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, RAID10, MULTIPATH, FAULTY, and CONTAINER.
MULTIPATH is not a Software RAID mechanism, but does involve multiple devices: each device is a path to one common physical storage device. New installations should not use md/multipath as it is not well supported and has no ongoing development. Use the Device Mapper based multipath-tools instead.
FAULTY is also not true RAID, and it only involves one device. It provides a layer over a true device that can be used to inject faults.
CONTAINER is different again. A CONTAINER is a collection of devices that are managed as a set. This is similar to the set of devices connected to a hardware RAID controller. The set of devices may contain a number of different RAID arrays each utilising some (or all) of the blocks from a number of the devices in the set. For example, two devices in a 5-device set might form a RAID1 using the whole devices. The remaining three might have a RAID5 over the first half of each device, and a RAID0 over the second half.
With a CONTAINER, there is one set of metadata that describes all of the arrays in the container. So when mdadm creates a CONTAINER device, the device just represents the metadata. Other normal arrays (RAID1 etc) can be created inside the container.
If a CONTAINER is passed to mdadm in this mode, then any arrays within that container will be assembled and started.
If a device is given before any options, or if the first option is one of --add, --re-add, --add-spare, --fail, --remove, or --replace, then the MANAGE mode is assumed. Anything other than these will cause the Misc mode to be assumed.
If the name given is of a directory, then mdadm will collect all the files contained in the directory with a name ending in .conf, sort them lexically, and process all of those files as config files.
Options are:
When creating an array, the homehost will be recorded in the metadata. For version-1 superblocks, it will be prefixed to the array name. For version-0.90 superblocks, part of the SHA1 hash of the hostname will be stored in the later half of the UUID.
When reporting information about an array, any array which is tagged for the given homehost will be reported as such.
When using Auto-Assemble, only arrays tagged for the given homehost will be allowed to use 'local' names (i.e. not ending in '_' followed by a digit string). See below under Auto Assembly.
The special name "any" can be used as a wild card. If an array is created with --homehost=any then the name "any" will be stored in the array and it can be assembled in the same way on any host. If an array is assembled with this option, then the homehost recorded on the array will be ignored.
This functionality is currently only provided by --detail and --monitor.
A suffix of 'K', 'M' or 'G' can be given to indicate Kilobytes, Megabytes or Gigabytes respectively.
Sometimes a replacement drive can be a little smaller than the original drives though this should be minimised by IDEMA standards. Such a replacement drive will be rejected by md. To guard against this it can be useful to set the initial size slightly smaller than the smaller device with the aim that it will still be larger than any replacement.
This value can be set with --grow for RAID level 1/4/5/6 though CONTAINER based arrays such as those with IMSM metadata may not be able to support this. If the array was created with a size smaller than the currently active drives, the extra space can be accessed using --grow. The size can be given as max which means to choose the largest size that fits on all current drives.
Before reducing the size of the array (with --grow --size=) you should make sure that space isn't needed. If the device holds a filesystem, you would need to resize the filesystem to use less space.
After reducing the array size you should check that the data stored in the device is still available. If the device holds a filesystem, then an 'fsck' of the filesystem is a minimum requirement. If there are problems the array can be made bigger again with no loss with another --grow --size= command.
This value cannot be used when creating a CONTAINER such as with DDF and IMSM metadata, though it perfectly valid when creating an array inside a container.
Setting the array-size causes the array to appear smaller to programs that access the data. This is particularly needed before reshaping an array so that it will be smaller. As the reshape is not reversible, but setting the size with --array-size is, it is required that the array size is reduced as appropriate before the number of devices in the array is reduced.
Before reducing the size of the array you should make sure that space isn't needed. If the device holds a filesystem, you would need to resize the filesystem to use less space.
After reducing the array size you should check that the data stored in the device is still available. If the device holds a filesystem, then an 'fsck' of the filesystem is a minimum requirement. If there are problems the array can be made bigger again with no loss with another --grow --array-size= command.
