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MDADM(8) System Manager's Manual MDADM(8)
mdadm - manage MD devices aka Linux Software RAID
mdadm [mode] <raiddevice> [options] <component-devices>
RAID devices are virtual devices created from two or more real
block devices. This allows multiple devices (typically disk
drives or partitions thereof) to be combined into a single device
to hold (for example) a single filesystem. Some RAID levels
include redundancy and so can survive some degree of device
failure.
Linux Software RAID devices are implemented through the md
(Multiple Devices) device driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping),
RAID1 (mirroring), RAID4, RAID5, RAID6, RAID10, MULTIPATH,
FAULTY, and CONTAINER.
MULTIPATH is not a Software RAID mechanism, but does involve
multiple devices: each device is a path to one common physical
storage device. New installations should not use md/multipath as
it is not well supported and has no ongoing development. Use the
Device Mapper based multipath-tools instead.
FAULTY is also not true RAID, and it only involves one device.
It provides a layer over a true device that can be used to inject
faults.
CONTAINER is different again. A CONTAINER is a collection of
devices that are managed as a set. This is similar to the set of
devices connected to a hardware RAID controller. The set of
devices may contain a number of different RAID arrays each
utilising some (or all) of the blocks from a number of the
devices in the set. For example, two devices in a 5-device set
might form a RAID1 using the whole devices. The remaining three
might have a RAID5 over the first half of each device, and a
RAID0 over the second half.
With a CONTAINER, there is one set of metadata that describes all
of the arrays in the container. So when mdadm creates a
CONTAINER device, the device just represents the metadata. Other
normal arrays (RAID1 etc) can be created inside the container.
mdadm has several major modes of operation:
Assemble
Assemble the components of a previously created array into
an active array. Components can be explicitly given or
can be searched for. mdadm checks that the components do
form a bona fide array, and can, on request, fiddle
superblock information so as to assemble a faulty array.
Build Build an array that doesn't have per-device metadata
(superblocks). For these sorts of arrays, mdadm cannot
differentiate between initial creation and subsequent
assembly of an array. It also cannot perform any checks
that appropriate components have been requested. Because
of this, the Build mode should only be used together with
a complete understanding of what you are doing.
Create Create a new array with per-device metadata (superblocks).
Appropriate metadata is written to each device, and then
the array comprising those devices is activated. A
'resync' process is started to make sure that the array is
consistent (e.g. both sides of a mirror contain the same
data) but the content of the device is left otherwise
untouched. The array can be used as soon as it has been
created. There is no need to wait for the initial resync
to finish.
Follow or Monitor
Monitor one or more md devices and act on any state
changes. This is only meaningful for RAID1, 4, 5, 6, 10
or multipath arrays, as only these have interesting state.
RAID0 or Linear never have missing, spare, or failed
drives, so there is nothing to monitor.
Grow Grow (or shrink) an array, or otherwise reshape it in some
way. Currently supported growth options including
changing the active size of component devices and changing
the number of active devices in Linear and RAID levels
0/1/4/5/6, changing the RAID level between 0, 1, 5, and 6,
and between 0 and 10, changing the chunk size and layout
for RAID 0,4,5,6,10 as well as adding or removing a write-
intent bitmap and changing the array's consistency policy.
Incremental Assembly
Add a single device to an appropriate array. If the
addition of the device makes the array runnable, the array
will be started. This provides a convenient interface to
a hot-plug system. As each device is detected, mdadm has
a chance to include it in some array as appropriate.
Optionally, when the --fail flag is passed in we will
remove the device from any active array instead of adding
it.
If a CONTAINER is passed to mdadm in this mode, then any
arrays within that container will be assembled and
started.
Manage This is for doing things to specific components of an
array such as adding new spares and removing faulty
devices.
Misc This is an 'everything else' mode that supports operations
on active arrays, operations on component devices such as
erasing old superblocks, and information-gathering
operations.
Auto-detect
This mode does not act on a specific device or array, but
rather it requests the Linux Kernel to activate any auto-
detected arrays.
-A, --assemble
Assemble a pre-existing array.
-B, --build
Build a legacy array without superblocks.
-C, --create
Create a new array.
-F, --follow, --monitor
Select Monitor mode.
-G, --grow
Change the size or shape of an active array.
-I, --incremental
Add/remove a single device to/from an appropriate array,
and possibly start the array.
--auto-detect
Request that the kernel starts any auto-detected arrays.
This can only work if md is compiled into the kernel — not
if it is a module. Arrays can be auto-detected by the
kernel if all the components are in primary MS-DOS
partitions with partition type FD, and all use v0.90
metadata. In-kernel autodetect is not recommended for new
installations. Using mdadm to detect and assemble arrays
— possibly in an initrd — is substantially more flexible
and should be preferred.
If a device is given before any options, or if the first option
is one of --add, --re-add, --add-spare, --fail, --remove, or
--replace, then the MANAGE mode is assumed. Anything other than
these will cause the Misc mode to be assumed.
-h, --help
Display a general help message or, after one of the above
options, a mode-specific help message.
--help-options
Display more detailed help about command-line parsing and
some commonly used options.
-V, --version
Print version information for mdadm.
-v, --verbose
Be more verbose about what is happening. This can be used
twice to be extra-verbose. The extra verbosity currently
only affects --detail --scan and --examine --scan.
-q, --quiet
Avoid printing purely informative messages. With this,
mdadm will be silent unless there is something really
important to report.
-f, --force
Be more forceful about certain operations. See the
various modes for the exact meaning of this option in
different contexts.
-c, --config=
Specify the config file or directory. If not specified,
the default config file and default conf.d directory will
be used. See mdadm.conf(5) for more details.
If the config file given is partitions then nothing will
be read, but mdadm will act as though the config file
contained exactly
DEVICE partitions containers
and will read /proc/partitions to find a list of devices
to scan, and /proc/mdstat to find a list of containers to
examine. If the word none is given for the config file,
then mdadm will act as though the config file were empty.
If the name given is of a directory, then mdadm will
collect all the files contained in the directory with a
name ending in .conf, sort them lexically, and process all
of those files as config files.
-s, --scan
Scan config file or /proc/mdstat for missing information.
In general, this option gives mdadm permission to get any
missing information (like component devices, array
devices, array identities, and alert destination) from the
configuration file (see previous option); one exception is
MISC mode when using --detail or --stop, in which case
--scan says to get a list of array devices from
/proc/mdstat.
-e, --metadata=
Declare the style of RAID metadata (superblock) to be
used. The default is 1.2 for --create, and to guess for
other operations. The default can be overridden by
setting the metadata value for the CREATE keyword in
mdadm.conf.
Options are:
0, 0.90
Use the original 0.90 format superblock. This
format limits arrays to 28 component devices and
limits component devices of levels 1 and greater to
2 terabytes. It is also possible for there to be
confusion about whether the superblock applies to a
whole device or just the last partition, if that
partition starts on a 64K boundary.
1, 1.0, 1.1, 1.2 default
Use the new version-1 format superblock. This has
fewer restrictions. It can easily be moved between
hosts with different endian-ness, and a recovery
operation can be checkpointed and restarted. The
different sub-versions store the superblock at
different locations on the device, either at the
end (for 1.0), at the start (for 1.1) or 4K from
the start (for 1.2). "1" is equivalent to "1.2"
(the commonly preferred 1.x format). "default" is
equivalent to "1.2".
ddf Use the "Industry Standard" DDF (Disk Data Format)
format defined by SNIA. When creating a DDF array
a CONTAINER will be created, and normal arrays can
be created in that container.
imsm Use the Intel(R) Matrix Storage Manager metadata
format. This creates a CONTAINER which is managed
in a similar manner to DDF, and is supported by an
option-rom on some platforms:
https://www.intel.com/content/www/us/en/support/products/122484
--homehost=
This will override any HOMEHOST setting in the config file
and provides the identity of the host which should be
considered the home for any arrays.
When creating an array, the homehost will be recorded in
the metadata. For version-1 superblocks, it will be
prefixed to the array name. For version-0.90 superblocks,
part of the SHA1 hash of the hostname will be stored in
the latter half of the UUID.
When reporting information about an array, any array which
is tagged for the given homehost will be reported as such.
When using Auto-Assemble, only arrays tagged for the given
homehost will be allowed to use 'local' names (i.e. not
ending in '_' followed by a digit string). See below
under Auto-Assembly.
The special name "any" can be used as a wild card. If an
array is created with --homehost=any then the name "any"
will be stored in the array and it can be assembled in the
same way on any host. If an array is assembled with this
option, then the homehost recorded on the array will be
ignored.
--prefer=
When mdadm needs to print the name for a device it
normally finds the name in /dev which refers to the device
and is the shortest. When a path component is given with
--prefer mdadm will prefer a longer name if it contains
that component. For example --prefer=by-uuid will prefer
a name in a subdirectory of /dev called by-uuid.
This functionality is currently only provided by --detail
and --monitor.
--home-cluster=
specifies the cluster name for the md device. The md
device can be assembled only on the cluster which matches
the name specified. If this option is not provided, mdadm
tries to detect the cluster name automatically.
-n, --raid-devices=
Specify the number of active devices in the array. This,
plus the number of spare devices (see below) must equal
the number of component-devices (including "missing"
devices) that are listed on the command line for --create.
Setting a value of 1 is probably a mistake and so requires
that --force be specified first. A value of 1 will then
be allowed for linear, multipath, RAID0 and RAID1. It is
never allowed for RAID4, RAID5 or RAID6.
This number can only be changed using --grow for RAID1,
RAID4, RAID5 and RAID6 arrays, and only on kernels which
provide the necessary support.
-x, --spare-devices=
Specify the number of spare (eXtra) devices in the initial
array. Spares can also be added and removed later. The
number of component devices listed on the command line
must equal the number of RAID devices plus the number of
spare devices.
-z, --size=
Amount (in Kilobytes) of space to use from each drive in
RAID levels 1/4/5/6/10 and for RAID 0 on external
metadata. This must be a multiple of the chunk size, and
must leave about 128Kb of space at the end of the drive
for the RAID superblock. If this is not specified (as it
normally is not) the smallest drive (or partition) sets
the size, though if there is a variance among the drives
of greater than 1%, a warning is issued.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
Sometimes a replacement drive can be a little smaller than
the original drives though this should be minimised by
IDEMA standards. Such a replacement drive will be
rejected by md. To guard against this it can be useful to
set the initial size slightly smaller than the smaller
device with the aim that it will still be larger than any
replacement.
This option can be used with --create for determining the
initial size of an array. For external metadata, it can be
used on a volume, but not on a container itself. Setting
the initial size of RAID 0 array is only valid for
external metadata.
This value can be set with --grow for RAID level
1/4/5/6/10 though DDF arrays may not be able to support
this. RAID 0 array size cannot be changed. If the array
was created with a size smaller than the currently active
drives, the extra space can be accessed using --grow. The
size can be given as max which means to choose the largest
size that fits on all current drives.
Before reducing the size of the array (with --grow
--size=) you should make sure that space isn't needed. If
the device holds a filesystem, you would need to resize
the filesystem to use less space.
After reducing the array size you should check that the
data stored in the device is still available. If the
device holds a filesystem, then an 'fsck' of the
filesystem is a minimum requirement. If there are
problems the array can be made bigger again with no loss
with another --grow --size= command.
-Z, --array-size=
This is only meaningful with --grow and its effect is not
persistent: when the array is stopped and restarted the
default array size will be restored.
Setting the array-size causes the array to appear smaller
to programs that access the data. This is particularly
needed before reshaping an array so that it will be
smaller. As the reshape is not reversible, but setting
the size with --array-size is, it is required that the
array size is reduced as appropriate before the number of
devices in the array is reduced.
Before reducing the size of the array you should make sure
that space isn't needed. If the device holds a
filesystem, you would need to resize the filesystem to use
less space.
After reducing the array size you should check that the
data stored in the device is still available. If the
device holds a filesystem, then an 'fsck' of the
filesystem is a minimum requirement. If there are
problems the array can be made bigger again with no loss
with another --grow --array-size= command.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
A value of max restores the apparent size of the array to
be whatever the real amount of available space is.
Clustered arrays do not support this parameter yet.
-c, --chunk=
Specify chunk size in kilobytes. The default when
creating an array is 512KB. To ensure compatibility with
earlier versions, the default when building an array with
no persistent metadata is 64KB. This is only meaningful
for RAID0, RAID4, RAID5, RAID6, and RAID10.
RAID4, RAID5, RAID6, and RAID10 require the chunk size to
be a power of 2, with minimal chunk size being 4KB.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
--rounding=
Specify the rounding factor for a Linear array. The size
of each component will be rounded down to a multiple of
this size. This is a synonym for --chunk but highlights
the different meaning for Linear as compared to other RAID
levels. The default is 64K if a kernel earlier than
2.6.16 is in use, and is 0K (i.e. no rounding) in later
kernels.
-l, --level=
Set RAID level. When used with --create, options are:
linear, raid0, 0, stripe, raid1, 1, mirror, raid4, 4,
raid5, 5, raid6, 6, raid10, 10, multipath, mp, faulty,
container. Obviously some of these are synonymous.
When a CONTAINER metadata type is requested, only the
container level is permitted, and it does not need to be
explicitly given.
When used with --build, only linear, stripe, raid0, 0,
raid1, multipath, mp, and faulty are valid.
Can be used with --grow to change the RAID level in some
cases. See LEVEL CHANGES below.
-p, --layout=
This option configures the fine details of data layout for
RAID5, RAID6, and RAID10 arrays, and controls the failure
modes for faulty. It can also be used for working around
a kernel bug with RAID0, but generally doesn't need to be
used explicitly.
The layout of the RAID5 parity block can be one of
left-asymmetric, left-symmetric, right-asymmetric,
right-symmetric, la, ra, ls, rs. The default is
left-symmetric.
It is also possible to cause RAID5 to use a RAID4-like
layout by choosing parity-first, or parity-last.
Finally for RAID5 there are DDF-compatible layouts,
ddf-zero-restart, ddf-N-restart, and ddf-N-continue.
These same layouts are available for RAID6. There are
also 4 layouts that will provide an intermediate stage for
converting between RAID5 and RAID6. These provide a
layout which is identical to the corresponding RAID5
layout on the first N-1 devices, and has the 'Q' syndrome
(the second 'parity' block used by RAID6) on the last
device. These layouts are: left-symmetric-6,
right-symmetric-6, left-asymmetric-6, right-asymmetric-6,
and parity-first-6.
When setting the failure mode for level faulty, the
options are: write-transient, wt, read-transient, rt,
write-persistent, wp, read-persistent, rp, write-all,
read-fixable, rf, clear, flush, none.
Each failure mode can be followed by a number, which is
used as a period between fault generation. Without a
number, the fault is generated once on the first relevant
request. With a number, the fault will be generated after
that many requests, and will continue to be generated
every time the period elapses.
Multiple failure modes can be current simultaneously by
using the --grow option to set subsequent failure modes.
"clear" or "none" will remove any pending or periodic
failure modes, and "flush" will clear any persistent
faults.
The layout options for RAID10 are one of 'n', 'o' or 'f'
followed by a small number signifying the number of copies
of each datablock. The default is 'n2'. The supported
options are:
'n' signals 'near' copies. Multiple copies of one data
block are at similar offsets in different devices.
'o' signals 'offset' copies. Rather than the chunks being
duplicated within a stripe, whole stripes are duplicated
but are rotated by one device so duplicate blocks are on
different devices. Thus subsequent copies of a block are
in the next drive, and are one chunk further down.
'f' signals 'far' copies (multiple copies have very
different offsets). See md(4) for more detail about
'near', 'offset', and 'far'.
As for the number of copies of each data block, 2 is
normal, 3 can be useful. This number can be at most equal
to the number of devices in the array. It does not need
to divide evenly into that number (e.g. it is perfectly
legal to have an 'n2' layout for an array with an odd
number of devices).
A bug introduced in Linux 3.14 means that RAID0 arrays
with devices of differing sizes started using a different
layout. This could lead to data corruption. Since Linux
5.4 (and various stable releases that received backports),
the kernel will not accept such an array unless a layout
is explicitly set. It can be set to 'original' or
'alternate'. When creating a new array, mdadm will select
'original' by default, so the layout does not normally
need to be set. An array created for either 'original' or
'alternate' will not be recognized by an (unpatched)
kernel prior to 5.4. To create a RAID0 array with devices
of differing sizes that can be used on an older kernel,
you can set the layout to 'dangerous'. This will use
whichever layout the running kernel supports, so the data
on the array may become corrupt when changing kernel from
pre-3.14 to a later kernel.
When an array is converted between RAID5 and RAID6 an
intermediate RAID6 layout is used in which the second
parity block (Q) is always on the last device. To convert
a RAID5 to RAID6 and leave it in this new layout (which
does not require re-striping) use --layout=preserve. This
will try to avoid any restriping.
The converse of this is --layout=normalise which will
change a non-standard RAID6 layout into a more standard
arrangement.
--parity=
same as --layout (thus explaining the p of -p).
-b, --bitmap=
Specify how to store a write-intent bitmap. Following
values are supported:
internal - the bitmap is stored with the metadata on the
array and so is replicated on all devices.
clustered - the array is created for a clustered
environment. One bitmap is created for each node as
defined by the --nodes parameter and are stored
internally.
none - create array with no bitmap or remove any present
bitmap (grow mode).
Setting bitmap for file is deprecated and should not be
used. The file should not exist unless --force is also
given. The same file should be provided when assembling
the array. The file name must contain at least one slash
('/'). Bitmap files are only known to work on ext2 and
ext3. Storing bitmap files on other filesystems may result
in serious problems.
When creating an array on devices which are 100G or
larger, mdadm automatically adds an internal bitmap as it
will usually be beneficial. This can be suppressed with
--bitmap=none or by selecting a different consistency
policy with --consistency-policy.
--bitmap-chunk=
Set the chunk size of the bitmap. Each bit corresponds to
that many Kilobytes of storage. When using a file-based
bitmap, the default is to use the smallest size that is at
least 4 and requires no more than 2^21 chunks. When using
an internal bitmap, the chunk size defaults to 64Meg, or
larger if necessary to fit the bitmap into the available
space.
A suffix of 'K', 'M', 'G' or 'T' can be given to indicate
Kilobytes, Megabytes, Gigabytes or Terabytes respectively.
-W, --write-mostly
subsequent devices listed in a --build, --create, or --add
command will be flagged as 'write-mostly'. This is valid
for RAID1 only and means that the 'md' driver will avoid
reading from these devices if at all possible. This can
be useful if mirroring over a slow link.
--write-behind=
Specify that write-behind mode should be enabled (valid
for RAID1 only). If an argument is specified, it will set
the maximum number of outstanding writes allowed. The
default value is 256. A write-intent bitmap is required
in order to use write-behind mode, and write-behind is
only attempted on drives marked as write-mostly.
--failfast
subsequent devices listed in a --create or --add command
will be flagged as 'failfast'. This is valid for RAID1
and RAID10 only. IO requests to these devices will be
encouraged to fail quickly rather than cause long delays
due to error handling. Also no attempt is made to repair
a read error on these devices.
If an array becomes degraded so that the 'failfast' device
is the only usable device, the 'failfast' flag will then
be ignored and extended delays will be preferred to
complete failure.
