.
-N, --name=
Specify the name of the array to assemble. It must be POSIX
PORTABLE NAME compatible and cannot be longer than 32 chars. This
must be the name that was specified when creating the array. It
must either match the name stored in the superblock exactly, or
it must match with the current homehost prefixed to the start of
the given name.
-f, --force
Assemble the array even if the metadata on some devices appears
to be out-of-date. If mdadm cannot find enough working devices
to start the array, but can find some devices that are recorded
as having failed, then it will mark those devices as working so
that the array can be started. This works only for native. For
external metadata it allows one to start dirty degraded RAID 4,
5, 6. An array which requires --force to be started may contain
data corruption. Use it carefully.
-R, --run
Attempt to start the array even if fewer drives were given than
were present last time the array was active. Normally if not all
the expected drives are found and --scan is not used, then the
array will be assembled but not started. With --run an attempt
will be made to start it anyway.
--no-degraded
This is the reverse of --run in that it inhibits the startup of
array unless all expected drives are present. This is only
needed with --scan, and can be used if the physical connections
to devices are not as reliable as you would like.
--backup-file=
If --backup-file was used while reshaping an array (e.g. changing
number of devices or chunk size) and the system crashed during
the critical section, then the same --backup-file must be pre-
sented to --assemble to allow possibly corrupted data to be re-
stored, 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 ir-
recoverably lost, but the rest of the array may still be recover-
able. This option should only be used as a last resort if there
is no way to recover the backup file.
-U, --update=
Update the superblock on each device while assembling the array.
The argument given to this flag can be one of summaries, uuid,
name, nodes, homehost, home-cluster, resync, byteorder, device-
size, no-bitmap, bbl, no-bbl, ppl, no-ppl, layout-original, lay-
out-alternate, layout-unspecified, metadata, or super-minor.
The super-minor option will update the preferred minor field on
each superblock to match the minor number of the array being as-
sembled. This can be useful if --examine reports a different
"Preferred Minor" to --detail. In some cases this update will be
performed automatically by the kernel driver. In particular, the
update happens automatically at the first write to an array with
redundancy (RAID level 1 or greater).
The uuid option will change the uuid of the array. If a UUID is
given with the --uuid option that UUID will be used as a new UUID
and will NOT be used to help identify the devices in the array.
If no --uuid is given, a random UUID is chosen.
The name option will change the name of the array as stored in
the superblock. This is only supported for version-1 su-
perblocks.
The nodes option will change the nodes of the array as stored in
the bitmap superblock. This option only works for a clustered en-
vironment.
The homehost option will change the homehost as recorded in the
superblock. For version-0 superblocks, this is the same as up-
dating the UUID. For version-1 superblocks, this involves updat-
ing the name.
The home-cluster option will change the cluster name as recorded
in the superblock and bitmap. This option only works for a clus-
tered 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 informa-
tion is correct.
The byteorder option allows arrays to be moved between machines
with different byte-order, such as from a big-endian machine like
a Sparc or some MIPS machines, to a little-endian x86_64 machine.
When assembling such an array for the first time after a move,
giving --update=byteorder will cause mdadm to expect superblocks
to have their byteorder reversed, and will correct that order be-
fore assembling the array. This is only valid with original
(Version 0.90) superblocks.
The summaries option will correct the summaries in the su-
perblock. That is the counts of total, working, active, failed,
and spare devices.
The devicesize option will rarely be of use. It applies to ver-
sion 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 meta-
data records the amount of the device that can be used to store
data, so if a device in a version 1.1 or 1.2 array becomes
larger, the metadata will still be visible, but the extra space
will not. In this case it might be useful to assemble the array
with --update=devicesize. This will cause mdadm to determine the
maximum usable amount of space on each device and update the rel-
evant field in the metadata.
The metadata option only works on v0.90 metadata arrays and will
convert them to v1.0 metadata. The array must not be dirty (i.e.
it must not need a sync) and it must not have a write-intent
bitmap.
The old metadata will remain on the devices, but will appear
older than the new metadata and so will usually be ignored. The
old metadata (or indeed the new metadata) can be removed by giv-
ing the appropriate --metadata= option to --zero-superblock.
The no-bitmap option can be used when an array has an internal
bitmap which is corrupt in some way so that assembling the array
normally fails. It will cause any internal bitmap to be ignored.
The bbl option will reserve space in each device for a bad block
list. This will be 4K in size and positioned near the end of any
free space between the superblock and the data.