A suffix of 'K', 'M' or 'G' can be given to indicate Kilobytes, Megabytes or Gigabytes respectively. A value of max restores the apparent size of the array to be whatever the real amount of available space is.
Clustered arrays do not support this parameter yet.
RAID4, RAID5, RAID6, and RAID10 require the chunk size to be a power of 2. In any case it must be a multiple of 4KB.
A suffix of 'K', 'M' or 'G' can be given to indicate Kilobytes, Megabytes or Gigabytes respectively.
When a CONTAINER metadata type is requested, only the container level is permitted, and it does not need to be explicitly given.
When used with --build, only linear, stripe, raid0, 0, raid1, multipath, mp, and faulty are valid.
Can be used with --grow to change the RAID level in some cases. See LEVEL CHANGES below.
The layout of the RAID5 parity block can be one of left-asymmetric, left-symmetric, right-asymmetric, right-symmetric, la, ra, ls, rs. The default is left-symmetric.
It is also possible to cause RAID5 to use a RAID4-like layout by choosing parity-first, or parity-last.
Finally for RAID5 there are DDF-compatible layouts, ddf-zero-restart, ddf-N-restart, and ddf-N-continue.
These same layouts are available for RAID6. There are also 4 layouts that will provide an intermediate stage for converting between RAID5 and RAID6. These provide a layout which is identical to the corresponding RAID5 layout on the first N-1 devices, and has the 'Q' syndrome (the second 'parity' block used by RAID6) on the last device. These layouts are: left-symmetric-6, right-symmetric-6, left-asymmetric-6, right-asymmetric-6, and parity-first-6.
When setting the failure mode for level faulty, the options are: write-transient, wt, read-transient, rt, write-persistent, wp, read-persistent, rp, write-all, read-fixable, rf, clear, flush, none.
Each failure mode can be followed by a number, which is used as a period between fault generation. Without a number, the fault is generated once on the first relevant request. With a number, the fault will be generated after that many requests, and will continue 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 persistent faults.
Finally, the layout options for RAID10 are one of 'n', 'o' or 'f' followed by a small number. The default is 'n2'. The supported options are:
'n' signals 'near' copies. Multiple copies of one data block are at similar offsets in different devices.
'o' signals 'offset' copies. Rather than the chunks being duplicated within a stripe, whole stripes are duplicated but are rotated by one device so duplicate blocks are on different devices. Thus subsequent copies of a block are in the next drive, and are one chunk further down.
'f' signals 'far' copies (multiple copies have very different offsets). See md(4) for more detail about 'near', 'offset', and 'far'.
The number is the number of copies of each datablock. 2 is normal, 3 can be useful. This number can be at most equal to the number of devices in the array. It does not need to divide evenly into that number (e.g. it is perfectly legal to have an 'n2' layout for an array with an odd number of devices).
When an array is converted between RAID5 and RAID6 an intermediate RAID6 layout is used in which the second parity block (Q) is always on the last device. To convert a RAID5 to RAID6 and leave it in this new layout (which does not require re-striping) use --layout=preserve. This will try to avoid any restriping.
The converse of this is --layout=normalise which will change a non-standard RAID6 layout into a more standard arrangement.
To help catch typing errors, the filename must contain at least one slash ('/') if it is a real file (not 'internal' or 'none').
Note: external bitmaps are only known to work on ext2 and ext3. Storing bitmap files on other filesystems may result in serious problems.
When creating an array on devices which are 100G or larger, mdadm automatically adds an internal bitmap as it will usually be beneficial. This can be suppressed with --bitmap=none or by selecting a different consistency policy with --consistency-policy.
A suffix of 'K', 'M' or 'G' can be given to indicate Kilobytes, Megabytes or Gigabytes respectively.
If an array becomes degraded so that the 'failfast' device is the only usable device, the 'failfast' flag will then be ignored and extended delays will be preferred to complete failure.
The 'failfast' flag is appropriate for storage arrays which have a low probability of true failure, but which may sometimes cause unacceptable delays due to internal maintenance functions.
