MDADM(8)MDADM(8)NAMEmdadm - manage MD devices aka Linux Software RAID
SYNOPSISmdadm [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 parti‐
tions thereof) to be combined into a single device to hold (for exam‐
ple) 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 CON‐
MULTIPATH is not a Software RAID mechanism, but does involve multiple
devices: each device is a path to one common physical storage device.
New installations should not use md/multipath as it is not well sup‐
ported and has no ongoing development. Use the Device Mapper based
FAULTY is also not true RAID, and it only involves one device. It pro‐
vides a layer over a true device that can be used to inject faults.
CONTAINER is different again. A CONTAINER is a collection of devices
that are managed as a set. This is similar to the set of devices con‐
nected to a hardware RAID controller. The set of devices may contain a
number of different RAID arrays each utilising some (or all) of the
blocks from a number of the devices in the set. For example, two
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.
MODESmdadm has several major modes of operation:
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 differ‐
entiate between initial creation and subsequent assembly of an
array. It also cannot perform any checks that appropriate com‐
ponents 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
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.
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.
This mode does not act on a specific device or array, but rather
it requests the Linux Kernel to activate any auto-detected
OPTIONSOptions for selecting a mode are:-A, --assemble
Assemble a pre-existing array.
Build a legacy array without superblocks.
Create a new array.
-F, --follow, --monitor
Select Monitor mode.
Change the size or shape of an active array.
Add/remove a single device to/from an appropriate array, and
possibly start the array.
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
--add, --fail, --remove, or --replace, then the MANAGE mode is assumed.
Anything other than these will cause the Misc mode to be assumed.
Options that are not mode-specific are:
Display general help message or, after one of the above options,
a mode-specific help message.
Display more detailed help about command line parsing and some
commonly used options.
Print version information for mdadm.
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.
Avoid printing purely informative messages. With this, mdadm
will be silent unless there is something really important to
Be more forceful about certain operations. See the various
modes for the exact meaning of this option in different con‐
Specify the config file or directory. Default is to use
/etc/mdadm.conf and /etc/mdadm.conf.d, or if those are missing
then /etc/mdadm/mdadm.conf and /etc/mdadm/mdadm.conf.d. 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
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 iden‐
tities, 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.
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.
Use the original 0.90 format superblock. This format
limits arrays to 28 component devices and limits compo‐
nent devices of levels 1 and greater to 2 terabytes. It
is also possible for there to be confusion about whether
the superblock applies to a whole device or just the last
partition, if that partition starts on a 64K boundary.
1, 1.0, 1.1, 1.2 default
Use the new version-1 format superblock. This has fewer
restrictions. It can easily be moved between hosts with
different endian-ness, and a recovery operation can be
checkpointed and restarted. The different sub-versions
store the superblock at different locations on the
device, either at the end (for 1.0), at the start (for
1.1) or 4K from the start (for 1.2). "1" is equivalent
to "1.2" (the commonly preferred 1.x format). "default"
is equivalent to "1.2".
ddf Use the "Industry Standard" DDF (Disk Data Format) format
defined by SNIA. When creating a DDF array a CONTAINER
will be created, and normal arrays can be created in that
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
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 later half of the UUID.
When reporting information about an array, any array which is
tagged for the given homehost will be reported as such.
When using Auto-Assemble, only arrays tagged for the given home‐
host will be allowed to use 'local' names (i.e. not ending in
'_' followed by a digit string). See below under Auto Assembly.
When mdadm needs to print the name for a device it normally
finds the name in /dev which refers to the device and is short‐
est. When a path component is given with --prefermdadm will
prefer a longer name if it contains that component. For example
--prefer=by-uuid will prefer a name in a subdirectory of /dev
This functionality is currently only provided by --detail and
For create, build, or grow:
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 prob‐
ably 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
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.
Amount (in Kibibytes) of space to use from each drive in RAID
levels 1/4/5/6. 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 warn‐
ing is issued.
A suffix of 'M' or 'G' can be given to indicate Megabytes or
Sometimes a replacement drive can be a little smaller than the
original drives though this should be minimised by IDEMA stan‐
dards. Such a replacement drive will be rejected by md. To
guard against this it can be useful to set the initial size
slightly smaller than the smaller device with the aim that it
will still be larger than any replacement.
This value can be set with --grow for RAID level 1/4/5/6 though
CONTAINER based arrays such as those with IMSM metadata may not
be able to support this. 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
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.
