MDADM(8) System Manager's Manual MDADM(8)
mdadm - manage MD devices aka Linux Software Raid.
mdadm [mode] <raiddevice> [options] <component-devices>
RAID devices are virtual devices created from two or more real block
devices. This allows multiple devices (typically disk drives or parti-
tions there-of) 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, MULTIPATH, and FAULTY.
MULTIPATH is not a Software RAID mechanism, but does involve multiple
devices. For MULTIPATH each device is a path to one common physical
FAULTY is also no true RAID, and it only involves one device. It pro-
vides a layer over a true device that can be used to inject faults.
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
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 and does not use one by default. 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 does not use /etc/raidtab, the raidtools configuration file, at
all. It has a different configuration file with a different format and
an different purpose.
mdadm has 7 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 exam-
ine MD superblocks, erasing old superblocks and stopping active
Follow or Monitor
Monitor one or more md devices and act on any state changes.
This is only meaningful for raid1, 4, 5, 6 or multipath arrays
as only these have interesting state. raid0 or linear never
have missing, spare, or failed drives, so there is nothing to
Grow Grow (or shrink) an array, or otherwise reshape it in some way.
Currently supported growth options including changing the active
size of componenet devices in RAID level 1/4/5/6 and changing
the number of active devices in RAID1.
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 dis-
covered is presented.
Print detail of one or more md devices.
Print content of md superblock on device(s).
-F, --follow, --monitor
Select Monitor mode.
Change the size or shape of an active array.
Display help message or, after above option, mode specific help
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.
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/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 iden-
tities, and alert destination from the configuration file:
/etc/mdadm/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. When used with --create, options are: linear,
raid0, 0, stripe, raid1, 1, mirror, raid4, 4, raid5, 5, raid6,
6, multipath, mp, fautly. Obviously some of these are synony-
When used with --build, only linear, raid0, 0, stripe are valid.
Set raid5 parity algorithm. Options are: left-asymmetric, left-
symmetric, right-asymmetric, right-symmetric, la, ra, ls, rs.
The default is left-symmetric.
This option is also used to set the failure mode for faulty.
The options are: write-transient, wt, read-transient, rt, write-
presistent, wp, read-persistent, rp, write-all, read-fixable,
rf, clear, flush, none.
Each mode can be followed by a number which is used as a period
between fault generation. Without a number, the fault is gener-
ated once on the first relevant request. With a number, the
fault will be generated after that many request, and will con-
tinue to be generated every time the period elapses.
Multiple failure modes can be current simultaneously by using
the "--grow" option to set subsequent failure modes.
"clear" or "none" will remove any pending or periodic failure
modes, and "flush" will clear any persistant faults.
To set the parity with "--grow", the level of the array
("faulty") must be specified before the fault mode is specified.
same as --parity
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 or raid5.
This number can only be changed using --grow for RAID1 arrays,
and only on kernels which provide necessary support.
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
RAID1/4/5/6. 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.
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.
Tell mdadm that the array pre-existed and is known to be clean.
This is only really useful for Building RAID1 array. Only use
this if you really know what you are doing. This is currently
only supported for --build.
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.
Instruct mdadm to create the device file if needed, possibly
allocating an unused minor number. "md" causes a non-partition-
able array to be used. "mdp", "part" or "p" causes a partition-
able array (2.6 and later) to be used. "yes" requires the named
md device to have a from this. See DEVICE NAMES below.
The argumentment can also come immediately after "-a". e.g.
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
allocted. 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.
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, will look for super blocks
with a minor number of 0.
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
See this option under Create and Build options.
Update the superblock on each device while assembling the array.
The argument given to this flag can be one of sparc2.2, sum-
maries, resync, 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 prefered minor field on
each superblock to match the minor number of the array being
assembled. This is not needed on 2.6 and later kernels as they
make this adjustment automatically.
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 summaries option will correct the summaries in the
superblock. That is the counts of total, working, active,
failed, and spare devices.
For Manage mode:
hotadd listed devices.
remove listed devices. They must not be active. i.e. they
should be failed or spare devices.
mark listed devices as faulty.
same as --fail.
For Examine mode:
In 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
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.
When used with --detail, the exit status of mdadm is set to
reflect the status of the device.
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 seconds before polling again. The default is 60
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 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 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 events. Running
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 to get through suc-
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 precisely one device is listed, but --scan is not given, then mdadm
acts as though --scan was given and identify information is extracted
from the configuration file.
The identity can be given with the --uuid option, with the --super-
minor option, can be found in the config file, or will be taken from
the super block on the first component-device listed on the command
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 com-
plete (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 or 6), give the --run flag.
If an auto option is given, either on the command line (--auto) or in
the configuration file (e.g. auto=part), then mdadm will create the md
device if necessary or will re-create it if it doesn't look usable as
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 conventions).
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 inuse, 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) noth-
ing, then mdadm will create a traditional, non-partitionable md array.
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 this option to create a partitionable array, the device
files for the first 4 partitions are also created. If a different num-
ber 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.
Usage: mdadm --build device --chunk=X --level=Y --raid-devices=Z
This usage is similar to --create. The difference is that it creates a
legacy array without a superblock. With these arrays there is no dif-
ference between initially creating the array and subsequently assem-
bling 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 the array.
This 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 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 over-ridden
with the --force option.
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 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 /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 and hence is unus-
able (raid4 or raid5).
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 is given, or --scan 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 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.
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 desti-
nation array has a failed drive but not 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 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 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.
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.
An md array that was rebuilding, isn't any more, either
because it finished normally or was aborted.
Fail An active component device of an array has been marked as
A spare component device which was being rebuilt to replace
a faulty device has failed.
A spare component device which was being rebuilt to replace
a faulty device as been successfully rebuild and has been
A new md array has been detected in the /proc/mdstat file.
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.
A spare drive has been moved from one array in a spare-group
to another to allow a failed drive to be replaced.
An array was found at startup, and the --test flag was
Only Fail , FailSpare , DegradedArray , and TestMessage 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 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 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.
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 may be added during 2.6 development, possibly
including restructuring a raid5 array to have more active devices.
Currently the only support available is to change the "size" attribute
for arrays with redundancy, and the raid-disks attribute of RAID1
Normally when an array is build 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 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 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 that which were in those slots must be failed and removed.
When the number of devices is increased, any hot spares that are
present may be activated immediately.
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 confile
file. This command will typically go in a system startup file.
mdadm --stop --scan
This will shut down all array 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 what arrays could be assembled from existign IDE and SCSI whole
drives (not partitions) and store the information is the format of a
config file. This file is very likely to contain unwanted detail, par-
ticularly 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 was 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 --create --help
Providew 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 con-
tain MD super block, and gives identifying information (e.g. UUID)
about known MD arrays. See mdadm.conf(5) for more details.
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) either of
where NN is a number. The standard names for partitionable arrays (as
available from 2.6 onwards) is one of
Partition numbers should be indicated by added "pMM" to these, thus
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)