The no-bbl option will cause any reservation of space for a bad
block list to be removed. If the bad block list contains en-
tries, this will fail, as removing the list could cause data cor-
ruption.
The ppl option will enable PPL for a RAID5 array and reserve
space for PPL on each device. There must be enough free space be-
tween the data and superblock and a write-intent bitmap or jour-
nal 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 ear-
lier, then to assemble the array on a new kernel, --update=lay-
out-original must be given. If the array was created and used
with a kernel from Linux 3.14 to Linux 5.3, then --update=lay-
out-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-orig-
nal or layout-alternate and allows the array to be again used on
a kernel prior to Linux 5.3. This option should be used with
great caution.
--freeze-reshape
This option is intended to be used in start-up scripts during the
initrd boot phase. When the array under reshape is assembled
during the initrd phase, this option stops the reshape after the
reshape-critical section has been restored. This happens before
the file system pivot operation and avoids loss of filesystem
context. Losing file system context would cause reshape to be
broken.
Reshape can be continued later using the --continue option for
the grow command.
For Manage mode:
-t, --test
Unless a more serious error occurred, mdadm will exit with a sta-
tus 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 speci-
fier such as missing, detached or faulty is used in requesting an
operation on the array. --test will report failure if these
specifiers didn't find any match.
-a, --add
hot-add listed devices. If a device appears to have recently
been part of the array (possibly it failed or was removed) the
device is re-added as described in the next point. If that fails
or the device was never part of the array, the device is added as
a hot-spare. If the array is degraded, it will immediately start
to rebuild data onto that spare.
Note that this and the following options are only meaningful on
array with redundancy. They don't apply to RAID0 or Linear.
--re-add
re-add a device that was previously removed from an array. If
the metadata on the device reports that it is a member of the ar-
ray, 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. How-
ever, based on the event count on the device, the recovery may
only require sections that are flagged by a write-intent bitmap
to be recovered or may not require any recovery at all.
When used on an array that has no metadata (i.e. it was built
with --build) it will be assumed that bitmap-based recovery is
enough to make the device fully consistent with the array.
--re-add can also be accompanied by --update=devicesize, --up-
date=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 de-
vices in the array that are marked faulty, remove them and at-
tempt 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 de-
vice. This is similar to marking a device as faulty, but the de-
vice remains in service during the recovery process to increase
resilience against multiple failures. When the replacement
process finishes, the replaced device will be marked as faulty.
--with This can follow a list of --replace devices. The devices listed
after --with will preferentially be used to replace the devices
listed after --replace. These devices must already be spare de-
vices 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 re-
ceiving node must acknowledge this message with --cluster-con-
firm. 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 interrupt-
ing 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 han-
dling where possible.
--nofailfast
Subsequent devices that are re-added will be re-added without the
'failfast' flag set.
Each of these options requires that the first device listed is the array
to be acted upon, and the remainder are component devices to be added,
removed, marked as faulty, etc. Several different operations can be
specified for different devices, e.g.
mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1
Each operation applies to all devices listed until the next operation.
If an array is using a write-intent bitmap, then devices which have been
removed can be re-added in a way that avoids a full reconstruction but
instead just updates the blocks that have changed since the device was
removed. For arrays with persistent metadata (superblocks) this is done
automatically. For arrays created with --build mdadm needs to be told
that this device we removed recently with --re-add.
Devices can only be removed from an array if they are not in active use,
i.e. that must be spares or failed devices. To remove an active device,
it must first be marked as faulty.
For Misc mode:
-Q, --query
Examine a device to see (1) if it is an md device and (2) if it
is a component of an md array. Information about what is discov-
ered is presented.
-D, --detail
Print details of one or more md devices.
--detail-platform
Print details of the platform's RAID capabilities (firmware /
hardware topology) for a given metadata format. If used without
an argument, mdadm will scan all controllers looking for their
capabilities. Otherwise, mdadm will only look at the controller
specified by the argument in the form of an absolute filepath or
a link, e.g. /sys/devices/pci0000:00/0000:00:1f.2.
-Y, --export
When used with --detail, --detail-platform, --examine, or --in-
cremental output will be formatted as key=value pairs for easy
import into the environment.
With --incremental The value MD_STARTED indicates whether an ar-
ray was started (yes) or not, which may include a reason (unsafe,
nothing, no). Also the value MD_FOREIGN indicates if the array
is expected on this host (no), or seems to be from elsewhere
(yes).
