MDADMSection: Maintenance Commands (8)
MDADMSection: Maintenance Commands (8)
mdadm [mode] <raiddevice> [options] <component-devices>
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:
Available options are:
When used with --build, only linear, raid0, 0, stripe are valid.
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.
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".
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.
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.
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.
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.
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:
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 includes a number if distinct operations that operate on distinct devices. The operations are:
Having --scan without listing any devices will cause all devices listed in the config file to be examined.
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:
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.
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.
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.
Provide general help.
If you're using the /proc filesystem, /proc/mdstat lists all active md devices with information about them. mdadm uses this to find arrays when --scan is given in Misc mode, and to monitor array reconstruction on Monitor mode.
The config file lists which devices may be scanned to see if they contain MD super block, and gives identifying information (e.g. UUID) about known MD arrays. See mdadm.conf(5) for more details.
for new releases of the RAID driver check out: