MDADM

Section: Maintenance Commands (8)
Updated:
 

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 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, and MULTIPATH.

MULTIPATH is not a Software RAID mechanism, but does involve multiple devices. For MULTIPATH 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 configuration 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 different configuration file with a different format and an different purpose.

 

MODES

mdadm has 7 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 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 monitor.

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.

 

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.

-G, --grow
Change the size or shape of an active array.

-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 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 arrays, and only on kernels which provide necessary support.

-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.

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.

--assume-clean
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.

-R, --run
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.

-f, --force
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.

-a, --auto{=no,yes,md,mdp,part,p}{NN}
Instruct mdadm to create the device file if needed, and to allocate an unused minor number. "yes" or "md" causes a non-partitionable array to be used. "mdp", "part" or "p" causes a partitionable array (2.6 and later) to be used. The argumentment can also come immediately after "-a". e.g. "-ap".

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 an underscore, a 'p', and a number, e.g. "/dev/home1_p3". If there is no trailing digit, then the partition names just have a number added, e.g. "/dev/scratch3".

 

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.

-a, --auto{=no,yes,md,mdp,part}
See this option under Create and Build options.

-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 seconds 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 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 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 possibly 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.

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 it is.

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) nothing, 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 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.

 

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 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 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.

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 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 on 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 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.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 multiple 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 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.

--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. 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 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:

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.

 

GROW MODE

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 arrays.

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 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 synchronised.

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 devices.

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.

 

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 '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 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 information 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)


 

Index

NAME
SYNOPSIS
DESCRIPTION
MODES
OPTIONS
For create or build:
For assemble:
For Manage mode:
For Examine mode:
For Misc mode:
For Monitor mode:
ASSEMBLE MODE
BUILD MODE
CREATE MODE
MANAGE MODE
MISC MODE
MONITOR MODE
GROW MODE
EXAMPLES
FILES
/proc/mdstat
/etc/mdadm.conf
NOTE
SEE ALSO
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