-E, --examine
Print contents of the metadata stored on the named device(s).
Note the contrast between --examine and --detail. --examine ap-
plies to devices which are components of an array, while --detail
applies to a whole array which is currently active.
-X, --examine-bitmap
Report information about a bitmap. The argument is an array com-
ponent. 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 par-
tially 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 over-
written 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 subar-
ray is deleted. Deleting all subarrays will leave an 'empty-con-
tainer' or spare superblock on the drives. See --zero-superblock
for completely removing a superblock. Note that some formats de-
pend on the subarray index for generating a UUID, this command
will fail if it would change the UUID of an active subarray.
--update-subarray=
If the device is a container and the argument to --update-subar-
ray specifies a subarray in the container, then attempt to update
the given superblock field in the subarray. See below in MISC
MODE for details.
-t, --test
When used with --detail, the exit status of mdadm is set to re-
flect the status of the device. See below in MISC MODE for de-
tails.
-W, --wait
For each md device given, wait for any resync, recovery, or re-
shape activity to finish before returning. mdadm will return
with success if it actually waited for every device listed, oth-
erwise 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 tran-
sitions 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 POL-
ICY.
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 automati-
cally added to the same array. This allows the failed device to
be automatically replaced by a new device without metadata if it
appears at specified path. This option is normally only set by
an udev script.
For Monitor mode:
-m, --mail
Give an mail address to send alerts to. Can be configured in
mdadm.conf as MAILADDR.
-p, --program, --alert
Give a program to be run whenever an event is detected. Can be
configured in mdadm.conf as PROGRAM.
-y, --syslog
Cause all events to be reported through 'syslog'. The messages
have facility of 'daemon' and varying priorities.
-d, --delay
Give a delay in seconds. The default is 60 seconds. mdadm polls
the md arrays and then waits this many seconds before polling
again if no event happened. Can be configured in mdadm.conf as
MONITORDELAY.
-r, --increment
Give a percentage increment. mdadm will generate RebuildNN
events with the given percentage increment.
-f, --daemonise
Tell mdadm to run as a background daemon if it decides to monitor
anything. This causes it to fork and run in the child, and to
disconnect from the terminal. The process id of the child is
written to stdout. This is useful with --scan which will only
continue monitoring if a mail address or alert program is found
in the config file.
-i, --pid-file
When mdadm is running in daemon mode, write the pid of the daemon
process to the specified file, instead of printing it on standard
output.
-1, --oneshot
Check arrays only once. This will generate NewArray events and
more significantly DegradedArray and SparesMissing events. Run-
ning
mdadm --monitor --scan -1
from a cron script will ensure regular notification of any de-
graded arrays.
-t, --test
Generate a TestMessage alert for every array found at startup.
This alert gets mailed and passed to the alert program. This can
be used for testing that alert message do get through success-
fully.
--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 compo-
nents. For each array, mdadm needs to know the md device, the identity
of the array, and the number of component devices. These can be found
in a number of ways.
In the first usage example (without the --scan) the first device given
is the md device. In the second usage example, all devices listed are
treated as md devices and assembly is attempted. In the third (where no
devices are listed) all md devices that are listed in the configuration
file are assembled. If no arrays are described by the configuration
file, then any arrays that can be found on unused devices will be assem-
bled.
If precisely one device is listed, but --scan is not given, then mdadm
acts as though --scan was given and identity information is extracted
from the configuration file.
The identity can be given with the --uuid option, the --name option, or
the --super-minor option, will be taken from the md-device record in the
config file, or will be taken from the super block of the first compo-
nent-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 re-
quested with (a possibly implicit) --scan. In the latter case, the de-
fault config file is used. See mdadm.conf(5) for more details.
If --scan is not given, then the config file will only be used to find
the identity of md arrays.
Normally the array will be started after it is assembled. However if
--scan is not given and not all expected drives were listed, then the
array is not started (to guard against usage errors). To insist that
the array be started in this case (as may work for RAID1, 4, 5, 6, or
10), give the --run flag.
If udev is active, mdadm does not create any entries in /dev but leaves
that to udev. It does record information in /run/mdadm/map which will
allow udev to choose the correct name.
If mdadm detects that udev is not configured, it will create the devices
in /dev itself.
Auto-Assembly
When --assemble is used with --scan and no devices are listed, mdadm
will first attempt to assemble all the arrays listed in the config file.
