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MDADM(8) MDADM(8)
NAME
mdadm - manage MD devices aka Linux Software Raid.
SYNOPSIS
mdadm [mode] <raiddevice> [options] <component-devices>
DESCRIPTION
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
driver.
Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4,
RAID5, RAID6, and MULTIPATH.
MULTIPATH is not a Software RAID mechanism, but does involve multiple devices. For MULTI-
PATH each device is a path to one common physical storage device.
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:
· mdadm is a single program and not a collection of programs.
· 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 configura-
tion file.
· 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 dif-
ferent configuration file with a different format and an different purpose.
MODES
mdadm has 6 major modes of operation:
Assemble
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. This is only meaning-
ful 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
monitor.
OPTIONS
Available options are:
-A, --assemble
Assemble a pre-existing array.
-B, --build
Build a legacy array without superblocks.
-C, --create
Create a new array.
-Q, --query
Examine a device to see (1) if it is an md device and (2) if it is a component of
an md array. Information about what is discovered is presented.
-D, --detail
Print detail of one or more md devices.
-E, --examine
Print content of md superblock on device(s).
-F, --follow, --monitor
Select Monitor mode.
-h, --help
Display help message or, after above option, mode specific help message.
--help-options
Display more detailed help about command line parsing and some commonly used
options.
-V, --version
Print version information for mdadm.
-v, --verbose
Be more verbose about what is happening.
-b, --brief
Be less verbose. This is used with --detail and --examine.
-f, --force
Be more forceful about certain operations. See the various modes of the exact
meaning of this option in different contexts.
-c, --config=
Specify the config file. Default is /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.
-s, --scan
scan config file or /proc/mdstat for missing information. In general, this option
gives mdadm permission to get any missing information, like component devices,
array devices, array identities, and alert destination from the configuration file:
/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:
-c, --chunk=
Specify chunk size of kibibytes. The default is 64.
--rounding=
Specify rounding factor for linear array (==chunk size)
-l, --level=
Set raid level. When used with --create, options are: linear, raid0, 0, stripe,
raid1, 1, mirror, raid4, 4, raid5, 5, raid6, 6, multipath, mp. Obviously some of
these are synonymous.
When used with --build, only linear, raid0, 0, stripe are valid.
-p, --parity=
Set raid5 parity algorithm. Options are: left-asymmetric, left-symmetric, right-
asymmetric, right-symmetric, la, ra, ls, rs. The default is left-symmetric.
--layout=
same as --parity
-n, --raid-devices=
Specify the number of active devices in the array. This, plus the number of spare
devices (see below) must equal the number of component-devices (including "missing"
devices) that are listed on the command line. 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.
Note that this number cannot be changed once the array has been created.
-x, --spare-devices=
Specify the number of spare (eXtra) devices in the initial array. Spares can also
be added and removed later. The number of component devices listed on the command
line must equal the number of raid devices plus the number of spare devices.
-z, --size=
Amount (in 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 warning is issued.
For assemble:
-u, --uuid=
uuid of array to assemble. Devices which don’t have this uuid are excluded
-m, --super-minor=
Minor number of device that array was created for. Devices which don’t have this
minor number are excluded. If you create an array as /dev/md1, then all
superblocks will contain the minor number 1, even if the array is later assembled
as /dev/md2.
Giving the literal word "dev" for --super-minor will cause mdadm to use the minor
number of the md device that is being assembled. e.g. when assembling /dev/md0,
will look for super blocks with a minor number of 0.
-f, --force
Assemble the array even if some superblocks appear out-of-date
-R, --run
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.
-U, --update=
Update the superblock on each device while assembling the array. The argument
given to this flag can be one of sparc2.2, summaries, or super-minor.
The sparc2.2 option will adjust the superblock of an array what was created on a
Sparc machine running a patched 2.2 Linux kernel. This kernel got the alignment of
part of the superblock wrong. You can use the --examine --sparc2.2 option to mdadm
to see what effect this would have.
The super-minor option will update the 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 summaries option will correct the summaries in the superblock. That is the
counts of total, working, active, failed, and spare devices.
For Manage mode:
-a, --add
hotadd listed devices.
-r, --remove
remove listed devices. They must not be active. i.e. they should be failed or
spare devices.
-f, --fail
mark listed devices as faulty.
--set-faulty
same as --fail.
For Examine mode:
--sparc2.2
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 --update=sparc2.2.