Setting the offset explicitly over-rides the default. The value given is in Kilobytes unless a suffix of 'K', 'M' or 'G' is used to explicitly indicate Kilobytes, Megabytes or Gigabytes respectively.
Since Linux 3.4, --data-offset can also be used with --grow for some RAID levels (initially on RAID10). This allows the data-offset to be changed as part of the reshape process. When the data offset is changed, no backup file is required as the difference in offsets is used to provide the same functionality.
When the new offset is earlier than the old offset, the number of devices in the array cannot shrink. When it is after the old offset, the number of devices in the array cannot increase.
When creating an array, --data-offset can be specified as variable. In the case each member device is expected to have a offset appended to the name, separated by a colon. This makes it possible to recreate exactly an array which has varying data offsets (as can happen when different versions of mdadm are used to add different devices).
The argument can also come immediately after "-a". e.g. "-ap".
If --auto is not given on the command line or in the config file, then the default will be --auto=yes.
If --scan is also given, then any auto= entries in the config file will override the --auto instruction given on the command line.
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/md/home1p3. If there is no trailing digit, then the partition names just have a number added, e.g. /dev/md/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 device number based on that name. If the device name is not in one of these formats, then a unused device number will be allocated. The device 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. Names that are not in 'standard' format are only allowed in "/dev/md/".
This is meaningful with --create or --build.
If the target array is a Linear array, then --add can be used to add one or more devices to the array. They are simply catenated on to the end of the array. Once added, the devices cannot be removed.
If the --raid-disks option is being used to increase the number of devices in an array, then --add can be used to add some extra devices to be included in the array. In most cases this is not needed as the extra devices can be added as spares first, and then the number of raid-disks can be changed. However for RAID0, it is not possible to add spares. So to increase the number of devices in a RAID0, it is necessary to set the new number of devices, and to add the new devices, in the same command.
Can be used with --grow to change the consistency policy of an active array in some cases. See CONSISTENCY POLICY CHANGES below.
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, --super-minor=dev will look for super blocks with a minor number of 0.
--super-minor is only relevant for v0.90 metadata, and should not normally be used. Using --uuid is much safer.
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 preferred minor field on each superblock to match the minor number of the array being assembled. This can be useful if --examine reports a different "Preferred Minor" to --detail. In some cases this update will be performed automatically by the kernel driver. In particular the update happens automatically at the first write to an array with redundancy (RAID level 1 or greater) on a 2.6 (or later) kernel.
The uuid option will change the uuid of the array. If a UUID is given with the --uuid option that UUID will be used as a new UUID and will NOT be used to help identify the devices in the array. If no --uuid is given, a random UUID is chosen.
The name option will change the name of the array as stored in the superblock. This is only supported for version-1 superblocks.
The nodes option will change the nodes of the array as stored in the bitmap superblock. This option only works for a clustered environment.
The homehost option will change the homehost as recorded in the superblock. For version-0 superblocks, this is the same as updating the UUID. For version-1 superblocks, this involves updating the name.
The home-cluster option will change the cluster name as recorded in the superblock and bitmap. This option only works for clustered environment.
The resync option will cause the array to be marked dirty meaning 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 redundant information is correct.
The byteorder option allows arrays to be moved between machines with different byte-order, such as from a big-endian machine like a Sparc or some MIPS machines, to a little-endian x86_64 machine. When assembling such an array for the first time after a move, giving --update=byteorder will cause mdadm to expect superblocks to have their byteorder reversed, and will correct that order before assembling the array. This is only valid with original (Version 0.90) superblocks.
The summaries option will correct the summaries in the superblock. That is the counts of total, working, active, failed, and spare devices.
The devicesize option will rarely be of use. It applies to version 1.1 and 1.2 metadata only (where the metadata is at the start of the device) and is only useful when the component device has changed size (typically become larger). The version 1 metadata records the amount of the device that can be used to store data, so if a device in a version 1.1 or 1.2 array becomes larger, the metadata will still be visible, but the extra space will not. In this case it might be useful to assemble the array with --update=devicesize. This will cause mdadm to determine the maximum usable amount of space on each device and update the relevant field in the metadata.