If no arrays are listed in the config (other than those marked <ignore>)
it will look through the available devices for possible arrays and will
try to assemble anything that it finds. Arrays which are tagged as be-
longing 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 writ-
ten 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 meta-
data, 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 con-
figuration 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 de-
nied 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 de-
vices
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 ar-
ray, 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 --as-
sume-clean with levels raid1 or raid10.
CREATE MODE
Usage: mdadm --create md-device --chunk=X --level=Y --raid-devices=Z de-
vices
This usage will initialize a new md array, associate some devices with
it, and activate the array.
md-device is a new device. This could be standard name or chosen name.
For details see: DEVICE NAMES
The named device will normally not exist when mdadm --create is run, but
will be created by udev once the array becomes active.
The max length md-device name is limited to 32 characters. Different
metadata types have more strict limitation (like IMSM where only 16
characters are allowed). For that reason, long name could be truncated
or rejected, it depends on metadata policy.
As devices are added, they are checked to see if they contain RAID su-
perblocks or filesystems. They are also checked to see if the variance
in device size exceeds 1%.
If any discrepancy is found, the array will not automatically be run,
though the presence of a --run can override this caution.
To create a "degraded" array in which some devices are missing, simply
give the word "missing" in place of a device name. This will cause
mdadm to leave the corresponding slot in the array empty. For a RAID4
or RAID5 array at most one slot can be "missing"; for a RAID6 array at
most two slots. For a RAID1 array, only one real device needs to be
given. All of the others can be "missing".
When creating a RAID5 array, mdadm will automatically create a degraded
array with an extra spare drive. This is because building the spare
into a degraded array is in general faster than resyncing the parity on
a non-degraded, but not clean, array. This feature can be overridden
with the --force option.
When creating a partition based array, using mdadm with version-1.x
metadata, the partition type should be set to 0xDA (non fs-data). This
type of selection allows for greater precision since using any other
[RAID auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create
problems in the event of array recovery through a live cdrom.
A new array will normally get a randomly assigned 128bit UUID which is
very likely to be unique. If you have a specific need, you can choose a
UUID for the array by giving the --uuid= option. Be warned that creat-
ing two arrays with the same UUID is a recipe for disaster. Also, using
--uuid= when creating a v0.90 array will silently override any --home-
host= setting.
Space for a bitmap will be reserved so that one can be added later with
--grow --bitmap=internal.
If the metadata type supports it (currently only 1.x and IMSM metadata),
space will be allocated to store a bad block list. This allows a modest
number of bad blocks to be recorded, allowing the drive to remain in
service while only partially functional.
When creating an array within a CONTAINER mdadm can be given either the
list of devices to use, or simply the name of the container. The former
case gives control over which devices in the container will be used for
the array. The latter case allows mdadm to automatically choose which
devices to use based on how much spare space is available.
The General Management options that are valid with --create are:
--run insist on running the array even if some devices look like they
might be in use.
--readonly
start the array in readonly mode.
MANAGE MODE
Usage: mdadm device options... devices...
This usage will allow individual devices in an array to be failed, re-
moved or added. It is possible to perform multiple operations with on
command. For example:
mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
will firstly mark /dev/hda1 as faulty in /dev/md0 and will then remove
it from the array and finally add it back in as a spare. However, only
one md array can be affected by a single command.
When a device is added to an active array, mdadm checks to see if it has
metadata on it which suggests that it was recently a member of the ar-
ray. If it does, it tries to "re-add" the device. If there have been
no changes since the device was removed, or if the array has a write-in-
tent bitmap which has recorded whatever changes there were, then the de-
vice will immediately become a full member of the array and those dif-
ferences recorded in the bitmap will be resolved.
MISC MODE
Usage: mdadm options ... devices ...
MISC mode includes a number of distinct operations that operate on dis-
tinct 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 de-
vice(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 un-
usable.
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-subar-
ray specifies a subarray in the container, then attempt to update
the given superblock field in the subarray. Similar to updating
an array in "assemble" mode, the field to update is selected by
-U or --update= option. The supported options are name, ppl,
no-ppl, bitmap and no-bitmap.
The name option updates the subarray name in the metadata. It
must be POSIX PORTABLE NAME compatible and cannot be longer than
32 chars. If successes, new value will be respected after next
assembly.
The ppl and no-ppl options enable and disable PPL in the meta-
data. 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 subar-
rays.
--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 compo-
nents of the one array are grouped together and reported in a
single entry suitable for inclusion in mdadm.conf.