The 'failfast' flag is appropriate for storage arrays
which have a low probability of true failure, but which
may sometimes cause unacceptable delays due to internal
maintenance functions.
--assume-clean
Tell mdadm that the array pre-existed and is known to be
clean. It can be useful when trying to recover from a
major failure as you can be sure that no data will be
affected unless you actually write to the array. It can
also be used when creating a RAID1 or RAID10 if you want
to avoid the initial resync, however this practice — while
normally safe — is not recommended. Use this only if you
really know what you are doing.
When the devices that will be part of a new array were
filled with zeros before creation the operator knows the
array is actually clean. If that is the case, such as
after running badblocks, this argument can be used to tell
mdadm the facts the operator knows.
When an array is resized to a larger size with --grow
--size= the new space is normally resynced in that same
way that the whole array is resynced at creation. From
Linux version 3.0, --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
successfull, will behave as if --assume-clean was
specified.
This is intended for use with devices that have hardware
offload for zeroing, but despite this zeroing can still
take several minutes for large disks. Thus a message is
printed before and after zeroing and each disk is zeroed
in parallel with the others.
This is only meaningful with --create.
--backup-file=
This is needed when --grow is used to increase the number
of raid devices in a RAID5 or RAID6 if there are no spare
devices available, or to shrink, change RAID level or
layout. See the GROW MODE section below on RAID-DEVICES
CHANGES. The file must be stored on a separate device,
not on the RAID array being reshaped.
--data-offset=
Arrays with 1.x metadata can leave a gap between the start
of the device and the start of array data. This gap can
be used for various metadata. The start of data is known
as the data-offset. Normally an appropriate data offset
is computed automatically. However it can be useful to
set it explicitly such as when re-creating an array which
was originally created using a different version of mdadm
which computed a different offset.
Setting the offset explicitly over-rides the default. The
value given is in Kilobytes unless a suffix of 'K', 'M',
'G' or 'T' is used to explicitly indicate Kilobytes,
Megabytes, Gigabytes or Terabytes respectively.
Since Linux 3.4, --data-offset can also be used with
--grow for some RAID levels (initially on RAID10). This
allows the data-offset to be changed as part of the
reshape process. When the data offset is changed, no
backup file is required as the difference in offsets is
used to provide the same functionality.
When the new offset is earlier than the old offset, the
number of devices in the array cannot shrink. When it is
after the old offset, the number of devices in the array
cannot increase.
When creating an array, --data-offset can be specified as
variable. In the case each member device is expected to
have an offset appended to the name, separated by a colon.
This makes it possible to recreate exactly an array which
has varying data offsets (as can happen when different
versions of mdadm are used to add different devices).
--continue
This option is complementary to the --freeze-reshape
option for assembly. It is needed when --grow operation is
interrupted and it is not restarted automatically due to
--freeze-reshape usage during array assembly. This option
is used together with -G , ( --grow ) command and device
for a pending reshape to be continued. All parameters
required for reshape continuation will be read from array
metadata. If initial --grow command had required
--backup-file= option to be set, continuation option will
require to have exactly the same backup file given as
well.
Any other parameter passed together with --continue option
will be ignored.
-N, --name=
Set a name for the array. It must be POSIX PORTABLE NAME
compatible and cannot be longer than 32 chars. This is
effective when creating an array with a v1 metadata, or an
external array.
If name is needed but not specified, it is taken from the
basename of the device that is being created. See DEVICE
NAMES
-R, --run
Insist that mdadm run the array, even if some of the
components appear to be active in another array or
filesystem. Normally mdadm will ask for confirmation
before including such components in an array. This option
causes that question to be suppressed.
-f, --force
Insist that mdadm accept the geometry and layout specified
without question. Normally mdadm will not allow the
creation of an array with only one device, and will try to
create a RAID5 array with one missing drive (as this makes
the initial resync work faster). With --force, mdadm will
not try to be so clever.
-o, --readonly
Start the array read only rather than read-write as
normal. No writes will be allowed to the array, and no
resync, recovery, or reshape will be started. It works
with Create, Assemble, Manage and Misc mode.
-a, --auto{=yes,md,mdp,part,p}{NN}
Instruct mdadm how to create the device file if needed,
possibly allocating an unused minor number. "md" causes a
non-partitionable array to be used (though since Linux
2.6.28, these array devices are in fact partitionable).
"mdp", "part" or "p" causes a partitionable array (2.6 and
later) to be used. "yes" requires the named md device to
have a 'standard' format, and the type and minor number
will be determined from this. With mdadm 3.0, device
creation is normally left up to udev so this option is
unlikely to be needed. See DEVICE NAMES below.
The argument can also come immediately after "-a". e.g.
"-ap".
If --auto is not given on the command line or in the
config file, then the default will be --auto=yes.
If --scan is also given, then any auto= entries in the
config file will override the --auto instruction given on
the command line.
For partitionable arrays, mdadm will create the device
file for the whole array and for the first 4 partitions.
A different number of partitions can be specified at the
end of this option (e.g. --auto=p7). If the device name
ends with a digit, the partition names add a 'p', and a
number, e.g. /dev/md/home1p3. If there is no trailing
digit, then the partition names just have a number added,
e.g. /dev/md/scratch3.
If the md device name is in a 'standard' format as
described in DEVICE NAMES, then it will be created, if
necessary, with the appropriate device number based on
that name. If the device name is not in one of these
formats, then an unused device number will be allocated.
The device number will be considered unused if there is no
active array for that number, and there is no entry in
/dev for that number and with a non-standard name. Names
that are not in 'standard' format are only allowed in
"/dev/md/".
This is meaningful with --create or --build.
-a, --add
This option can be used in Grow mode in two cases.
If the target array is a Linear array, then --add can be
used to add one or more devices to the array. They are
simply catenated on to the end of the array. Once added,
the devices cannot be removed.
If the --raid-disks option is being used to increase the
number of devices in an array, then --add can be used to
add some extra devices to be included in the array. In
most cases this is not needed as the extra devices can be
added as spares first, and then the number of raid disks
can be changed. However, for RAID0 it is not possible to
add spares. So to increase the number of devices in a
RAID0, it is necessary to set the new number of devices,
and to add the new devices, in the same command.
--nodes
Only works when the array is created for a clustered
environment. It specifies the maximum number of nodes in
the cluster that will use this device simultaneously. If
not specified, this defaults to 4.
--write-journal
Specify journal device for the RAID-4/5/6 array. The
journal device should be an SSD with a reasonable
lifetime.
-k, --consistency-policy=
Specify how the array maintains consistency in the case of
an unexpected shutdown. Only relevant for RAID levels
with redundancy. Currently supported options are:
resync Full resync is performed and all redundancy is
regenerated when the array is started after an
unclean shutdown.
bitmap Resync assisted by a write-intent bitmap.
Implicitly selected when using --bitmap.
journal
For RAID levels 4/5/6, the journal device is used
to log transactions and replay after an unclean
shutdown. Implicitly selected when using
--write-journal.
ppl For RAID5 only, Partial Parity Log is used to close
the write hole and eliminate resync. PPL is stored
in the metadata region of RAID member drives, no
additional journal drive is needed.
Can be used with --grow to change the consistency policy
of an active array in some cases. See CONSISTENCY POLICY
CHANGES below.
-u, --uuid=
uuid of array to assemble. Devices which don't have this
uuid are excluded
-m, --super-minor=
Minor number of device that array was created for.
Devices which don't have this minor number are excluded.
If you create an array as /dev/md1, then all superblocks
will contain the minor number 1, even if the array is
later assembled as /dev/md2.
Giving the literal word "dev" for --super-minor will cause
mdadm to use the minor number of the md device that is
being assembled. e.g. when assembling /dev/md0,
--super-minor=dev will look for super blocks with a minor
number of 0.
--super-minor is only relevant for v0.90 metadata, and
should not normally be used. Using --uuid is much safer.
-N, --name=
Specify the name of the array to assemble. It must be
POSIX PORTABLE NAME compatible and cannot be longer than
32 chars. This must be the name that was specified when
creating the array. It must either match the name stored
in the superblock exactly, or it must match with the
current homehost prefixed to the start of the given name.
-f, --force
Assemble the array even if the metadata on some devices
appears to be out-of-date. If mdadm cannot find enough
working devices to start the array, but can find some
devices that are recorded as having failed, then it will
mark those devices as working so that the array can be
started. This works only for native. For external metadata
it allows 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.
-a, --auto{=no,yes,md,mdp,part}
See this option under Create and Build options.
-b, --bitmap=
Specify the bitmap file that was given when the array was
created. If an array has an internal bitmap, there is no
need to specify this when assembling the array.
--backup-file=
If --backup-file was used while reshaping an array (e.g.
changing number of devices or chunk size) and the system
crashed during the critical section, then the same
--backup-file must be presented to --assemble to allow
possibly corrupted data to be restored, and the reshape to
be completed.
--invalid-backup
If the file needed for the above option is not available
for any reason an empty file can be given together with
this option to indicate that the backup file is invalid.
In this case the data that was being rearranged at the
time of the crash could be irrecoverably lost, but the
rest of the array may still be recoverable. This option
should only be used as a last resort if there is no way to
recover the backup file.
-U, --update=
Update the superblock on each device while assembling the
array. The argument given to this flag can be one of
sparc2.2, summaries, uuid, name, nodes, homehost, home-
cluster, resync, byteorder, devicesize, no-bitmap, bbl,
no-bbl, ppl, no-ppl, layout-original, layout-alternate,
layout-unspecified, metadata, or super-minor.
The sparc2.2 option will adjust the superblock of an array
what was created on a Sparc machine running a patched 2.2
Linux kernel. This kernel got the alignment of part of
the superblock wrong. You can use the --examine
--sparc2.2 option to mdadm to see what effect this would
have.