The metadata option only works on v0.90 metadata arrays and will convert them to v1.0 metadata. The array must not be dirty (i.e. it must not need a sync) and it must not have a write-intent bitmap.
The old metadata will remain on the devices, but will appear older than the new metadata and so will usually be ignored. The old metadata (or indeed the new metadata) can be removed by giving the appropriate --metadata= option to --zero-superblock.
The no-bitmap option can be used when an array has an internal bitmap which is corrupt in some way so that assembling the array normally fails. It will cause any internal bitmap to be ignored.
The bbl option will reserve space in each device for a bad block list. This will be 4K in size and positioned near the end of any free space between the superblock and the data.
The no-bbl option will cause any reservation of space for a bad block list to be removed. If the bad block list contains entries, this will fail, as removing the list could cause data corruption.
The ppl option will enable PPL for a RAID5 array and reserve space for PPL on each device. There must be enough free space between the data and superblock and a write-intent bitmap or journal must not be used.
The no-ppl option will disable PPL in the superblock.
Reshape can be continued later using the --continue option for the grow command.
Note that this and the following options are only meaningful on array with redundancy. They don't apply to RAID0 or Linear.
When used on an array that has no metadata (i.e. it was built with --build) it will be assumed that bitmap-based recovery is enough to make the device fully consistent with the array.
When used with v1.x metadata, --re-add can be accompanied by --update=devicesize, --update=bbl, or --update=no-bbl. See the description of these option when used in Assemble mode for an explanation of their use.
If the device name given is missing then mdadm will try to find any device that looks like it should be part of the array but isn't and will try to re-add all such devices.
If the device name given is faulty then mdadm will find all devices in the array that are marked faulty, remove them and attempt to immediately re-add them. This can be useful if you are certain that the reason for failure has been resolved.
As well as the name of a device file (e.g. /dev/sda1) the words failed, detached and names like set-A can be given to --remove. The first causes all failed device to be removed. The second causes any device which is no longer connected to the system (i.e an 'open' returns ENXIO) to be removed. The third will remove a set as describe below under --fail.
For RAID10 arrays where the number of copies evenly divides the number of devices, the devices can be conceptually divided into sets where each set contains a single complete copy of the data on the array. Sometimes a RAID10 array will be configured so that these sets are on separate controllers. In this case all the devices in one set can be failed by giving a name like set-A or set-B to --fail. The appropriate set names are reported by --detail.
Each of these options requires that the first device listed is the array to be acted upon, and the remainder are component devices to be added, removed, marked as faulty, etc. Several different operations can be specified for different devices, e.g. mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1 Each operation applies to all devices listed until the next operation.
If an array is using a write-intent bitmap, then devices which have been removed can be re-added in a way that avoids a full reconstruction but instead just updates the blocks that have changed since the device was removed. For arrays with persistent metadata (superblocks) this is done automatically. For arrays created with --build mdadm needs to be told that this device we removed recently with --re-add.
Devices can only be removed from an array if they are not in active use, i.e. that must be spares or failed devices. To remove an active device, it must first be marked as faulty.
With --incremental The value MD_STARTED indicates whether an array was started (yes) or not, which may include a reason (unsafe, nothing, no). Also the value MD_FOREIGN indicates if the array is expected on this host (no), or seems to be from elsewhere (yes).
Note: Be careful to call --zero-superblock with clustered raid, make sure array isn't used or assembled in other cluster node before execute it.
Details of check and repair can be found it md(4) under SCRUBBING AND MISMATCHES.
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 no arrays are described by the configuration file, then any arrays that can be found on unused devices will be assembled.
If precisely one device is listed, but --scan is not given, then mdadm acts as though --scan was given and identity information is extracted from the configuration file.