Having --scan without listing any devices will cause all devices
listed in the config file to be examined.
--dump=directory
If the device contains RAID metadata, a file will be created in
the directory and the metadata will be written to it. The file
will be the same size as the device and will have the metadata
written at the same location as it exists in the device. How-
ever, the file will be "sparse" so that only those blocks con-
taining 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 op-
eration.
If a file name is given instead of a directory then mdadm will
restore from that file to a single device, always provided the
size of the file matches that of the device, and the file con-
tains 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 opera-
tion to be applied to all arrays listed in /proc/mdstat. For
--examine, --scan causes all devices listed in the config file to
be examined.
-b, --brief
Be less verbose. This is used with --detail and --examine. Us-
ing --brief with --verbose gives an intermediate level of ver-
bosity.
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 des-
tination array has a failed drive but no spares.
The result of monitoring the arrays is the generation of events. These
events are passed to a separate program (if specified) and may be mailed
to a given E-mail address.
When passing events to a program, the program is run once for each
event, and is given 2 or 3 command-line arguments: the first is the name
of the event (see below), the second is the name of the md device which
is affected, and the third is the name of a related device if relevant
(such as a component device that has failed).
If --scan is given, then a program or an e-mail address must be speci-
fied on the command line or in the config file. If neither are avail-
able, then mdadm will not monitor anything. For devices given directly
in command line, without program or email specified, each event is re-
ported to stdout.
Note: On systems where mdadm monitoring is managed through systemd, the
mdmonitor.service should be present. This service is designed to be the
primary solution for array monitoring. It is configured to operate in
system-wide mode. It is initiated by udev when start criteria are met,
e.g. mdadm.conf exists and necessary configuration parameters are set.
It is kept alive as long as a redundant RAID array is active; it stops
otherwise. User should customize MAILADDR in mdadm.conf to receive mail
notifications. MONITORDELAY, MAILFROM and PROGRAM are optional. See
mdadm.conf(5) for detailed description of these options. Use systemctl
status mdmonitor.service to verify status or determine if additional
configuration is needed.
The different events are:
DeviceDisappeared
An md array which previously was configured appears to no
longer be configured. (syslog priority: Critical)
If mdadm was told to monitor an array which is RAID0 or Lin-
ear, then it will report DeviceDisappeared with the extra in-
formation Wrong-Level. This is because RAID0 and Linear do
not support the device-failed, hot-spare and resync opera-
tions 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 in-
crement 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 be-
cause it finished normally or was aborted. (syslog priority:
Warning)
Fail An active component device of an array has been marked as
faulty. (syslog priority: Critical)
FailSpare
A spare component device which was being rebuilt to replace a
faulty device has failed. (syslog priority: Critical)
SpareActive
A spare component device which was being rebuilt to replace a
faulty device has been successfully rebuilt and has been made
active. (syslog priority: Info)
NewArray
A new md array has been detected in the /proc/mdstat file.
(syslog priority: Info)
DegradedArray
A newly noticed array appears to be degraded. This message
is not generated when mdadm notices a drive failure which
causes degradation, but only when mdadm notices that an array
is degraded when it first sees the array. (syslog priority:
Critical)
MoveSpare
A spare drive has been moved from one array in a spare-group
or domain to another to allow a failed drive to be replaced.
(syslog priority: Info)
SparesMissing
If mdadm has been told, via the config file, that an array
should have a certain number of spare devices, and mdadm de-
tects that it has fewer than this number when it first sees
the array, it will report a SparesMissing message. (syslog
priority: Warning)
TestMessage
An array was found at startup, and the --test flag was given.
(syslog priority: Info)
Only Fail, FailSpare, DegradedArray, SparesMissing and TestMessage cause
Email to be sent. All events cause the program to be run. The program
is run with two or three arguments: the event name, the array device and
possibly a second device.
Each event has an associated array device (e.g. /dev/md1) and possibly
a second device. For Fail, FailSpare, and SpareActive the second device
is the relevant component device. For MoveSpare the second device is
the array that the spare was moved from.
For mdadm to move spares from one array to another, the different arrays
need to be labeled with the same spare-group or the spares must be al-
lowed 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 de-
vices 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 re-
move the spare from the second array and add it to the first. If the
removal succeeds but the adding fails, then it is added back to the
original array.
If the spare group for a degraded array is not defined, mdadm will look
at the rules of spare migration specified by POLICY lines in mdadm.conf
and then follow similar steps as above if a matching spare is found.