The super-minor option will update the preferred minor
field on each superblock to match the minor number of the
array being assembled. This can be useful if --examine
reports a different "Preferred Minor" to --detail. In
some cases this update will be performed automatically by
the kernel driver. In particular, the update happens
automatically at the first write to an array with
redundancy (RAID level 1 or greater) on a 2.6 (or later)
kernel.
The uuid option will change the uuid of the array. If a
UUID is given with the --uuid option that UUID will be
used as a new UUID and will NOT be used to help identify
the devices in the array. If no --uuid is given, a random
UUID is chosen.
The name option will change the name of the array as
stored in the superblock. This is only supported for
version-1 superblocks.
The nodes option will change the nodes of the array as
stored in the bitmap superblock. This option only works
for a clustered environment.
The homehost option will change the homehost as recorded
in the superblock. For version-0 superblocks, this is the
same as updating the UUID. For version-1 superblocks,
this involves updating the name.
The home-cluster option will change the cluster name as
recorded in the superblock and bitmap. This option only
works for a clustered environment.
The resync option will cause the array to be marked dirty
meaning that any redundancy in the array (e.g. parity for
RAID5, copies for RAID1) may be incorrect. This will
cause the RAID system to perform a "resync" pass to make
sure that all redundant information is correct.
The byteorder option allows arrays to be moved between
machines with different byte-order, such as from a big-
endian machine like a Sparc or some MIPS machines, to a
little-endian x86_64 machine. When assembling such an
array for the first time after a move, giving
--update=byteorder will cause mdadm to expect superblocks
to have their byteorder reversed, and will correct that
order before assembling the array. This is only valid
with original (Version 0.90) superblocks.
The summaries option will correct the summaries in the
superblock. That is the counts of total, working, active,
failed, and spare devices.
The devicesize option will rarely be of use. It applies
to version 1.1 and 1.2 metadata only (where the metadata
is at the start of the device) and is only useful when the
component device has changed size (typically become
larger). The version 1 metadata records the amount of the
device that can be used to store data, so if a device in a
version 1.1 or 1.2 array becomes larger, the metadata will
still be visible, but the extra space will not. In this
case it might be useful to assemble the array with
--update=devicesize. This will cause mdadm to determine
the maximum usable amount of space on each device and
update the relevant field in the metadata.
The metadata option only works on v0.90 metadata arrays
and will convert them to v1.0 metadata. The array must
not be dirty (i.e. it must not need a sync) and it must
not have a write-intent bitmap.
The old metadata will remain on the devices, but will
appear older than the new metadata and so will usually be
ignored. The old metadata (or indeed the new metadata) can
be removed by giving the appropriate --metadata= option to
--zero-superblock.
The no-bitmap option can be used when an array has an
internal bitmap which is corrupt in some way so that
assembling the array normally fails. It will cause any
internal bitmap to be ignored.
The bbl option will reserve space in each device for a bad
block list. This will be 4K in size and positioned near
the end of any free space between the superblock and the
data.
The no-bbl option will cause any reservation of space for
a bad block list to be removed. If the bad block list
contains entries, this will fail, as removing the list
could cause data corruption.
The ppl option will enable PPL for a RAID5 array and
reserve space for PPL on each device. There must be enough
free space between the data and superblock and a write-
intent bitmap or journal must not be used.
The no-ppl option will disable PPL in the superblock.
The layout-original and layout-alternate options are for
RAID0 arrays with non-uniform devices size that were in
use before Linux 5.4. If the array was being used with
Linux 3.13 or earlier, then to assemble the array on a new
kernel, --update=layout-original must be given. If the
array was created and used with a kernel from Linux 3.14
to Linux 5.3, then --update=layout-alternate must be
given. This only needs to be given once. Subsequent
assembly of the array will happen normally. For more
information, see md(4).
The layout-unspecified option reverts the effect of
layout-orignal or layout-alternate and allows the array to
be again used on a kernel prior to Linux 5.3. This option
should be used with great caution.
--freeze-reshape
This option is intended to be used in start-up scripts
during the initrd boot phase. When the array under
reshape is assembled during the initrd phase, this option
stops the reshape after the reshape-critical section has
been restored. This happens before the file system pivot
operation and avoids loss of filesystem context. Losing
file system context would cause reshape to be broken.
Reshape can be continued later using the --continue option
for the grow command.
-t, --test
Unless a more serious error occurred, mdadm will exit with
a status of 2 if no changes were made to the array and 0
if at least one change was made. This can be useful when
an indirect specifier such as missing, detached or faulty
is used in requesting an operation on the array. --test
will report failure if these specifiers didn't find any
match.
-a, --add
hot-add listed devices. If a device appears to have
recently been part of the array (possibly it failed or was
removed) the device is re-added as described in the next
point. If that fails or the device was never part of the
array, the device is added as a hot-spare. If the array
is degraded, it will immediately start to rebuild data
onto that spare.
Note that this and the following options are only
meaningful on array with redundancy. They don't apply to
RAID0 or Linear.
--re-add
re-add a device that was previously removed from an array.
If the metadata on the device reports that it is a member
of the array, and the slot that it used is still vacant,
then the device will be added back to the array in the
same position. This will normally cause the data for that
device to be recovered. However, based on the event count
on the device, the recovery may only require sections that
are flagged by a write-intent bitmap to be recovered or
may not require any recovery at all.
When used on an array that has no metadata (i.e. it was
built with --build) it will be assumed that bitmap-based
recovery is enough to make the device fully consistent
with the array.
--re-add can also be accompanied by --update=devicesize,
--update=bbl, or --update=no-bbl. See descriptions of
these options when used in Assemble mode for an
explanation of their use.
If the device name given is missing then mdadm will try to
find any device that looks like it should be part of the
array but isn't and will try to re-add all such devices.
If the device name given is faulty then mdadm will find
all devices in the array that are marked faulty, remove
them and attempt to immediately re-add them. This can be
useful if you are certain that the reason for failure has
been resolved.
--add-spare
Add a device as a spare. This is similar to --add except
that it does not attempt --re-add first. The device will
be added as a spare even if it looks like it could be a
recent member of the array.
-r, --remove
remove listed devices. They must not be active. i.e.
they should be failed or spare devices.
As well as the name of a device file (e.g. /dev/sda1) the
words failed, detached and names like set-A can be given
to --remove. The first causes all failed devices to be
removed. The second causes any device which is no longer
connected to the system (i.e an 'open' returns ENXIO) to
be removed. The third will remove a set as described
below under --fail.
-f, --fail
Mark listed devices as faulty. As well as the name of a
device file, the word detached or a set name like set-A
can be given. The former will cause any device that has
been detached from the system to be marked as failed. It
can then be removed.
For RAID10 arrays where the number of copies evenly
divides the number of devices, the devices can be
conceptually divided into sets where each set contains a
single complete copy of the data on the array. Sometimes
a RAID10 array will be configured so that these sets are
on separate controllers. In this case, all the devices in
one set can be failed by giving a name like set-A or set-B
to --fail. The appropriate set names are reported by
--detail.
--set-faulty
same as --fail.
--replace
Mark listed devices as requiring replacement. As soon as
a spare is available, it will be rebuilt and will replace
the marked device. This is similar to marking a device as
faulty, but the device remains in service during the
recovery process to increase resilience against multiple
failures. When the replacement process finishes, the
replaced device will be marked as faulty.
--with This can follow a list of --replace devices. The devices
listed after --with will preferentially be used to replace
the devices listed after --replace. These devices must
already be spare devices in the array.
--write-mostly
Subsequent devices that are added or re-added will have
the 'write-mostly' flag set. This is only valid for RAID1
and means that the 'md' driver will avoid reading from
these devices if possible.
--readwrite
Subsequent devices that are added or re-added will have
the 'write-mostly' flag cleared.
--cluster-confirm
Confirm the existence of the device. This is issued in
response to an --add request by a node in a cluster. When
a node adds a device it sends a message to all nodes in
the cluster to look for a device with a UUID. This
translates to a udev notification with the UUID of the
device to be added and the slot number. The receiving node
must acknowledge this message with --cluster-confirm.
Valid arguments are <slot>:<devicename> in case the device
is found or <slot>:missing in case the device is not
found.
--add-journal
Add a journal to an existing array, or recreate journal
for a RAID-4/5/6 array that lost a journal device. To
avoid interrupting ongoing write operations, --add-journal
only works for array in Read-Only state.
--failfast
Subsequent devices that are added or re-added will have
the 'failfast' flag set. This is only valid for RAID1 and
RAID10 and means that the 'md' driver will avoid long
timeouts on error handling where possible.
--nofailfast
Subsequent devices that are re-added will be re-added
without the 'failfast' flag set.
Each of these options requires that the first device listed is
the array to be acted upon, and the remainder are component
devices to be added, removed, marked as faulty, etc. Several
different operations can be specified for different devices, e.g.
mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove
/dev/sdb1
Each operation applies to all devices listed until the next
operation.
If an array is using a write-intent bitmap, then devices which
have been removed can be re-added in a way that avoids a full
reconstruction but instead just updates the blocks that have
changed since the device was removed. For arrays with persistent
metadata (superblocks) this is done automatically. For arrays
created with --build mdadm needs to be told that this device we
removed recently with --re-add.
Devices can only be removed from an array if they are not in
active use, i.e. that must be spares or failed devices. To
remove an active device, it must first be marked as faulty.
-Q, --query
Examine a device to see (1) if it is an md device and (2)
if it is a component of an md array. Information about
what is discovered is presented.
-D, --detail
Print details of one or more md devices.