The identity can be given with the --uuid option, the --name option, or the --super-minor option, will be taken from the md-device record in the config file, or will be taken from the super block of 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 or /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 not all expected drives were listed, 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, 6, or 10), give the --run flag.
If udev is active, mdadm does not create any entries in /dev but leaves that to udev. It does record information in /run/mdadm/map which will allow udev to choose the correct name.
If mdadm detects that udev is not configured, it will create the devices in /dev itself.
In Linux kernels prior to version 2.6.28 there were two distinctly different types of md devices that could be created: one that could be partitioned using standard partitioning tools and one that could not. Since 2.6.28 that distinction is no longer relevant as both type of devices can be partitioned. mdadm will normally create the type that originally could not be partitioned as it has a well defined major number (9).
Prior to 2.6.28, it is important that mdadm chooses the correct type of array device to use. This can be controlled with the --auto option. In particular, a value of "mdp" or "part" or "p" tells mdadm to use a partitionable device rather than the default.
In the no-udev case, the value given to --auto can be suffixed by a number. This tells mdadm to create that number of partition devices rather than the default of 4.
The value given to --auto can also be given in the configuration file as a word starting auto= on the ARRAY line for the relevant array.
If no arrays are listed in the config (other than those marked <ignore>) it will look through the available devices for possible arrays and will try to assemble anything that it finds. Arrays which are tagged as belonging to the given homehost will be assembled and started normally. Arrays which do not obviously belong to this host are given names that are expected not to conflict with anything local, and are started "read-auto" so that nothing is written to any device until the array is written to. i.e. automatic resync etc is delayed.
If mdadm finds a consistent set of devices that look like they should comprise an array, and if the superblock is tagged as belonging to the given home host, it will automatically choose a device name and try to assemble the array. If the array uses version-0.90 metadata, then the minor number as recorded in the superblock is used to create a name in /dev/md/ so for example /dev/md/3. If the array uses version-1 metadata, then the name from the superblock is used to similarly create a name in /dev/md/ (the name will have any 'host' prefix stripped first).
This behaviour can be modified by the AUTO line in the mdadm.conf configuration file. This line can indicate that specific metadata type should, or should not, be automatically assembled. If an array is found which is not listed in mdadm.conf and has a metadata format that is denied by the AUTO line, then it will not be assembled. The AUTO line can also request that all arrays identified as being for this homehost should be assembled regardless of their metadata type. See mdadm.conf(5) for further details.
Note: Auto assembly cannot be used for assembling and activating some arrays which are undergoing reshape. In particular as the backup-file cannot be given, any reshape which requires a backup-file to continue cannot be started by auto assembly. An array which is growing to more devices and has passed the critical section can be assembled using auto-assembly.
This usage is similar to --create. The difference is that it creates an 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 raid0, linear, raid1, raid10, multipath, or faulty, or one of their synonyms. All devices must be listed and the array will be started once complete. It will often be appropriate to use --assume-clean with levels raid1 or raid10.
This usage will initialise a new md array, associate some devices with it, and activate the array.
The named device will normally not exist when mdadm --create is run, but will be created by udev once the array becomes active.
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 overridden with the --force option.
When creating an array with version-1 metadata a name for the array is required. If this is not given with the --name option, mdadm will choose a name based on the last component of the name of the device being created. So if /dev/md3 is being created, then the name 3 will be chosen. If /dev/md/home is being created, then the name home will be used.
When creating a partition based array, using mdadm with version-1.x metadata, the partition type should be set to 0xDA (non fs-data). This type selection allows for greater precision since using any other [RAID auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create problems in the event of array recovery through a live cdrom.
A new array will normally get a randomly assigned 128bit UUID which is very likely to be unique. If you have a specific need, you can choose a UUID for the array by giving the --uuid= option. Be warned that creating two arrays with the same UUID is a recipe for disaster. Also, using --uuid= when creating a v0.90 array will silently override any --homehost= setting.