GROW MODE
The GROW mode is used for changing the size or shape of an active array.
The following changes are supported:
• change the "size" attribute for RAID1, RAID4, RAID5 and RAID6.
• increase or decrease the "raid-devices" attribute of RAID0, RAID1,
RAID4, RAID5, and RAID6.
• change the chunk-size and layout of RAID0, RAID4, RAID5, RAID6 and
RAID10.
• convert between RAID1 and RAID5, between RAID5 and RAID6, between
RAID0, RAID4, and RAID5, and between RAID0 and RAID10 (in the near-2
mode).
• add a write-intent bitmap to any array which supports these bitmaps,
or remove a write-intent bitmap from such an array.
• change the array's consistency policy.
Using GROW on containers is currently supported only for Intel's IMSM
container format. The number of devices in a container can be increased
- which affects all arrays in the container - or an array in a container
can be converted between levels where those levels are supported by the
container, and the conversion is on of those listed above.
Notes:
• Intel's native checkpointing doesn't use --backup-file option and it
is transparent for assembly feature.
• Roaming between Windows(R) and Linux systems for IMSM metadata is
not supported during grow process.
• When growing a raid0 device, the new component disk size (or exter-
nal 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, re-
moved and replaced with larger drives, then you could have an array of
large drives with only a small amount used. In this situation, changing
the "size" with "GROW" mode will allow the extra space to start being
used. If the size is increased in this way, a "resync" process will
start to make sure the new parts of the array are synchronised.
Note that when an array changes size, any filesystem that may be stored
in the array will not automatically grow or shrink to use or vacate the
space. The filesystem will need to be explicitly told to use the extra
space after growing, or to reduce its size prior to shrinking the array.
Also, the size of an array cannot be changed while it has an active
bitmap. If an array has a bitmap, it must be removed before the size
can be changed. Once the change is complete a new bitmap can be created.
RAID-DEVICES CHANGES
A RAID1 array can work with any number of devices from 1 upwards (though
1 is not very useful). There may be times which you want to increase or
decrease the number of active devices. Note that this is different to
hot-add or hot-remove which changes the number of inactive devices.
When reducing the number of devices in a RAID1 array, the slots which
are to be removed from the array must already be vacant. That is, the
devices which were in those slots must be failed and removed.
When the number of devices is increased, any hot spares that are present
will be activated immediately.
Changing the number of active devices in a RAID5 or RAID6 is much more
effort. Every block in the array will need to be read and written back
to a new location. Linux Kernel is able to increase or decrease the num-
ber of devices in a RAID5 and RAID6 safely, including restarting an in-
terrupted "reshape".
The Linux Kernel is able to convert a RAID0 into a RAID4 or RAID5.
mdadm uses this functionality and the ability to add devices to a RAID4
to allow devices to be added to a RAID0. When requested to do this,
mdadm will convert the RAID0 to a RAID4, add the necessary disks and
make the reshape happen, and then convert the RAID4 back to RAID0.
When decreasing the number of devices, the size of the array will also
decrease. If there was data in the array, it could get destroyed and
this is not reversible, so you should firstly shrink the filesystem on
the array to fit within the new size. To help prevent accidents, mdadm
requires that the size of the array be decreased first with mdadm --grow
--array-size. This is a reversible change which simply makes the end of
the array inaccessible. The integrity of any data can then be checked
before the non-reversible reduction in the number of devices is request.
When relocating the first few stripes on a RAID5 or RAID6, it is not
possible to keep the data on disk completely consistent and crash-proof.
To provide the required safety, mdadm disables writes to the array while
this "critical section" is reshaped, and takes a backup of the data that
is in that section. For grows, this backup may be stored in any spare
devices that the array has, however it can also be stored in a separate
file specified with the --backup-file option, and is required to be
specified for shrinks, RAID level changes and layout changes. If this
option is used, and the system does crash during the critical period,
the same file must be passed to --assemble to restore the backup and re-
assemble the array. When shrinking rather than growing the array, the
reshape is done from the end towards the beginning, so the "critical
section" is at the end of the reshape.