--detail-platform
Print details of the platform's RAID capabilities
(firmware / hardware topology) for a given metadata
format. If used without an argument, mdadm will scan all
controllers looking for their capabilities. Otherwise,
mdadm will only look at the controller specified by the
argument in the form of an absolute filepath or a link,
e.g. /sys/devices/pci0000:00/0000:00:1f.2.
-Y, --export
When used with --detail, --detail-platform, --examine, or
--incremental output will be formatted as key=value pairs
for easy import into the environment.
With --incremental The value MD_STARTED indicates whether
an array was started (yes) or not, which may include a
reason (unsafe, nothing, no). Also the value MD_FOREIGN
indicates if the array is expected on this host (no), or
seems to be from elsewhere (yes).
-E, --examine
Print contents of the metadata stored on the named
device(s). Note the contrast between --examine and
--detail. --examine applies to devices which are
components of an array, while --detail applies to a whole
array which is currently active.
--sparc2.2
If an array was created on a SPARC machine with a 2.2
Linux kernel patched with RAID support, the superblock
will have been created incorrectly, or at least
incompatibly with 2.4 and later kernels. Using the
--sparc2.2 flag with --examine will fix the superblock
before displaying it. If this appears to do the right
thing, then the array can be successfully assembled using
--assemble --update=sparc2.2.
-X, --examine-bitmap
Report information about a bitmap file. The argument is
either an external bitmap file or an array component in
case of an internal bitmap. Note that running this on an
array device (e.g. /dev/md0) does not report the bitmap
for that array.
--examine-badblocks
List the bad-blocks recorded for the device, if a bad-
blocks list has been configured. Currently only 1.x and
IMSM metadata support bad-blocks lists.
--dump=directory
--restore=directory
Save metadata from lists devices, or restore metadata to
listed devices.
-R, --run
start a partially assembled array. If --assemble did not
find enough devices to fully start the array, it might
leaving it partially assembled. If you wish, you can then
use --run to start the array in degraded mode.
-S, --stop
deactivate array, releasing all resources.
-o, --readonly
mark array as readonly.
-w, --readwrite
mark array as readwrite.
--zero-superblock
If the device contains a valid md superblock, the block is
overwritten with zeros. With --force the block where the
superblock would be is overwritten even if it doesn't
appear to be valid.
Note: Be careful when calling --zero-superblock with
clustered raid. Make sure the array isn't used or
assembled in another cluster node before executing it.
--kill-subarray=
If the device is a container and the argument to
--kill-subarray specifies an inactive subarray in the
container, then the subarray is deleted. Deleting all
subarrays will leave an 'empty-container' or spare
superblock on the drives. See --zero-superblock for
completely removing a superblock. Note that some formats
depend on the subarray index for generating a UUID, this
command will fail if it would change the UUID of an active
subarray.
--update-subarray=
If the device is a container and the argument to
--update-subarray specifies a subarray in the container,
then attempt to update the given superblock field in the
subarray. See below in MISC MODE for details.
-t, --test
When used with --detail, the exit status of mdadm is set
to reflect the status of the device. See below in MISC
MODE for details.
-W, --wait
For each md device given, wait for any resync, recovery,
or reshape activity to finish before returning. mdadm
will return with success if it actually waited for every
device listed, otherwise it will return failure.
--wait-clean
For each md device given, or each device in /proc/mdstat
if --scan is given, arrange for the array to be marked
clean as soon as possible. mdadm will return with success
if the array uses external metadata and we successfully
waited. For native arrays, this returns immediately as
the kernel handles dirty-clean transitions at shutdown.
No action is taken if safe-mode handling is disabled.
--action=
Set the "sync_action" for all md devices given to one of
idle, frozen, check, repair. Setting to idle will abort
any currently running action though some actions will
automatically restart. Setting to frozen will abort any
current action and ensure no other action starts
automatically.
Details of check and repair can be found it md(4) under
SCRUBBING AND MISMATCHES.
--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. If any such
array is listed in mdadm.conf as requiring an external
bitmap, that bitmap will be attached first.
--fail, -f
This allows the hot-plug system to remove devices that
have fully disappeared from the kernel. It will first
fail and then remove the device from any array it belongs
to. The device name given should be a kernel device name
such as "sda", not a name in /dev.
--path=
Only used with --fail. The 'path' given will be recorded
so that if a new device appears at the same location it
can be automatically added to the same array. This allows
the failed device to be automatically replaced by a new
device without metadata if it appears at specified path.
This option is normally only set by an udev script.
-m, --mail
Give a mail address to send alerts to.
-p, --program, --alert
Give a program to be run whenever an event is detected.
-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. mdadm polls the md arrays and
then waits this many seconds before polling again. The
default is 60 seconds. Since 2.6.16, there is no need to
reduce this as the kernel alerts mdadm immediately when
there is any change.
-r, --increment
Give a percentage increment. mdadm will generate
RebuildNN events with the given percentage increment.
-f, --daemonise
Tell mdadm to run as a background daemon if it decides to
monitor anything. This causes it to fork and run in the
child, and to disconnect from the terminal. The process
id of the child is written to stdout. This is useful with
--scan which will only continue monitoring if a mail
address or alert program is found in the config file.
-i, --pid-file
When mdadm is running in daemon mode, write the pid of the
daemon process to the specified file, instead of printing
it on standard output.
-1, --oneshot
Check arrays only once. This will generate NewArray
events and more significantly DegradedArray and
SparesMissing events. Running
mdadm --monitor --scan -1
from a cron script will ensure regular notification of any
degraded arrays.
-t, --test
Generate a TestMessage alert for every array found at
startup. This alert gets mailed and passed to the alert
program. This can be used for testing that alert message
do get through successfully.
--no-sharing
This inhibits the functionality for moving spares between
arrays. Only one monitoring process started with --scan
but without this flag is allowed, otherwise the two could
interfere with each other.
Usage: mdadm --assemble md-device options-and-component-
devices...
Usage: mdadm --assemble --scan md-devices-and-options...
Usage: mdadm --assemble --scan options...
This usage assembles one or more RAID arrays from pre-existing
components. For each array, mdadm needs to know the md device,
the identity of the array, and the number of component devices.
These can be found in a number of ways.
In the first usage example (without the --scan) the first device
given is the md device. In the second usage example, all devices
listed are treated as md devices and assembly is attempted. In
the third (where no devices are listed) all md devices that are
listed in the configuration file are assembled. If no arrays are
described by the configuration file, then any arrays that can be
found on unused devices will be assembled.
If precisely one device is listed, but --scan is not given, then
mdadm acts as though --scan was given and identity information is
extracted from the configuration file.
The identity can be given with the --uuid option, the --name
option, or the --super-minor option, will be taken from the md-
device record in the config file, or will be taken from the super
block of the first component-device listed on the command line.
Devices can be given on the --assemble command line or in the
config file. Only devices which have an md superblock which
contains the right identity will be considered for any array.
The config file is only used if explicitly named with --config or
requested with (a possibly implicit) --scan. In the latter case,
the default config file is used. See mdadm.conf(5) for more
details.
If --scan is not given, then the config file will only be used to
find the identity of md arrays.
Normally the array will be started after it is assembled.
However if --scan is not given and not all expected drives were
listed, then the array is not started (to guard against usage
errors). To insist that the array be started in this case (as
may work for RAID1, 4, 5, 6, or 10), give the --run flag.
If udev is active, mdadm does not create any entries in /dev but
leaves that to udev. It does record information in
/run/mdadm/map which will allow udev to choose the correct name.
If mdadm detects that udev is not configured, it will create the
devices in /dev itself.
In Linux kernels prior to version 2.6.28 there were two distinct
types of md devices that could be created: one that could be
partitioned using standard partitioning tools and one that could
not. Since 2.6.28 that distinction is no longer relevant as both
types of devices can be partitioned. mdadm will normally create
the type that originally could not be partitioned as it has a
well-defined major number (9).
Prior to 2.6.28, it is important that mdadm chooses the correct
type of array device to use. This can be controlled with the
--auto option. In particular, a value of "mdp" or "part" or "p"
tells mdadm to use a partitionable device rather than the
default.
In the no-udev case, the value given to --auto can be suffixed by
a number. This tells mdadm to create that number of partition
devices rather than the default of 4.
The value given to --auto can also be given in the configuration
file as a word starting auto= on the ARRAY line for the relevant
array.
Auto-Assembly
When --assemble is used with --scan and no devices are listed,
mdadm will first attempt to assemble all the arrays listed in the
config file.
If no arrays are listed in the config (other than those marked
<ignore>) it will look through the available devices for possible
arrays and will try to assemble anything that it finds. Arrays
which are tagged as belonging to the given homehost will be
assembled and started normally. Arrays which do not obviously
belong to this host are given names that are expected not to
conflict with anything local, and are started "read-auto" so that
nothing is written to any device until the array is written to.
i.e. automatic resync etc is delayed.
If mdadm finds a consistent set of devices that look like they
should comprise an array, and if the superblock is tagged as
belonging to the given home host, it will automatically choose a
device name and try to assemble the array. If the array uses
version-0.90 metadata, then the minor number as recorded in the
superblock is used to create a name in /dev/md/ so for example
/dev/md/3. If the array uses version-1 metadata, then the name
from the superblock is used to similarly create a name in
/dev/md/ (the name will have any 'host' prefix stripped first).
This behaviour can be modified by the AUTO line in the mdadm.conf
configuration file. This line can indicate that specific
metadata type should, or should not, be automatically assembled.
If an array is found which is not listed in mdadm.conf and has a
metadata format that is denied by the AUTO line, then it will not
be assembled. The AUTO line can also request that all arrays
identified as being for this homehost should be assembled
regardless of their metadata type. See mdadm.conf(5) for further
details.