This value cannot be used when creating a CONTAINER such as with
DDF and IMSM metadata, though it perfectly valid when creating
an array inside a container.
This is only meaningful with --grow and its effect is not per‐
sistent: 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 appro‐
priate 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 'M' or 'G' can be given to indicate Megabytes or
Gigabytes respectively. A value of max restores the apparent
size of the array to be whatever the real amount of available
Specify chunk size of kibibytes. The default when creating an
array is 512KB. To ensure compatibility with earlier versions,
the default when Building and 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. In any case it must be a multiple of 4KB.
A suffix of 'M' or 'G' can be given to indicate Megabytes or
Specify 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.
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.
This option configures the fine details of data layout for
RAID5, RAID6, and RAID10 arrays, and controls the failure modes
The layout of the RAID5 parity block can be one of left-asymmet‐
ric, left-symmetric, right-asymmetric, right-symmetric, la, ra,
ls, rs. The default is left-symmetric.
It is also possible to cause RAID5 to use a RAID4-like layout by
choosing parity-first, or parity-last.
Finally for RAID5 there are DDF-compatible layouts,
ddf-zero-restart, ddf-N-restart, and ddf-N-continue.
These same layouts are available for RAID6. There are also 4
layouts that will provide an intermediate stage for converting
between RAID5 and RAID6. These provide a layout which is iden‐
tical 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-sym‐
metric-6, right-symmetric-6, left-asymmetric-6, right-asymmet‐
ric-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,
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.
Finally, the layout options for RAID10 are one of 'n', 'o' or
'f' followed by a small number. The default is 'n2'. The sup‐
ported 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
The number is the number of copies of each datablock. 2 is nor‐
mal, 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).
When an array is converted between RAID5 and RAID6 an intermedi‐
ate RAID6 layout is used in which the second parity block (Q) is
always on the last device. To convert a RAID5 to RAID6 and
leave it in this new layout (which does not require re-striping)
use --layout=preserve. This will try to avoid any restriping.
The converse of this is --layout=normalise which will change a
non-standard RAID6 layout into a more standard arrangement.
same as --layout (thus explaining the p of -p).
Specify a file to store a write-intent bitmap in. The file
should not exist unless --force is also given. The same file
should be provided when assembling the array. If the word
internal is given, then the bitmap is stored with the metadata
on the array, and so is replicated on all devices. If the word
none is given with --grow mode, then any bitmap that is present
To help catch typing errors, the filename must contain at least
one slash ('/') if it is a real file (not 'internal' or 'none').
Note: external bitmaps are only known to work on ext2 and ext3.
Storing bitmap files on other filesystems may result in serious
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 .
Set the chunksize 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 bit‐
map, the chunksize defaults to 64Meg, or larger if necessary to
fit the bitmap into the available space.
A suffix of 'M' or 'G' can be given to indicate Megabytes or
subsequent devices listed in a --build, --create, or --add com‐
mand will be flagged as 'write-mostly'. This is valid for RAID1
only and means that the 'md' driver will avoid reading from
these devices if at all possible. This can be useful if mirror‐
ing over a slow link.
Specify that write-behind mode should be enabled (valid for
RAID1 only). If an argument is specified, it will set the maxi‐
mum 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
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 actu‐
ally write to the array. It can also be used when creating a
RAID1 or RAID10 if you want to avoid the initial resync, however
this practice — while normally safe — is not recommended. Use
this only if you really know what you are doing.
When the devices that will be part of a new array were filled
with zeros before creation the operator knows the array is actu‐
ally clean. If that is the case, such as after running bad‐
blocks, this argument can be used to tell mdadm the facts the
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 auto‐
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
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
Setting the offset explicitly over-rides the default. The value
given is in Kilobytes unless an 'M' or 'G' suffix is given.
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 vari‐
able. In the case each member device is expected to have a off‐
set 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).
This option is complementary to the --freeze-reshape option for
assembly. It is needed when --grow operation is interrupted and
it is not restarted automatically due to --freeze-reshape usage
during array assembly. This option is used together with -G , (
--grow ) command and device for a pending reshape to be contin‐
ued. All parameters required for reshape continuation will be
read from array metadata. If initial --grow command had
required --backup-file= option to be set, continuation option
will require to have exactly the same backup file given as well.