LEVEL CHANGES
Changing the RAID level of any array happens instantaneously. However
in the RAID5 to RAID6 case this requires a non-standard layout of the
RAID6 data, and in the RAID6 to RAID5 case that non-standard layout is
required before the change can be accomplished. So while the level
change is instant, the accompanying layout change can take quite a long
time. A --backup-file is required. If the array is not simultaneously
being grown or shrunk, so that the array size will remain the same - for
example, reshaping a 3-drive RAID5 into a 4-drive RAID6 - the backup
file will be used not just for a "critical section" but throughout the
reshape operation, as described below under LAYOUT CHANGES.
CHUNK-SIZE AND LAYOUT CHANGES
Changing the chunk-size or layout without also changing the number of
devices as the same time will involve re-writing all blocks in-place.
To ensure against data loss in the case of a crash, a --backup-file must
be provided for these changes. Small sections of the array will be
copied to the backup file while they are being rearranged. This means
that all the data is copied twice, once to the backup and once to the
new layout on the array, so this type of reshape will go very slowly.
If the reshape is interrupted for any reason, this backup file must be
made available to mdadm --assemble so the array can be reassembled.
Consequently, the file cannot be stored on the device being reshaped.
BITMAP CHANGES
A write-intent bitmap can be added to, or removed from, an active array.
CONSISTENCY POLICY CHANGES
The consistency policy of an active array can be changed by using the
--consistency-policy option in Grow mode. Currently this works only for
the ppl and resync policies and allows one to enable or disable the
RAID5 Partial Parity Log (PPL).
INCREMENTAL MODE
Usage: mdadm --incremental [--run] [--quiet] component-device [optional-
aliases-for-device]
Usage: mdadm --incremental --fail component-device
Usage: mdadm --incremental --rebuild-map
Usage: mdadm --incremental --run --scan
This mode is designed to be used in conjunction with a device discovery
system. As devices are found in a system, they can be passed to mdadm
--incremental to be conditionally added to an appropriate array.
Conversely, it can also be used with the --fail flag to do just the op-
posite 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 con-
ditionally starts the array.
Note that mdadm will normally only add devices to an array which were
previously working (active or spare) parts of that array. The support
for automatic inclusion of a new drive as a spare in some array requires
a configuration through POLICY in config file.
The tests that mdadm makes are as follow:
+ Is the device permitted by mdadm.conf? That is, is it listed in
a DEVICES line in that file. If DEVICES is absent then the de-
fault it to allow any device. Similarly if DEVICES contains the
special word partitions then any device is allowed. Otherwise
the device name given to mdadm, or one of the aliases given, or
an alias found in the filesystem, must match one of the names or
patterns in a DEVICES line.
This is the only context where the aliases are used. They are
usually provided by a udev rules mentioning $env{DEVLINKS}.
+ Does the device have a valid md superblock? If a specific meta-
data version is requested with --metadata or -e then only that
style of metadata is accepted, otherwise mdadm finds any known
version of metadata. If no md metadata is found, the device may
be still added to an array as a spare if POLICY allows.
mdadm keeps a list of arrays that it has partially assembled in
/run/mdadm/map. If no array exists which matches the metadata on the
new device, mdadm must choose a device name and unit number. It does
this based on any name given in mdadm.conf or any name information
stored in the metadata. If this name suggests a unit number, that num-
ber 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 de-
vices 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 de-
vices are missing the array will not be restarted.
As an alternative, --run may be passed to mdadm in which case the array
will be run as soon as there are enough devices present for the data to
be accessible. For a RAID1, that means one device will start the array.
For a clean RAID5, the array will be started as soon as all but one
drive is present.
Note that neither of these approaches is really ideal. If it can be
known that all device discovery has completed, then
mdadm -IRs
can be run which will try to start all arrays that are being incremen-
tally 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 oper-
ates.
MDADM_NO_MDMON
Setting this value to 1 will prevent mdadm from automatically
launching mdmon. This variable is intended primarily for debug-
ging mdadm/mdmon.
MDADM_NO_UDEV
Normally, mdadm does not create any device nodes in /dev, but
leaves that task to udev. If udev appears not to be configured,
or if this environment variable is set to '1', the mdadm will
create and devices that are needed.
MDADM_NO_SYSTEMCTL
If mdadm detects that systemd is in use it will normally request
systemd to start various background tasks (particularly mdmon)
rather than forking and running them in the background. This can
be suppressed by setting MDADM_NO_SYSTEMCTL=1.
IMSM_NO_PLATFORM
A key value of IMSM metadata is that it allows interoperability
with boot ROMs on Intel platforms, and with other major operating
systems. Consequently, mdadm will only allow an IMSM array to be
created or modified if detects that it is running on an Intel
platform which supports IMSM, and supports the particular config-
uration of IMSM that is being requested (some functionality re-
quires 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 com-
promised by setting this value.