Note: Auto-assembly cannot be used for assembling and activating
some arrays which are undergoing reshape. In particular as the
backup-file cannot be given, any reshape which requires a backup
file to continue cannot be started by auto-assembly. An array
which is growing to more devices and has passed the critical
section can be assembled using auto-assembly.
Usage: mdadm --build md-device --chunk=X --level=Y
--raid-devices=Z devices
This usage is similar to --create. The difference is that it
creates an array without a superblock. With these arrays there
is no difference between initially creating the array and
subsequently assembling the array, except that hopefully there is
useful data there in the second case.
The level may raid0, linear, raid1, raid10, multipath, or faulty,
or one of their synonyms. All devices must be listed and the
array will be started once complete. It will often be
appropriate to use --assume-clean with levels raid1 or raid10.
Usage: mdadm --create md-device --chunk=X --level=Y
--raid-devices=Z devices
This usage will initialize a new md array, associate some devices
with it, and activate the array.
md-device is a new device. This could be standard name or chosen
name. For details see: DEVICE NAMES
The named device will normally not exist when mdadm --create is
run, but will be created by udev once the array becomes active.
The max length md-device name is limited to 32 characters.
Different metadata types have more strict limitation (like IMSM
where only 16 characters are allowed). For that reason, long
name could be truncated or rejected, it depends on metadata
policy.
As devices are added, they are checked to see if they contain
RAID superblocks or filesystems. They are also checked to see if
the variance in device size exceeds 1%.
If any discrepancy is found, the array will not automatically be
run, though the presence of a --run can override this caution.
To create a "degraded" array in which some devices are missing,
simply give the word "missing" in place of a device name. This
will cause mdadm to leave the corresponding slot in the array
empty. For a RAID4 or RAID5 array at most one slot can be
"missing"; for a RAID6 array at most two slots. For a RAID1
array, only one real device needs to be given. All of the others
can be "missing".
When creating a RAID5 array, mdadm will automatically create a
degraded array with an extra spare drive. This is because
building the spare into a degraded array is in general faster
than resyncing the parity on a non-degraded, but not clean,
array. This feature can be overridden with the --force option.
When creating a partition based array, using mdadm with
version-1.x metadata, the partition type should be set to 0xDA
(non fs-data). This type of selection allows for greater
precision since using any other [RAID auto-detect (0xFD) or a
GNU/Linux partition (0x83)], might create problems in the event
of array recovery through a live cdrom.
A new array will normally get a randomly assigned 128bit UUID
which is very likely to be unique. If you have a specific need,
you can choose a UUID for the array by giving the --uuid= option.
Be warned that creating two arrays with the same UUID is a recipe
for disaster. Also, using --uuid= when creating a v0.90 array
will silently override any --homehost= setting.
If the array type supports a write-intent bitmap, and if the
devices in the array exceed 100G is size, an internal write-
intent bitmap will automatically be added unless some other
option is explicitly requested with the --bitmap option or a
different consistency policy is selected with the
--consistency-policy option. In any case, 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.
Usage: mdadm device options... devices...
This usage will allow individual devices in an array to be
failed, removed or added. It is possible to perform multiple
operations with on command. For example:
mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
will firstly mark /dev/hda1 as faulty in /dev/md0 and will then
remove it from the array and finally add it back in as a spare.
However, only one md array can be affected by a single command.
When a device is added to an active array, mdadm checks to see if
it has metadata on it which suggests that it was recently a
member of the array. If it does, it tries to "re-add" the
device. If there have been no changes since the device was
removed, or if the array has a write-intent bitmap which has
recorded whatever changes there were, then the device will
immediately become a full member of the array and those
differences recorded in the bitmap will be resolved.
Usage: mdadm options ... devices ...
MISC mode includes a number of distinct operations that operate
on distinct devices. The operations are:
--query
The device is examined to see if it is (1) an active md
array, or (2) a component of an md array. The information
discovered is reported.
--detail
The device should be an active md device. mdadm will
display a detailed description of the array. --brief or
--scan will cause the output to be less detailed and the
format to be suitable for inclusion in mdadm.conf. The
exit status of mdadm will normally be 0 unless mdadm
failed to get useful information about the device(s);
however, if the --test option is given, then the exit
status will be:
0 The array is functioning normally.
1 The array has at least one failed device.
2 The array has multiple failed devices such that it
is unusable.
4 There was an error while trying to get information
about the device.
--detail-platform
Print detail of the platform's RAID capabilities (firmware
/ hardware topology). If the metadata is specified with
-e or --metadata= then the return status will be:
0 metadata successfully enumerated its platform
components on this system
1 metadata is platform independent
2 metadata failed to find its platform components on
this system
--update-subarray=
If the device is a container and the argument to
--update-subarray specifies a subarray in the container,
then attempt to update the given superblock field in the
subarray. Similar to updating an array in "assemble"
mode, the field to update is selected by -U or --update=
option. The supported options are name, ppl, no-ppl,
bitmap and no-bitmap.
The name option updates the subarray name in the metadata.
It must be POSIX PORTABLE NAME compatible and cannot be
longer than 32 chars. If successes, new value will be
respected after next assembly.
The ppl and no-ppl options enable and disable PPL in the
metadata. Currently supported only for IMSM subarrays.
The bitmap and no-bitmap options enable and disable write-
intent bitmap in the metadata. Currently supported only
for IMSM subarrays.
--examine
The device should be a component of an md array. mdadm
will read the md superblock of the device and display the
contents. If --brief or --scan is given, then multiple
devices that are components of the one array are grouped
together and reported in a single entry suitable for
inclusion in mdadm.conf.
Having --scan without listing any devices will cause all
devices listed in the config file to be examined.
--dump=directory
If the device contains RAID metadata, a file will be
created in the directory and the metadata will be written
to it. The file will be the same size as the device and
will have the metadata written at the same location as it
exists in the device. However, the file will be "sparse"
so that only those blocks containing metadata will be
allocated. The total space used will be small.
The filename used in the directory will be the base name
of the device. Further, if any links appear in
/dev/disk/by-id which point to the device, then hard links
to the file will be created in directory based on these
by-id names.
Multiple devices can be listed and their metadata will all
be stored in the one directory.
--restore=directory
This is the reverse of --dump. mdadm will locate a file
in the directory that has a name appropriate for the given
device and will restore metadata from it. Names that
match /dev/disk/by-id names are preferred, however if two
of those refer to different files, mdadm will not choose
between them but will abort the operation.
If a file name is given instead of a directory then mdadm
will restore from that file to a single device, always
provided the size of the file matches that of the device,
and the file contains valid metadata.
--stop The devices should be active md arrays which will be
deactivated, as long as they are not currently in use.
--run This will fully activate a partially assembled md array.
--readonly
This will mark an active array as read-only, providing
that it is not currently being used.
--readwrite
This will change a readonly array back to being
read/write.
--scan For all operations except --examine, --scan will cause the
operation to be applied to all arrays listed in
/proc/mdstat. For --examine, --scan causes all devices
listed in the config file to be examined.
-b, --brief
Be less verbose. This is used with --detail and
--examine. Using --brief with --verbose gives an
intermediate level of verbosity.
Usage: mdadm --monitor options... devices...
Monitor option can work in two modes:
• system wide mode, follow all md devices based on
/proc/mdstat,
• follow only specified MD devices in command line.
--scan - indicates system wide mode. Option causes the monitor to
track all md devices that appear in /proc/mdstat. If it is not
set, then at least one device must be specified.
Monitor usage causes mdadm to periodically poll a number of md
arrays and to report on any events noticed.
In both modes, monitor will work as long as there is an active
array with redundancy and it is defined to follow (for --scan
every array is followed).
As well as reporting events, mdadm may move a spare drive from
one array to another if they are in the same spare-group or
domain and if the destination array has a failed drive but no
spares.
The result of monitoring the arrays is the generation of events.
These events are passed to a separate program (if specified) and
may be mailed to a given E-mail address.
When passing events to a program, the program is run once for
each event, and is given 2 or 3 command-line arguments: the first
is the name of the event (see below), the second is the name of
the md device which is affected, and the third is the name of a
related device if relevant (such as a component device that has
failed).
If --scan is given, then a program or an e-mail address must be
specified on the command line or in the config file. If neither
are available, then mdadm will not monitor anything. For devices
given directly in command line, without program or email
specified, each event is reported to stdout.
Note: For systems where is configured via systemd,
mdmonitor(mdmonitor.service) should be configured. The service is
designed to be primary solution for array monitoring, it is
configured to work in system wide mode. It is automatically
started and stopped according to current state and types of MD
arrays in system. The service may require additional
configuration, like e-mail or delay. That should be done in
mdadm.conf.
The different events are:
DeviceDisappeared
An md array which previously was configured appears to
no longer be configured. (syslog priority: Critical)
If mdadm was told to monitor an array which is RAID0
or Linear, then it will report DeviceDisappeared with
the extra information Wrong-Level. This is because
RAID0 and Linear do not support the device-failed,
hot-spare and resync operations which are monitored.
RebuildStarted
An md array started reconstruction (e.g. recovery,
resync, reshape, check, repair). (syslog priority:
Warning)
RebuildNN
Where NN is a two-digit number (eg. 05, 48). This
indicates that the rebuild has reached that percentage
of the total. The events are generated at a fixed
increment from 0. The increment size may be specified
with a command-line option (the default is 20).
(syslog priority: Warning)
RebuildFinished
An md array that was rebuilding, isn't any more,
either because it finished normally or was aborted.
