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, and FAULTY.
MULTIPATH is not a Software RAID mechanism, but does involve multiple
devices: each device is a path to one common physical storage device.
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.
mdadm 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 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 superblocks.
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 RAID levels 1/4/5/6, as well as adding or removing a
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.
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
Options for selecting a mode are:
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 a single device into 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. 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, or --remove, then the MANAGE mode is assume. 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 less verbose. This is used with --detail and --examine.
Using --brief with --verbose gives an intermediate level of
Be more forceful about certain operations. See the various
modes for the exact meaning of this option in different
Specify the config file. Default is to use
/etc/mdadm/mdadm.conf, or if that is missing, then
/etc/mdadm.conf. 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 and will read
/proc/partitions to find a list of devices to scan. If the word
none is given for the config file, then mdadm will act as though
the config file were empty.
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 superblock (raid metadata) to be used. The
default is 0.90 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.
0, 0.90, default
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.
1, 1.0, 1.1, 1.2
Use the new version-1 format superblock. This has few
restrictions. 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).
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
superblock. 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
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 assembled.
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
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 or raid5.
This number can only be changed using --grow for RAID1, RAID5
and RAID6 arrays, and only on kernels which provide necessary
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
level 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
warning is issued.
This value can be set with --grow for RAID level 1/4/5/6. 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.
Specify chunk size of kibibytes. The default is 64.
Specify rounding factor for linear array (==chunk size)
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. Obviously some of these
When used with --build, only linear, stripe, raid0, 0, raid1,
multipath, mp, and faulty are valid.
Not yet supported with --grow.
This option configures the fine details of data layout for
raid5, and raid10 arrays, and controls the failure modes for
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.
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.
To set the parity with --grow, the level of the array ("faulty")
must be specified before the fault mode is specified.
Finally, the layout options for RAID10 are one of ’n’, ’o’ or
’f’ followed by a small number. 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’ and ’far’.
The number is the number of copies of each datablock. 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).
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
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
bitmap, the chunksize is automatically determined to make best
use of available space.
subsequent devices lists 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.
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.
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
This is needed when --grow is used to increase the number of
raid-devices in a RAID5 if there are no spare devices
available. See the section below on RAID_DEVICE CHANGES. The
file should be stored on a separate device, not on the raid
array being reshaped.
Set a name for the array. This is currently only effective when
creating an array with a version-1 superblock. The name is a
simple textual string that can be used to identify array
components when assembling.
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
without 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.
Instruct mdadm to create the device file if needed, possibly
allocating an unused minor number. "md" causes a non-
partitionable array to be used. "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. See DEVICE NAMES
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/home1p3".
If there is no trailing digit, then the partition names just
have a number added, e.g. "/dev/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 number based on that name. If the device name is
not in one of these formats, then a unused minor number will be
allocated. The minor 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.
Normally when --auto causes mdadm to create devices in /dev/md/
it will also create symlinks from /dev/ with names starting with
md or md_. Use --symlink=no to suppress this, or --symlink=yes
to enforce this even if it is suppressing mdadm.conf.
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
assembled. e.g. when assembling /dev/md0, --super-minor=dev
will look for super blocks with a minor number of 0.
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 some superblocks appear out-of-date
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
created. If an array has an internal bitmap, there is no need
to specify this when assembling the array.
If --backup-file was used to grow the number of raid-devices in
a RAID5, 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.
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, homehost, resync, byteorder, devicesize,
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)
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
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. 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 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.
This flag is only meaningful with auto-assembly (see discussion
below). In that situation, if no suitable arrays are found for
this homehost, mdadm will rescan for any arrays at all and will
assemble them and update the homehost to match the current host.
For Manage mode:
hot-add listed devices.
re-add a device that was recently removed from an array.
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 and detached 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. This will only succeed for devices that are spares or
have already been marked as failed.
mark listed devices as faulty. As well as the name of a device
file, the word detached can be given. This will cause any
device that has been detached from the system to be marked as
failed. It can then be removed.
same as --fail.
Each of these options require that the first device listed is the array
to be acted upon, and the remainder are component devices to be added,
removed, or marked as faulty. Several different operations can be
specified for different devices, e.g.
mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1
Each operation applies to all devices listed until the next operation.
If an array is using a write-intent bitmap, then devices which have
been removed can be re-added in a way that avoids a full reconstruction
but instead just updates the blocks that have changed since the device
was removed. For arrays with persistent metadata (superblocks) this is
done automatically. For arrays created with --build mdadm needs to be
told that this device we removed recently with --re-add.
Devices can only be removed from an array if they are not in active
use, i.e. that must be spares or failed devices. To remove an active
device, it must first be marked as faulty.
For Misc mode:
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.
Print detail of one or more md devices.
When used with --detail or --examine, output will be formatted
as key=value pairs for easy import into the environment.
Print content of md superblock on device(s).
If an array was created on 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
Report information about a bitmap file. The argument is either
an external bitmap file or an array component in case of an
start a partially built array.
deactivate array, releasing all resources.
mark array as readonly.
mark array as readwrite.
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
When used with --detail, the exit status of mdadm is set to
reflect the status of the device.
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.
For Incremental Assembly mode:
Rebuild the map file (/var/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
For Monitor mode:
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
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 form 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
daemon 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.
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
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 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
configuration file are 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, with 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/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 insufficient drives were listed to start a
complete (non-degraded) array, 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 the md device does not exist, then it will be created providing the
intent is clear. i.e. the name must be in a standard form, or the
--auto option must be given to clarify how and whether the device
should be created. This can be useful for handling partitioned devices
(which don’t have a stable device number — it can change after a
reboot) and when using "udev" to manage your /dev tree (udev cannot
handle md devices because of the unusual device initialisation
If the option to "auto" is "mdp" or "part" or (on the command line
only) "p", then mdadm will create a partitionable array, using the
first free one that is not in use and does not already have an entry in
/dev (apart from numeric /dev/md* entries).