For Misc mode:
-R, --run
start a partially built array.
-S, --stop
deactivate array, releasing all resources.
-o, --readonly
mark array as readonly.
-w, --readwrite
mark array as readwrite.
--zero-superblock
If the device contains a valid md superblock, the block is 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.
-t, --test
When used with --detail, the exit status of mdadm is set to reflect the status of
the device.
For Monitor mode:
-m, --mail
Give a mail address to send alerts to.
-p, --program, --alert
Give a program to be run whenever an event is detected.
-d, --delay
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.
-f, --daemonise
Tell mdadm to run as a background daemon if it decides to monitor anything. This
causes it to fork and run in the child, and to disconnect 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.
-1, --oneshot
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 degraded arrays.
-t, --test
Generate a TestMessage alert for every array found at startup. This alert gets
mailed and passed to the alert program. This can be used for testing that alert
message to get through successfully.
ASSEMBLE MODE
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 com-
ponent-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, that 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 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 possi-
bly implicit) --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
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 or 6), give the --run flag.
BUILD MODE
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.
CREATE MODE
Usage: mdadm --create device --chunk=X --level=Y
--raid-devices=Z devices
This usage will initialise a new md array, associate some devices with it, and activate
the array.
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
"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 -I --force option.
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.
--readonly
start the array readonly - not supported yet.
MANAGE MODE
Usage: mdadm device options... devices...
This usage will allow individual devices in an array to be failed, removed or added. It
is possible to perform multiple operations with on command. For example:
mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
will firstly mark /dev/hda1 as faulty in /dev/md0 and will then remove it from the array
and finally add it back in as a spare. However only one md array can be affected by a
single command.
MISC MODE
Usage: mdadm options ... devices ...
MISC mode includes a number if distinct operations that operate on distinct devices. The
operations are:
--query
The device is examined to see if it is (1) an active md array, or (2) a component
of an md array. The information discovered is reported.
--detail
The device should be an active md device. mdadm will display a detailed 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 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 unusable (raid4 or
raid5).
4 There was an error while trying to get information about the device.
--examine
The device should be a component of an md array. mdadm will read the md superblock
of the device and display the contents. If --brief 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 The devices should be active md arrays which will be deactivated, as long as they
are not currently in use.
--run This will fully activate a partially assembled md array.
--readonly
This will mark an active array as read-only, providing that it is not currently
being used.
--readwrite
This will change a readonly array back to being read/write.
--scan For all operations except --examine, --scan will cause the operation to be applied
to all arrays listed in /proc/mdstat. For --examine, --scan causes all devices
listed in the config file to be examined.
MONITOR MODE
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
spares.
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:
DeviceDisappeared
An md array which previously was configured appears to no longer be configured.
RebuildStarted
An md array started reconstruction.
RebuildNN
Where NN is 20, 40, 60, or 80, this indicates that rebuild has passed that many
percentage of the total.
RebuildFinished
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 faulty.
FailSpare
A spare component device which was being rebuilt to replace a faulty device has
failed.
SpareActive
A spare component device which was being rebuilt to replace a faulty device as
been successfully rebuild and has been made active.
NewArray
A new md array has been detected in the /proc/mdstat file.
DegradedArray
A newly noticed array appears to be degraded. This message is not generated
when mdadm notices a drive failure which causes degradation, but only when
mdadm notices that an array is degraded when it first sees the array.
MoveSpare
A spare drive has been moved from one array in a spare-group to another to
allow a failed drive to be replaced.
TestMessage
An array was found at startup, and the --test flag was given.
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 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.
EXAMPLES
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 currently in used).
This will typically going 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 â€â€™DEVICE /dev/hd*[0-9] /dev/sd*[0-9]â€â€™ > 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 â€â€™DEVICE /dev/hd[a-z] /dev/sd*[a-z]â€â€™ > 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, 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 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.
mdadm --help
Provide general help.
FILES
/proc/mdstat
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.
/etc/mdadm.conf
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.
NOTE
mdadm was previously known as mdctl.
SEE ALSO
For information on the various levels of RAID, check out:
http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/
for new releases of the RAID driver check out:
ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches
or
http://www.cse.unsw.edu.au/~neilb/patches/linux-stable/
mdadm.conf(5), md(4).
raidtab(5), raid0run(8), raidstop(8), mkraid(8)
v1.5.0 MDADM(8)
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