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 there-of) to be combined into a sin-
gle 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
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4
Recent kernels (2002) also support a mode known as MULTIPATH. mdadm only provides limited
support for MULTIPATH as yet.
mdadm is a program that can be used to create, manage, and monitor MD devices. As such it
provides a similar set of functionality to the raidtools packages. The key differences
between mdadm and raidtools are:
o mdadm is a single program and not a collection of programs.
o mdadm can perform (almost) all of its functions without having a configuration file.
Also mdadm helps with management of the configuration file.
o mdadm can provide information about your arrays (through Query, Detail, and Examine)
that raidtools cannot.
mdadm has 6 major modes of operation:
Assemble the parts 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 a legacy array without per-device superblocks.
Create Create a new array with per-device superblocks.
Manage This is for doing things to specific components of an array such as adding new
spares and removing faulty devices.
Misc This mode allows operations on independent devices such as examine MD superblocks,
erasing old superblocks and stopping active arrays.
Follow or Monitor
Monitor one or more md devices and act on any state changes.
Available options are:
Assemble a pre-existing array.
Build a legacy array without superblocks.
Create a new array.
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.
Print content of md superblock on device(s).
-F, --follow, --monitor
Select Monitor mode.
Display help message or, after above option, mode specific help message.
Print version information for mdadm.
Be more verbose about what is happening.
Be less verbose. This is used with --detail and --examine.
Be more forceful about certain operations. See the various modes of the exact
meaning of this option in different contexts.
Specify the config file. Default is /etc/mdadm.conf.
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:
/etc/mdadm.conf. 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.
For create or build:
Specify chunk size of kibibytes. The default is 64.
Specify rounding factor for linear array (==chunk size)
Set raid level. Options are: linear, raid0, 0, stripe, raid1, 1, mirror, raid5, 4,
raid5, 5, multipath, mp. Obviously some of these are synonymous. Only the first 4
are valid when Building.
Set raid5 parity algorithm. Options are: left-asymmetric, left-symmetric, right-
asymmetric, right-symmetric, la, ra, ls, rs. The default is left-symmetric.
same as --parity
number of active devices in array.
number of spare (eXtra) devices in initial array. Spares can be added and removed
Amount (in Kibibytes) of space to use from each drive in RAID1/4/5. 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.
uuid of array to assemble. Devices which don't have this uuid are excluded
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
Assemble the array even if some superblocks appear out-of-date
Attempt to start the array even if fewer drives were given than are needed for a
full array. Normally if not all 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.
For Manage mode:
hotadd listed devices.
remove listed devices. They must not be active. i.e. they should be failed or
mark listed devices as faulty.
same as --fail.
For Misc mode:
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 over-written with zeros.
With --force the block where the superblock would be is over-written even if it
doesn't appear to be valid.
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.
Give a delay in seconds. mdadm polls the md arrays and then waits this many sec-
onds before polling again. The default is 60 seconds.
Usage: mdadm --assemble device 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 com-
ponent-devices. These can be found in a number of ways.
The md device is either given before --scan or is found from the config file. In the lat-
ter case, multiple md devices can be started with a single mdadm command.
The identity can be given with the --uuid option, with the --super-minor option, can be
found in in the config file, or will be taken from the super block on the first component-
device listed on the command line.
Devices can be given on the --assemble command line or from the config file. Only devices
which have an md superblock which contains the right identity will be considered for any
The config file is only used if explicitly named with --config or requested with --scan.
In the later case, /etc/mdadm.conf is used.
If --scan is not given, then the config file will only be used to find the identity of md
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 or RAID5), give the --run flag.
Usage: mdadm --build device --chunk=X --level=Y --raid-devices=Z devices
This usage is similar to --create. The difference is that it creates a legacy array with-
out a superblock. With these arrays there is no difference between initially creating the
array and subsequently assembling the array, except that hopefully there is useful data
there in the second case.
The level may only be 0, raid0, or linear. All devices must be listed and the array will
be started once complete.
Usage: mdadm --create device --chunk=X --level=Y
This usage will initialise a new md array, associate some devices with it, and activate
As devices are added, they are checked to see if they contain raid superblocks or filesys-
tems. 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 miss-
ing 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 RAID1
array, only one real device needs to be given. All of the others can be missing.
The General Management options that are valid with --create are:
--run insist of 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
Usage: mdadm options ... devices ...
MISC mode includes a number if distinct operations that operate on distinct devices. The
The device is examined to see if it is (1) an active md array, or (2) a component
of an md array. The information discovered is reported.
The device should be an active md device. mdadm will display a detailed descrip-
tion 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.conf.
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 is given, or --scan then multi-
ple devices that are components of the one array are grouped together and reported
in a single entry suitable for inclusion in /etc/mdadm.conf.
Having --scan without listing any devices will cause all devices listed in the con-
fig file to be examined.
--stop This devices should active md arrays which will be deactivated, if 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
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 the background.
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 not
If any devices are listed on the command line, mdadm will only monitor those devices. Oth-
erwise 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 event to program, the program is run once for each event and is given 2 or 3
command-line arguements. 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 any-
thing. 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.
An md array started reconstruction.
Where NN is 20, 40, 60, or 80, this indicates that rebuild has passed that many
percentage of the total.
Fail An active component device of an array has been marked as faulty.
A spare component device which was being rebuilt to replace a faulty device has
A spare component device which was being rebuilt to replace a faulty device as
been successfully rebuild and has been made active.
A new md array has been detected in the /proc/mdstat file.
A spare drive has been moved from one array in a spare-group to another to
allow a failed drive to be replaced.
Only Fail and FailSpare cause Email to be sent. All events cause the program to be run.
The program is run with two or three arguments, they being 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
labelled 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 which is 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.
To find out if a devices is a raid array or part of one:
mdadm -Q /dev/name-of-device
To assemble and start all array listed in the standard config file:
To shut down all arrays (that are not still in used):
mdadm --stop --scan
To monitor all arrays if (and only if) an email address or program was given in the config
file, but poll every 2 minutes:
mdadm -Fs --delay=120
To create /dev/md0 as a RAID1 array with /dev/hda1 and /dev/hdc1:
mdadm -C /dev/md0 -l1 -n2 /dev/hd[ac]1
To create prototype a config file that describes currently active arrays that are known to
be made from partitions of IDE or SCSI drives:
echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.conf
mdadm --detail --scan >> mdadm.conf
This file should be reviewed before being used as it may contain unwanted detail.
To find out what raid arrays could be assembled from existing IDE and SCSI whole drives
echo 'DEVICE /dev/hd[a-z] /dev/sd*[a-z]' > mdadm.conf
mdadm -Es -c mdadm.conf >> mdadm.conf This file is very likely to contain unwanted
detail, particularly the devices= entries.
To get help about Create mode:
mdadm --create --help
To get help about the format of the config file:
mdadm --config --help
To get general help:
If you're using the /proc filesystem, /proc/mdstat lists all active md devices with infor-
mation 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.
mdadm was previously known as mdctl.
For information on the various levels of RAID, check out:
for new releases of the RAID driver check out:
raidtab(5), raid0run(8), raidstop(8), mkraid(8)