(syslog priority: Warning)
Fail An active component device of an array has been marked
as faulty. (syslog priority: Critical)
FailSpare
A spare component device which was being rebuilt to
replace a faulty device has failed. (syslog priority:
Critical)
SpareActive
A spare component device which was being rebuilt to
replace a faulty device has been successfully rebuilt
and has been made active. (syslog priority: Info)
NewArray
A new md array has been detected in the /proc/mdstat
file. (syslog priority: Info)
DegradedArray
A newly noticed array appears to be degraded. This
message is not generated when mdadm notices a drive
failure which causes degradation, but only when mdadm
notices that an array is degraded when it first sees
the array. (syslog priority: Critical)
MoveSpare
A spare drive has been moved from one array in a
spare-group or domain to another to allow a failed
drive to be replaced. (syslog priority: Info)
SparesMissing
If mdadm has been told, via the config file, that an
array should have a certain number of spare devices,
and mdadm detects that it has fewer than this number
when it first sees the array, it will report a
SparesMissing message. (syslog priority: Warning)
TestMessage
An array was found at startup, and the --test flag was
given. (syslog priority: Info)
Only Fail, FailSpare, DegradedArray, SparesMissing and
TestMessage cause Email to be sent. All events cause the program
to be run. The program is run with two or three arguments: the
event name, the array device and possibly a second device.
Each event has an associated array device (e.g. /dev/md1) and
possibly a second device. For Fail, FailSpare, and SpareActive
the second device is the relevant component device. For
MoveSpare the second device is the array that the spare was moved
from.
For mdadm to move spares from one array to another, the different
arrays need to be labeled with the same spare-group or the spares
must be allowed to migrate through matching POLICY domains in the
configuration file. The spare-group name can be any string; it
is only necessary that different spare groups use different
names.
When mdadm detects that an array in a spare group has fewer
active devices than necessary for the complete array, and has no
spare devices, it will look for another array in the same spare
group that has a full complement of working drives and a spare.
It will then attempt to remove the spare from the second array
and add it to the first. If the removal succeeds but the adding
fails, then it is added back to the original array.
If the spare group for a degraded array is not defined, mdadm
will look at the rules of spare migration specified by POLICY
lines in mdadm.conf and then follow similar steps as above if a
matching spare is found.
The GROW mode is used for changing the size or shape of an active
array.
During the kernel 2.6 era the following changes were added:
• change the "size" attribute for RAID1, RAID4, RAID5 and
RAID6.
• increase or decrease the "raid-devices" attribute of RAID0,
RAID1, RAID4, RAID5, and RAID6.
• change the chunk-size and layout of RAID0, RAID4, RAID5,
RAID6 and RAID10.
• convert between RAID1 and RAID5, between RAID5 and RAID6,
between RAID0, RAID4, and RAID5, and between RAID0 and RAID10
(in the near-2 mode).
• add a write-intent bitmap to any array which supports these
bitmaps, or remove a write-intent bitmap from such an array.
• change the array's consistency policy.
Using GROW on containers is currently supported only for Intel's
IMSM container format. The number of devices in a container can
be increased - which affects all arrays in the container - or an
array in a container can be converted between levels where those
levels are supported by the container, and the conversion is on
of those listed above.
Notes:
• Intel's native checkpointing doesn't use --backup-file option
and it is transparent for assembly feature.
• Roaming between Windows(R) and Linux systems for IMSM
metadata is not supported during grow process.
• When growing a raid0 device, the new component disk size (or
external backup size) should be larger than LCM(old, new) *
chunk-size * 2, where LCM() is the least common multiple of
the old and new count of component disks, and "* 2" comes
from the fact that mdadm refuses to use more than half of a
spare device for backup space.
SIZE CHANGES
Normally when an array is built the "size" is taken from the
smallest of the drives. If all the small drives in an arrays
are, over time, removed and replaced with larger drives, then you
could have an array of large drives with only a small amount
used. In this situation, changing the "size" with "GROW" mode
will allow the extra space to start being used. If the size is
increased in this way, a "resync" process will start to make sure
the new parts of the array are synchronised.
Note that when an array changes size, any filesystem that may be
stored in the array will not automatically grow or shrink to use
or vacate the space. The filesystem will need to be explicitly
told to use the extra space after growing, or to reduce its size
prior to shrinking the array.
Also, the size of an array cannot be changed while it has an
active bitmap. If an array has a bitmap, it must be removed
before the size can be changed. Once the change is complete a new
bitmap can be created.
Note: --grow --size is not yet supported for external file
bitmap.
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. From 2.6.17, the Linux Kernel is
able to increase the number of devices in a RAID5 safely,
including restarting an interrupted "reshape". From 2.6.31, the
Linux Kernel is able to increase or decrease the number of
devices in a RAID5 or RAID6.
From 2.6.35, the Linux Kernel is able to convert a RAID0 in to a
RAID4 or RAID5. mdadm uses this functionality and the ability to
add devices to a RAID4 to allow devices to be added to a RAID0.
When requested to do this, mdadm will convert the RAID0 to a
RAID4, add the necessary disks and make the reshape happen, and
then convert the RAID4 back to RAID0.
When decreasing the number of devices, the size of the array will
also decrease. If there was data in the array, it could get
destroyed and this is not reversible, so you should firstly
shrink the filesystem on the array to fit within the new size.
To help prevent accidents, mdadm requires that the size of the
array be decreased first with mdadm --grow --array-size. This is
a reversible change which simply makes the end of the array
inaccessible. The integrity of any data can then be checked
before the non-reversible reduction in the number of devices is
request.
When relocating the first few stripes on a RAID5 or RAID6, it is
not possible to keep the data on disk completely consistent and
crash-proof. To provide the required safety, mdadm disables
writes to the array while this "critical section" is reshaped,
and takes a backup of the data that is in that section. For
grows, this backup may be stored in any spare devices that the
array has, however it can also be stored in a separate file
specified with the --backup-file option, and is required to be
specified for shrinks, RAID level changes and layout changes. If
this option is used, and the system does crash during the
critical period, the same file must be passed to --assemble to
restore the backup and reassemble the array. When shrinking
rather than growing the array, the reshape is done from the end
towards the beginning, so the "critical section" is at the end of
the reshape.
LEVEL CHANGES
Changing the RAID level of any array happens instantaneously.
However in the RAID5 to RAID6 case this requires a non-standard
layout of the RAID6 data, and in the RAID6 to RAID5 case that
non-standard layout is required before the change can be
accomplished. So while the level change is instant, the
accompanying layout change can take quite a long time. A
--backup-file is required. If the array is not simultaneously
being grown or shrunk, so that the array size will remain the
same - for example, reshaping a 3-drive RAID5 into a 4-drive
RAID6 - the backup file will be used not just for a "critical
section" but throughout the reshape operation, as described below
under LAYOUT CHANGES.
CHUNK-SIZE AND LAYOUT CHANGES
Changing the chunk-size or layout without also changing the
number of devices as the same time will involve re-writing all
blocks in-place. To ensure against data loss in the case of a
crash, a --backup-file must be provided for these changes. Small
sections of the array will be copied to the backup file while
they are being rearranged. This means that all the data is
copied twice, once to the backup and once to the new layout on
the array, so this type of reshape will go very slowly.
If the reshape is interrupted for any reason, this backup file
must be made available to mdadm --assemble so the array can be
reassembled. Consequently, the file cannot be stored on the
device being reshaped.
BITMAP CHANGES
A write-intent bitmap can be added to, or removed from, an active
array. Either internal bitmaps, or bitmaps stored in a separate
file, can be added. Note that if you add a bitmap stored in a
file which is in a filesystem that is on the RAID array being
affected, the system will deadlock. The bitmap must be on a
separate filesystem.
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 to enable
or disable the RAID5 Partial Parity Log (PPL).
Usage: mdadm --incremental [--run] [--quiet] component-device
[optional-aliases-for-device]
Usage: mdadm --incremental --fail component-device
Usage: mdadm --incremental --rebuild-map
Usage: mdadm --incremental --run --scan
This mode is designed to be used in conjunction with a device
discovery system. As devices are found in a system, they can be
passed to mdadm --incremental to be conditionally added to an
appropriate array.
Conversely, it can also be used with the --fail flag to do just
the opposite and find whatever array a particular device is part
of and remove the device from that array.
If the device passed is a CONTAINER device created by a previous
call to mdadm, then rather than trying to add that device to an
array, all the arrays described by the metadata of the container
will be started.
mdadm performs a number of tests to determine if the device is
part of an array, and which array it should be part of. If an
appropriate array is found, or can be created, mdadm adds the
device to the array and conditionally starts the array.
Note that mdadm will normally only add devices to an array which
were previously working (active or spare) parts of that array.
The support for automatic inclusion of a new drive as a spare in
some array requires a configuration through POLICY in config
file.
The tests that mdadm makes are as follow:
+ Is the device permitted by mdadm.conf? That is, is it
listed in a DEVICES line in that file. If DEVICES is
absent then the default it to allow any device. Similarly
if DEVICES contains the special word partitions then any
device is allowed. Otherwise the device name given to
mdadm, or one of the aliases given, or an alias found in
the filesystem, must match one of the names or patterns in
a DEVICES line.
This is the only context where the aliases are used. They
are usually provided by a udev rules mentioning
$env{DEVLINKS}.
+ Does the device have a valid md superblock? If a specific
metadata version is requested with --metadata or -e then
only that style of metadata is accepted, otherwise mdadm
finds any known version of metadata. If no md metadata is
found, the device may be still added to an array as a
spare if POLICY allows.
mdadm keeps a list of arrays that it has partially assembled in
/run/mdadm/map. If no array exists which matches the metadata on
the new device, mdadm must choose a device name and unit number.
It does this based on any name given in mdadm.conf or any name
information stored in the metadata. If this name suggests a unit
number, that number will be used, otherwise a free unit number
will be chosen. Normally mdadm will prefer to create a
partitionable array, however if the CREATE line in mdadm.conf
suggests that a non-partitionable array is preferred, that will
be honoured.