Any other parameter passed together with --continue option will
Set a name for the array. This is currently only effective when
creating an array with a version-1 superblock, or an array in a
DDF container. The name is a simple textual string that can be
used to identify array components when assembling. If name is
needed but not specified, it is taken from the basename of the
device that is being created. e.g. when creating /dev/md/home
the name will default to home.
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.
Insist that mdadm accept the geometry and layout specified with‐
out question. Normally mdadm will not allow 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.
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.
Instruct mdadm how to create the device file if needed, possibly
allocating an unused minor number. "md" causes a non-partition‐
able 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
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 a 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
This is meaningful with --create or --build.
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
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 com‐
For assemble:-u, --uuid=
uuid of array to assemble. Devices which don't have this uuid
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
Giving the literal word "dev" for --super-minor will cause mdadm
to use the minor number of the md device that is being assem‐
bled. 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.
Specify the name of the array to assemble. 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
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. An array which requires --force
to be started may contain data corruption. Use it carefully.
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.
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.
See this option under Create and Build options.
Specify the bitmap file that was given when the array was cre‐
ated. If an array has an internal bitmap, there is no need to
specify this when assembling the array.
If --backup-file was used while reshaping an array (e.g. chang‐
ing number of devices or chunk size) and the system crashed dur‐
ing 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.
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.
Update the superblock on each device while assembling the array.
The argument given to this flag can be one of sparc2.2, sum‐
maries, uuid, naml, homehost, resync, byteorder, devicesize,
no-bitmap, bbl, no-, 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 ker‐
nel. 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)
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
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 resync option will cause the array to be marked dirty mean‐
ing 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 redun‐
dant information is correct.
The byteorder option allows arrays to be moved between machines
with different byte-order. 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)
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 ver‐
sion 1.1 and 1.2 metadata only (where the metadata is at the
start of the device) and is only useful when the component
device has changed size (typically become larger). The version
1 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-
The old metadata will remain on the devices, but will appear
older than the new metadata and so will usually be ignored. The
old metadata (or indeed the new metadata) can be removed by giv‐
ing the appropriate --metadata= option to --zero-superblock.
The no-bitmap option can be used when an array has an internal
bitmap which is corrupt in some way so that assembling the array
normally fails. It will cause any internal bitmap to be
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
Option is intended to be used in start-up scripts during initrd
boot phase. When array under reshape is assembled during initrd
phase, this option stops reshape after reshape critical section
is being restored. This happens before file system pivot opera‐
tion and avoids loss of file system context. Losing file system
context would cause reshape to be broken.
Reshape can be continued later using the --continue option for
the grow command.
For Manage mode:-t, --test
Unless a more serious error occurred, mdadm will exit with a
status of 2 if no changes were made to the array and 0 if at
least one change was made. This can be useful when an indirect
specifier such as missing, detached or faulty is used in
requesting an operation on the array. --test will report fail‐
ure if these specifiers didn't find any match.
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 immedi‐
ately 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 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 recov‐
ered. However based on the event count on the device, the
recovery may only require sections that are flagged a write-
intent bitmap to be recovered or may not require any recovery at
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.
When used with v1.x metadata, --re-add can be accompanied by
--update=devicesize, --update=bbl, or --update=no-bbl. See the
description of these option 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.
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 device 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 describe below under --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
same as --fail.
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
--with This can follow a list of --replace devices. The devices listed
after --with will be preferentially used to replace the devices
listed after --replace. These device must already be spare
devices in the array.
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
Subsequent devices that are added or re-added will have the
'write-mostly' flag cleared.
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 --buildmdadm needs to be
told that this device we removed recently with --re-add.
Devices can only be removed from an array if they are not in active
use, i.e. that must be spares or failed devices. To remove an active
device, it must first be marked as faulty.
For Misc mode:-Q, --query
Examine a device to see (1) if it is an md device and (2) if it
is a component of an md array. Information about what is dis‐
covered is presented.
Print details of one or more md devices.
Print details of the platform's RAID capabilities (firmware /
hardware topology) for a given metadata format. If used without
argument, mdadm will scan all controllers looking for their
capabilities. Otherwise, mdadm will only look at the controller
specified by the argument in form of an absolute filepath or a
link, e.g. /sys/devices/pci0000:00/0000:00:1f.2.
When used with --detail , --detail-platform or --examine, output
will be formatted as key=value pairs for easy import into the
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.
If an array was created on a SPARC machine with a 2.2 Linux ker‐
nel 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
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.