If the option to "auto" is "yes" or "md" or (on the command line)
nothing, then mdadm will create a traditional, non-partitionable md
It is expected that the "auto" functionality will be used to create
device entries with meaningful names such as "/dev/md/home" or
"/dev/md/root", rather than names based on the numerical array number.
When using option "auto" to create a partitionable array, the device
files for the first 4 partitions are also created. If a different
number is required it can be simply appended to the auto option. e.g.
"auto=part8". Partition names are created by appending a digit string
to the device name, with an intervening "p" if the device name ends
with a digit.
The --auto option is also available in Build and Create modes. As
those modes do not use a config file, the "auto=" config option does
not apply to these modes.
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 a homehost has been specified (either in the config file or on the
command line), mdadm will look further for possible arrays and will try
to assemble anything that it finds which is tagged as belonging to the
given homehost. This is the only situation where mdadm will assemble
arrays without being given specific device name or identity information
for the array.
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
If mdadm cannot find any array for the given host at all, and if
--auto-update-homehost is given, then mdadm will search again for any
array (not just an array created for this host) and will assemble each
assuming --update=homehost. This will change the host tag in the
superblock so that on the next run, these arrays will be found without
the second pass. The intention of this feature is to support
transitioning a set of md arrays to using homehost tagging.
The reason for requiring arrays to be tagged with the homehost for auto
assembly is to guard against problems that can arise when moving
devices from one host to another.
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 difference
between initially creating the array and subsequently assembling the
array, except that hopefully there is useful data there in the second
The level may raid0, linear, multipath, or faulty, or one of their
synonyms. All devices must be listed and the array will be started once
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.
If the --auto option is given (as described in more detail in the
section on Assemble mode), then the md device will be created with a
suitable device number if necessary.
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 an array with version-1 metadata a name for the host is
required. If this is not given with the --name option, mdadm will
chose 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
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
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.
Usage: mdadm options ... devices ...
MISC mode includes a number of distinct operations that operate on
distinct 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 /etc/mdadm/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
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 /etc/mdadm/mdadm.conf.
Having --scan without listing any devices will cause all devices
listed in the config file to be examined.
--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.
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
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.
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 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
specified on the command line or in the config file. If neither are
available, 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
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.
An md array started reconstruction. (syslog priority:
Where NN is 20, 40, 60, or 80, this indicates that rebuild
has passed that many percentage of the total. (syslog
An md array that was rebuilding, isn’t any more, either
because it finished normally or was aborted. (syslog
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
to another to allow a failed drive to be replaced. (syslog
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 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 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.
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,
including restructuring a raid5 array to have more active devices.
Currently the only support available is to
· change the "size" attribute for RAID1, RAID5 and RAID6.
· increase the "raid-disks" attribute of RAID1, RAID5, and RAID6.
· add a write-intent bitmap to any array which supports these
bitmaps, or remove a write-intent bitmap from such an array.
Normally when an array is built the "size" it 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
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
Note that when an array changes size, any filesystem that may be stored
in the array will not automatically grow to use the space. The
filesystem will need to be explicitly told to use the extra space.
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.
Increasing the number of active devices in a RAID5 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 do this safely,
including restart and interrupted "reshape".
When relocating the first few stripes on a raid5, 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. This backup is normally 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. 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
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 --rebuild
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.
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 only add devices to an array which were previously
working (active or spare) parts of that array. It does not currently
support automatic inclusion of a new drive as a spare in some array.
mdadm --incremental requires a bug-fix in all kernels through 2.6.19.
Hopefully, this will be fixed in 2.6.20; alternately, apply the patch
which is included with the mdadm source distribution. If mdadm detects
that this bug is present, it will abort any attempt to use
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
metadata version is request 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 is
+ Does the metadata match an expected array? The metadata can
match in two ways. Either there is an array listed in
mdadm.conf which identifies the array (either by UUID, by name,
by device list, or by minor-number), or the array was created
with a homehost specified and that homehost matches the one in
mdadm.conf or on the command line. If mdadm is not able to
positively identify the array as belonging to the current host,
the device will be rejected.
+ mdadm keeps a list of arrays that it has partially assembled in
/var/run/mdadm/map (or /var/run/mdadm.map if the directory
doesn’t exist). 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.
+ 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
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.
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 > /var/run/mdadm
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 /var/run/mdadm.
mdadm -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as
mdadm --incremental --rebuild --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 --create --help
Provide help about the Create mode.
mdadm --config --help
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
contain MD super block, and gives identifying information (e.g. UUID)
about known MD arrays. See mdadm.conf(5) for more details.
When --incremental mode is used, this file gets a list of arrays
currently being created. If /var/run/mdadm does not exist as a
directory, then /var/run/mdadm.map is used instead.
While entries in the /dev directory can have any format you like, mdadm
has an understanding of ’standard’ formats which it uses to guide its
behaviour when creating device files via the --auto option.
The standard names for non-partitioned arrays (the only sort of md
array available in 2.4 and earlier) are either of
where NN is a number. The standard names for partitionable arrays (as
available from 2.6 onwards) are either of
Partition numbers should be indicated by added "pMM" to these, thus
mdadm was previously known as mdctl.
mdadm is completely separate from the raidtools package, and does not
use the /etc/raidtab configuration file at all.
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
raidtab(5), raid0run(8), raidstop(8), mkraid(8).