If the array is not found in the config file and its metadata
does not identify it as belonging to the "homehost", then mdadm
will choose a name for the array which is certain not to conflict
with any array which does belong to this host. It does this be
adding an underscore and a small number to the name preferred by
the metadata.
Once an appropriate array is found or created and the device is
added, mdadm must decide if the array is ready to be started. It
will normally compare the number of available (non-spare) devices
to the number of devices that the metadata suggests need to be
active. If there are at least that many, the array will be
started. This means that if any devices are missing the array
will not be restarted.
As an alternative, --run may be passed to mdadm in which case the
array will be run as soon as there are enough devices present for
the data to be accessible. For a RAID1, that means one device
will start the array. For a clean RAID5, the array will be
started as soon as all but one drive is present.
Note that neither of these approaches is really ideal. If it can
be known that all device discovery has completed, then
mdadm -IRs
can be run which will try to start all arrays that are being
incrementally assembled. They are started in "read-auto" mode in
which they are read-only until the first write request. This
means that no metadata updates are made and no attempt at resync
or recovery happens. Further devices that are found before the
first write can still be added safely.
This section describes environment variables that affect how
mdadm operates.
MDADM_NO_MDMON
Setting this value to 1 will prevent mdadm from
automatically launching mdmon. This variable is intended
primarily for debugging mdadm/mdmon.
MDADM_NO_UDEV
Normally, mdadm does not create any device nodes in /dev,
but leaves that task to udev. If udev appears not to be
configured, or if this environment variable is set to '1',
the mdadm will create and devices that are needed.
MDADM_NO_SYSTEMCTL
If mdadm detects that systemd is in use it will normally
request systemd to start various background tasks
(particularly mdmon) rather than forking and running them
in the background. This can be suppressed by setting
MDADM_NO_SYSTEMCTL=1.
IMSM_NO_PLATFORM
A key value of IMSM metadata is that it allows
interoperability with boot ROMs on Intel platforms, and
with other major operating systems. Consequently, mdadm
will only allow an IMSM array to be created or modified if
detects that it is running on an Intel platform which
supports IMSM, and supports the particular configuration
of IMSM that is being requested (some functionality
requires newer OROM support).
These checks can be suppressed by setting
IMSM_NO_PLATFORM=1 in the environment. This can be useful
for testing or for disaster recovery. You should be aware
that interoperability may be compromised by setting this
value.
These change can also be suppressed by adding
mdadm.imsm.test=1 to the kernel command line. This makes
it easy to test IMSM code in a virtual machine that
doesn't have IMSM virtual hardware.
MDADM_GROW_ALLOW_OLD
If an array is stopped while it is performing a reshape
and that reshape was making use of a backup file, then
when the array is re-assembled mdadm will sometimes
complain that the backup file is too old. If this happens
and you are certain it is the right backup file, you can
over-ride this check by setting MDADM_GROW_ALLOW_OLD=1 in
the environment.
MDADM_CONF_AUTO
Any string given in this variable is added to the start of
the AUTO line in the config file, or treated as the whole
AUTO line if none is given. It can be used to disable
certain metadata types when mdadm is called from a boot
script. For example
export MDADM_CONF_AUTO='-ddf -imsm'
will make sure that mdadm does not automatically assemble
any DDF or IMSM arrays that are found. This can be useful
on systems configured to manage such arrays with dmraid.
mdadm --query /dev/name-of-device
This will find out if a given device is a RAID array, or is part
of one, and will provide brief information about the device.
mdadm --assemble --scan
This will assemble and start all arrays listed in the standard
config file. This command will typically go in a system startup
file.
mdadm --stop --scan
This will shut down all arrays that can be shut down (i.e. are
not currently in use). This will typically go in a system
shutdown script.
mdadm --follow --scan --delay=120
If (and only if) there is an Email address or program given in
the standard config file, then monitor the status of all arrays
listed in that file by polling them ever 2 minutes.
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1
Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and
/dev/hdc1.
echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf
mdadm --detail --scan >> mdadm.conf
This will create a prototype config file that describes currently
active arrays that are known to be made from partitions of IDE or
SCSI drives. This file should be reviewed before being used as
it may contain unwanted detail.
echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf
mdadm --examine --scan --config=mdadm.conf >> mdadm.conf
This will find arrays which could be assembled from existing IDE
and SCSI whole drives (not partitions), and store the information
in the format of a config file. This file is very likely to
contain unwanted detail, particularly the devices= entries. It
should be reviewed and edited before being used as an actual
config file.
mdadm --examine --brief --scan --config=partitions
mdadm -Ebsc partitions
Create a list of devices by reading /proc/partitions, scan these
for RAID superblocks, and printout a brief listing of all that
were found.
mdadm -Ac partitions -m 0 /dev/md0
Scan all partitions and devices listed in /proc/partitions and
assemble /dev/md0 out of all such devices with a RAID superblock
with a minor number of 0.
mdadm --monitor --scan --daemonise > /run/mdadm/mon.pid
If config file contains a mail address or alert program, run
mdadm in the background in monitor mode monitoring all md
devices. Also write pid of mdadm daemon to /run/mdadm/mon.pid.
mdadm -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as
appropriate.
mdadm --incremental --rebuild-map --run --scan
Rebuild the array map from any current arrays, and then start any
that can be started.
mdadm /dev/md4 --fail detached --remove detached
Any devices which are components of /dev/md4 will be marked as
faulty and then remove from the array.
mdadm --grow /dev/md4 --level=6 --backup-file=/root/backup-md4
The array /dev/md4 which is currently a RAID5 array will be
converted to RAID6. There should normally already be a spare
drive attached to the array as a RAID6 needs one more drive than
a matching RAID5.
mdadm --create /dev/md/ddf --metadata=ddf --raid-disks 6
/dev/sd[a-f]
Create a DDF array over 6 devices.
mdadm --create /dev/md/home -n3 -l5 -z 30000000 /dev/md/ddf
Create a RAID5 array over any 3 devices in the given DDF set.
Use only 30 gigabytes of each device.
mdadm -A /dev/md/ddf1 /dev/sd[a-f]
Assemble a pre-exist ddf array.
mdadm -I /dev/md/ddf1
Assemble all arrays contained in the ddf array, assigning names
as appropriate.
mdadm --create --help
Provide help about the Create mode.
mdadm --config --help
Provide help about the format of the config file.
mdadm --help
Provide general help.
/proc/mdstat
If you're using the /proc filesystem, /proc/mdstat lists all
active md devices with information about them. mdadm uses this
to find arrays when --scan is given in Misc mode, and to monitor
array reconstruction on Monitor mode.
/etc/mdadm.conf (or /etc/mdadm/mdadm.conf)
Default config file. See mdadm.conf(5) for more details.
/etc/mdadm.conf.d (or /etc/mdadm/mdadm.conf.d)
Default directory containing configuration files. See
mdadm.conf(5) for more details.
/run/mdadm/map
When --incremental mode is used, this file gets a list of arrays
currently being created.
A valid name can only consist of characters "A-Za-z0-9.-_". The
name cannot start with a leading "-" and cannot exceed 255 chars.
mdadm understand two sorts of names for array devices.
The first is the so-called 'standard' format name, which matches
the names used by the kernel and which appear in /proc/mdstat.
The second sort can be freely chosen, but must reside in
/dev/md/. When giving a device name to mdadm to create or
assemble an array, either full path name such as /dev/md0 or
/dev/md/home can be given, or just the suffix of the second sort
of name, such as home can be given.
In every style, raw name must be compatible with POSIX PORTABLE
NAME and has to be no longer than 32 chars.
When mdadm chooses device names during auto-assembly or
incremental assembly, it will sometimes add a small sequence
number to the end of the name to avoid conflicted between
multiple arrays that have the same name. If mdadm can reasonably
determine that the array really is meant for this host, either by
a hostname in the metadata, or by the presence of the array in
mdadm.conf, then it will leave off the suffix if possible. Also
if the homehost is specified as <ignore> mdadm will only use a
suffix if a different array of the same name already exists or is
listed in the config file.
The standard names for non-partitioned arrays (the only sort of
md array available in 2.4 and earlier) are of the form
/dev/mdNN
where NN is a number. The standard names for partitionable
arrays (as available from 2.6 onwards) are of the form:
/dev/md_dNN
Partition numbers should be indicated by adding "pMM" to these,
thus "/dev/md/d1p2".
From kernel version 2.6.28 the "non-partitioned array" can
actually be partitioned. So the "md_dNN" names are no longer
needed, and partitions such as "/dev/mdNNpXX" are possible.
From kernel version 2.6.29 standard names can be non-numeric
following the form:
/dev/md_XXX
where XXX is any string. These names are supported by mdadm
since version 3.3 provided they are enabled in mdadm.conf.
EXAMINE
checkpoint
Checkpoint value is reported when array is performing some
action including resync, recovery or reshape. Checkpoints
allow resuming action from certain point if it was
interrupted.
Checkpoint is reported as combination of two values:
current migration unit and number of blocks per unit. By
multiplying those values and dividing by array size
checkpoint progress percentage can be obtained in relation
to current progress reported in /proc/mdstat. Checkpoint
is also related to (and sometimes based on) sysfs entry
sync_completed but depending on action units may differ.
Even if units are the same, it should not be expected that
checkpoint and sync_completed will be exact match nor
updated simultaneously.
mdadm was previously known as mdctl.
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).
This page is part of the mdadm (Tool for managing md arrays in
Linux) project. Information about the project can be found at
⟨http://neil.brown.name/blog/mdadm⟩. If you have a bug report for
this manual page, send it to [email protected]. This
page was obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-13.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
v4.3 MDADM(8)
Pages that refer to this page: md(4), mdadm.conf(5), blkdeactivate(8), mdmon(8), raid6check(8)
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