List the bad-blocks recorded for the device, if a bad-blocks
list has been configured. Currently only 1.x metadata supports
Save metadata from lists devices, or restore metadata to listed
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.
deactivate array, releasing all resources.
mark array as readonly.
mark array as readwrite.
If the device contains a valid md superblock, the block is over‐
written with zeros. With --force the block where the superblock
would be is overwritten even if it doesn't appear to be valid.
If the device is a container and the argument to --kill-subarray
specifies an inactive subarray in the container, then the subar‐
ray is deleted. Deleting all subarrays will leave an 'empty-
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
If the device is a container and the argument to --update-subar‐
ray specifies a subarray in the container, then attempt to
update the given superblock field in the subarray. See below in
MISC MODE for details.
When used with --detail, the exit status of mdadm is set to
reflect the status of the device. See below in MISC MODE for
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, oth‐
erwise it will return failure.
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.
For Incremental Assembly mode:--rebuild-map, -r
Rebuild the map file (/run/mdadm/map) that mdadm uses to help
track which arrays are currently being assembled.
Run any array assembled as soon as a minimal number of devices
are available, rather than waiting until all expected devices
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
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
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 nor‐
mally only set by a udev script.
For Monitor mode:-m, --mail
Give a mail address to send alerts to.
-p, --program, --alert
Give a program to be run whenever an event is detected.
Cause all events to be reported through 'syslog'. The messages
have facility of 'daemon' and varying priorities.
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.
Give a percentage increment. mdadm will generate RebuildNN
events with the given percentage increment.
Tell mdadm to run as a background daemon if it decides to moni‐
tor 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.
When mdadm is running in daemon mode, write the pid of the dae‐
mon process to the specified file, instead of printing it on
Check arrays only once. This will generate NewArray events and
more significantly DegradedArray and SparesMissing events. Run‐
mdadm--monitor --scan -1
from a cron script will ensure regular notification of any
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 suc‐
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 compo‐
nents. For each array, mdadm needs to know the md device, the identity
of the array, and a 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 configura‐
tion file are assembled. If no arrays are described by the configura‐
tion file, then any arrays that can be found on unused devices will be
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 later case,
/etc/mdadm.conf or /etc/mdadm/mdadm.conf is used.
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 distinctly dif‐
ferent 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 type of
devices can be partitioned. mdadm will normally create the type that
originally could not be partitioned as it has a well defined major num‐
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.
When --assemble is used with --scan and no devices are listed, mdadm
will first attempt to assemble all the arrays listed in the config
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 meta‐
data, then the name from the superblock is used to similarly create a
name in /dev/md/ (the name will have any 'host' prefix stripped first).
This behaviour can be modified by the AUTO line in the mdadm.conf con‐
figuration file. This line can indicate that specific metadata type
should, or should not, be automatically assembled. If an array is
found which is not listed in mdadm.conf and has a metadata format that
is 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
Usage: mdadm--build md-device --chunk=X --level=Y --raid-devices=Z
This usage is similar to --create. The difference is that it creates
an array without a superblock. With these arrays there is no differ‐
ence between initially creating the array and subsequently assembling
the array, except that hopefully there is useful data there in the sec‐
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
This usage will initialise a new md array, associate some devices with
it, and activate the array.
The named device will normally not exist when mdadm--create is run,
but will be created by udev once the array becomes active.
As devices are added, they are checked to see if they contain RAID
superblocks or filesystems. They are also checked to see if the vari‐
ance 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 an array with version-1 metadata a name for the array is
required. If this is not given with the --name option, mdadm will
choose a name based on the last component of the name of the device
being created. So if /dev/md3 is being created, then the name 3 will
be chosen. If /dev/md/home is being created, then the name home will
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 selection allows for greater precision since using any other [RAID
auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create prob‐
lems 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 cre‐
ating two arrays with the same UUID is a recipe for disaster. Also,
using --uuid= when creating a v0.90 array will silently override any
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. In any case space for a bitmap will be
reserved so that one can be added layer with --grow --bitmap=internal.
If the metadata type supports it (currently only 1.x metadata), space
will be allocated to store a bad block list. This allows a modest num‐
ber of bad blocks to be recorded, allowing the drive to remain in ser‐
vice 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 for‐
mer 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.
start the array readonly — not supported yet.