These change can also be suppressed by adding mdadm.imsm.test=1
to the kernel command line. This makes it easy to test IMSM code
in a virtual machine that doesn't have IMSM virtual hardware.
MDADM_GROW_ALLOW_OLD
If an array is stopped while it is performing a reshape and that
reshape was making use of a backup file, then when the array is
re-assembled mdadm will sometimes complain that the backup file
is too old. If this happens and you are certain it is the right
backup file, you can over-ride this check by setting
MDADM_GROW_ALLOW_OLD=1 in the environment.
MDADM_CONF_AUTO
Any string given in this variable is added to the start of the
AUTO line in the config file, or treated as the whole AUTO line
if none is given. It can be used to disable certain metadata
types when mdadm is called from a boot script. For example
export MDADM_CONF_AUTO='-ddf -imsm'
will make sure that mdadm does not automatically assemble any DDF
or IMSM arrays that are found. This can be useful on systems
configured to manage such arrays with dmraid.
EXAMPLES
mdadm --query /dev/name-of-device
This will find out if a given device is a RAID array, or is part of one,
and will provide brief information about the device.
mdadm --assemble --scan
This will assemble and start all arrays listed in the standard config
file. This command will typically go in a system startup file.
mdadm --stop --scan
This will shut down all arrays that can be shut down (i.e. are not cur-
rently 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 stan-
dard config file, then monitor the status of all arrays listed in that
file by polling them ever 2 minutes.
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1
Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and /dev/hdc1.
echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf
mdadm --detail --scan >> mdadm.conf
This will create a prototype config file that describes currently active
arrays that are known to be made from partitions of IDE or SCSI drives.
This file should be reviewed before being used as it may contain un-
wanted detail.
echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf
mdadm --examine --scan --config=mdadm.conf >> mdadm.conf
This will find arrays which could be assembled from existing IDE and
SCSI whole drives (not partitions), and store the information in the
format of a config file. This file is very likely to contain unwanted
detail, particularly the devices= entries. It should be reviewed and
edited before being used as an actual config file.
mdadm --examine --brief --scan --config=partitions
mdadm -Ebsc partitions
Create a list of devices by reading /proc/partitions, scan these for
RAID superblocks, and printout a brief listing of all that were found.
mdadm -Ac partitions -m 0 /dev/md0
Scan all partitions and devices listed in /proc/partitions and assemble
/dev/md0 out of all such devices with a RAID superblock with a minor
number of 0.
mdadm --monitor --scan --daemonise > /run/mdadm/mon.pid
If config file contains a mail address or alert program, run mdadm in
the background in monitor mode monitoring all md devices. Also write
pid of mdadm daemon to /run/mdadm/mon.pid.
mdadm -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as appropri-
ate.
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 ap-
propriate.
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 cur-
rently being created.
POSIX PORTABLE NAME
A valid name can only consist of characters "A-Za-z0-9.-_". The name
cannot start with a leading "-" and cannot exceed 255 chars.
DEVICE NAMES
mdadm understand two sorts of names for array devices.
The first is the so-called 'standard' format name, which matches the
names used by the kernel and which appear in /proc/mdstat.
The second sort can be freely chosen, but must reside in /dev/md/. When
giving a device name to mdadm to create or assemble an array, either
full path name such as /dev/md0 or /dev/md/home can be given, or just
the suffix of the second sort of name, such as home can be given.
In every style, raw name must be compatible with POSIX PORTABLE NAME and
has to be no longer than 32 chars.
When mdadm chooses device names during auto-assembly or incremental as-
sembly, it will sometimes add a small sequence number to the end of the
name to avoid conflicted between multiple arrays that have the same
name. If mdadm can reasonably determine that the array really is meant
for this host, either by a hostname in the metadata, or by the presence
of the array in mdadm.conf, then it will leave off the suffix if possi-
ble. 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 ver-
sion 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 mi-
gration unit and number of blocks per unit. By multiplying those
values and dividing by array size checkpoint progress percentage
can be obtained in relation to current progress reported in
/proc/mdstat. Checkpoint is also related to (and sometimes based
on) sysfs entry sync_completed but depending on action units may
differ. Even if units are the same, it should not be expected
that checkpoint and sync_completed will be exact match nor up-
dated 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)
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