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 dis‐
tinct devices. The operations are:
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
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
4 There was an error while trying to get information about
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
If the device is a container and the argument to --update-subar‐
ray specifies a subarray in the container, then attempt to
update the given superblock field in the subarray. Similar to
updating an array in "assemble" mode, the field to update is
selected by -U or --update= option. Currently only name is sup‐
The name option updates the subarray name in the metadata, it
may not affect the device node name or the device node symlink
until the subarray is re-assembled. If updating name would
change the UUID of an active subarray this operation is blocked,
and the command will end in an error.
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.
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 have the metadata writ‐
ten in the file at the same locate that it exists in the device.
However the file will be "sparse" so that only those blocks con‐
taining metadata will be allocated. The total space used will be
The file name 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.
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
If a file name is given instead of a directory then mdadm will
restore from that file to a single device, always provided the
size of the file matches that of the device, and the file con‐
tains valid metadata.
--stop The devices should be active md arrays which will be deacti‐
vated, as long as they are not currently in use.
--run This will fully activate a partially assembled md array.
This will mark an active array as read-only, providing that it
is not currently being used.
This will change a readonly array back to being read/write.
--scan For all operations except --examine, --scan will cause the oper‐
ation to be applied to all arrays listed in /proc/mdstat. For
--examine, --scan causes all devices listed in the config file
to be examined.
Be less verbose. This is used with --detail and --examine.
Using --brief with --verbose gives an intermediate level of ver‐
Usage: mdadm--monitor options... devices...
This usage causes mdadm to periodically poll a number of md arrays and
to report on any events noticed. mdadm will never exit once it decides
that there are arrays to be checked, so it should normally be run in
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.
If any devices are listed on the command line, mdadm will only monitor
those devices. Otherwise all arrays listed in the configuration file
will be monitored. Further, if --scan is given, then any other md
devices that appear in /proc/mdstat will also be monitored.
The result of monitoring the arrays is the generation of events. These
events are passed to a separate program (if specified) and may be
mailed to a given E-mail address.
When passing events to a program, the program is run once for each
event, and is given 2 or 3 command-line arguments: the first is the
name of the event (see below), the second is the name of the md device
which is affected, and the third is the name of a related device if
relevant (such as a component device that has failed).
If --scan is given, then a program or an E-mail address must be speci‐
fied on the command line or in the config file. If neither are avail‐
able, then mdadm will not monitor anything. Without --scan, mdadm will
continue monitoring as long as something was found to monitor. If no
program or email is given, then each event is reported to stdout.
The different events are:
An md array which previously was configured appears to no
longer be configured. (syslog priority: Critical)
If mdadm was told to monitor an array which is RAID0 or Lin‐
ear, then it will report DeviceDisappeared with the extra
information Wrong-Level. This is because RAID0 and Linear
do not support the device-failed, hot-spare and resync oper‐
ations which are monitored.
An md array started reconstruction. (syslog priority: Warn‐
Where NN is a two-digit number (ie. 05, 48). This indicates
that rebuild has passed that many percent of the total. The
events are generated with fixed increment since 0. Increment
size may be specified with a commandline option (default is
20). (syslog priority: Warning)
An md array that was rebuilding, isn't any more, either
because it finished normally or was aborted. (syslog prior‐
Fail An active component device of an array has been marked as
faulty. (syslog priority: Critical)
A spare component device which was being rebuilt to replace
a faulty device has failed. (syslog priority: Critical)
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)
A new md array has been detected in the /proc/mdstat file.
(syslog priority: Info)
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
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)
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)
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 configura‐
tion file. The spare-group name can be any string; it is only neces‐
sary 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 drive and a spare. It will then
attempt to remove the spare from the second drive 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. For this to work, the kernel must support the necessary change.
Various types of growth are being added during 2.6 development.
Currently the supported changes include
· 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
· convert between RAID1 and RAID5, between RAID5 and RAID6, between
RAID0, RAID4, and RAID5, and between RAID0 and RAID10 (in the
· add a write-intent bitmap to any array which supports these bit‐
maps, or remove a write-intent bitmap from such an array.
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 sup‐
ported by the container, and the conversion is on of those listed
above. Resizing arrays in an IMSM container with --grow --size is not
Grow functionality (e.g. expand a number of raid devices) for Intel's
IMSM container format has an experimental status. It is guarded by the
MDADM_EXPERIMENTAL environment variable which must be set to '1' for a
GROW command to succeed. This is for the following reasons:
1. Intel's native IMSM check-pointing is not fully tested yet.
This can causes IMSM incompatibility during the grow process: an
array which is growing cannot roam between Microsoft Windows(R)
and Linux systems.
2. Interrupting a grow operation is not recommended, because it has
not been fully tested for Intel's IMSM container format yet.
Note: Intel's native checkpointing doesn't use --backup-file option and
it is transparent for assembly feature.
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, one at a
time, removed and replaced with larger drives, then you could have an
array of large drives with only a small amount used. In this situa‐
tion, 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 synchro‐
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
Also the size of an array cannot be changed while it has an active bit‐
map. 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.
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
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 inter‐
rupted "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
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
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 grow‐
ing the array, the reshape is done from the end towards the beginning,
so the "critical section" is at the end of the reshape.
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 "cricital section" but through‐
out the reshape operation, as described below under LAYOUT CHANGES.
CHUNK-SIZE AND LAYOUT CHANGES
Changing the chunk-size of 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
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.
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.
Usage: mdadm--incremental [--run] [--quiet] component-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. Similar if DEVICES contains the
special word partitions then any device is allowed. Otherwise
the device name given to mdadm must match one of the names or
patterns in a DEVICES line.
+ Does the device have a valid md superblock? If a specific meta‐
data version is requested with --metadata or -e then only that
style of metadata is accepted, otherwise mdadm finds any known
version of metadata. If no md metadata is found, the device may
be still added to an array as a spare if POLICY allows.
mdadm keeps a list of arrays that it has partially assembled in
/run/mdadm/map. If no array exists which matches the metadata on the
new device, mdadm must choose a device name and unit number. It does
this based on any name given in mdadm.conf or any name information
stored in the metadata. If this name suggests a unit number, that num‐
ber will be used, otherwise a free unit number will be chosen. Nor‐
mally 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 nor‐
mally 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
can be run which will try to start all arrays that are being incremen‐
tally assembled. They are started in "read-auto" mode in which they
are read-only until the first write request. This means that no meta‐
data updates are made and no attempt at resync or recovery happens.
Further devices that are found before the first write can still be
This section describes environment variables that affect how mdadm
Setting this value to 1 will prevent mdadm from automatically
launching mdmon. This variable is intended primarily for debug‐
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.
A key value of IMSM metadata is that it allows interoperability
with boot ROMs on Intel platforms, and with other major operat‐
ing 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 functional‐
ity 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 com‐
promised by setting this value.
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 sys‐
tems configured to manage such arrays with dmraid.
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.
This will assemble and start all arrays listed in the standard config
file. This command will typically go in a system startup file.
This will shut down all arrays that can be shut down (i.e. are not cur‐
rently in use). This will typically go in a system shutdown script.
mdadm--follow --scan --delay=120
If (and only if) there is an Email address or program given in the
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 con‐
tain 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
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.
Try to incorporate newly discovered device into some array as appropri‐
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.
Assemble all arrays contained in the ddf array, assigning names as
Provide help about the Create mode.
Provide help about the format of the config file.
Provide general help.
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.
The config file lists which devices may be scanned to see if they con‐
tain MD super block, and gives identifying information (e.g. UUID)
about known MD arrays. See mdadm.conf(5) for more details.
A directory containing configuration files which are read in lexical
When --incremental mode is used, this file gets a list of arrays cur‐
rently being created.
DEVICE NAMESmdadm 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.
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 possi‐
ble. Also if the homehost is specified as <ignore> mdadm will only use
a suffix if a different array of the same name already exists or is
listed in the config file.
The standard names for non-partitioned arrays (the only sort of md
array available in 2.4 and earlier) are of the form
where NN is a number. The standard names for partitionable arrays (as
available from 2.6 onwards) are of the form:
Partition numbers should be indicated by adding "pMM" to these, thus
From kernel version 2.6.28 the "non-partitioned array" can actually be
partitioned. So the "md_dNN" names are no longer needed, and parti‐
tions such as "/dev/mdNNpXX" are possible.
From kernel version 2.6.29 standard names can be non-numeric following
where XXX is any string. These names are supported by mdadm since ver‐
sion 3.3 provided they are enabled in mdadm.conf.
NOTEmdadm was previously known as mdctl.
For further information on mdadm usage, MD and the various levels of
(based upon Jakob Østergaard's Software-RAID.HOWTO)
The latest version of mdadm should always be available from
Related man pages:
mdmon(8), mdadm.conf(5), md(4).