If the array type supports a write-intent bitmap, and if the devices in the array exceed 100G is size, an internal write-intent bitmap will automatically be added unless some other option is explicitly requested with the --bitmap option or a different consistency policy is selected with the --consistency-policy option. In any case space for a bitmap will be reserved so that one can be added later with --grow --bitmap=internal.
If the metadata type supports it (currently only 1.x and IMSM metadata), space will be allocated to store a bad block list. This allows a modest number of bad blocks to be recorded, allowing the drive to remain in service while only partially functional.
When creating an array within a CONTAINER mdadm can be given either the list of devices to use, or simply the name of the container. The former case gives control over which devices in the container will be used for the array. The latter case allows mdadm to automatically choose which devices to use based on how much spare space is available.
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.
When a device is added to an active array, mdadm checks to see if it has metadata on it which suggests that it was recently a member of the array. If it does, it tries to "re-add" the device. If there have been no changes since the device was removed, or if the array has a write-intent bitmap which has recorded whatever changes there were, then the device will immediately become a full member of the array and those differences recorded in the bitmap will be resolved.
MISC mode includes a number of distinct operations that operate on distinct devices. The operations are:
The name option updates the subarray name in the metadata, it may not affect the device node name or the device node symlink until the subarray is re-assembled. If updating name would change the UUID of an active subarray this operation is blocked, and the command will end in an error.
The ppl and no-ppl options enable and disable PPL in the metadata. Currently supported only for IMSM subarrays.
Having --scan without listing any devices will cause all devices listed in the config file to be examined.
The file name used in the directory will be the base name of the device. Further if any links appear in /dev/disk/by-id which point to the device, then hard links to the file will be created in directory based on these by-id names.
Multiple devices can be listed and their metadata will all be stored in the one directory.
If a file name is given instead of a directory then mdadm will restore from that file to a single device, always provided the size of the file matches that of the device, and the file contains valid metadata.
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 or domain and if the destination array has a failed drive but no 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 events to a program, the program is run once for each event, and is given 2 or 3 command-line arguments: 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:
If mdadm was told to monitor an array which is RAID0 or Linear, then it will report DeviceDisappeared with the extra information Wrong-Level. This is because RAID0 and Linear do not support the device-failed, hot-spare and resync operations which are monitored.
Only Fail, FailSpare, DegradedArray, SparesMissing and TestMessage cause Email to be sent. All events cause the program to be run. The program is run with two or three arguments: 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 labeled with the same spare-group or the spares must be allowed to migrate through matching POLICY domains 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 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.
If the spare group for a degraded array is not defined, mdadm will look at the rules of spare migration specified by POLICY lines in mdadm.conf and then follow similar steps as above if a matching spare is found.
Currently the supported changes include
Using GROW on containers is currently supported only for Intel's IMSM container format. The number of devices in a container can be increased - which affects all arrays in the container - or an array in a container can be converted between levels where those levels are supported by the container, and the conversion is on of those listed above. Resizing arrays in an IMSM container with --grow --size is not yet supported.
Notes:
Note that when an array changes size, any filesystem that may be stored in the array will not automatically grow or shrink to use or vacate the space. The filesystem will need to be explicitly told to use the extra space after growing, or to reduce its size prior to shrinking the array.
Also the size of an array cannot be changed while it has an active bitmap. If an array has a bitmap, it must be removed before the size can be changed. Once the change is complete a new bitmap can be created.
Note: --grow --size is not yet supported for external file bitmap.
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 which were in those slots must be failed and removed.
When the number of devices is increased, any hot spares that are present will be activated immediately.
Changing the number of active devices in a RAID5 or RAID6 is much more effort. Every block in the array will need to be read and written back to a new location. From 2.6.17, the Linux Kernel is able to increase the number of devices in a RAID5 safely, including restarting an interrupted "reshape". From 2.6.31, the Linux Kernel is able to increase or decrease the number of devices in a RAID5 or RAID6.
From 2.6.35, the Linux Kernel is able to convert a RAID0 in to a RAID4 or RAID5. mdadm uses this functionality and the ability to add devices to a RAID4 to allow devices to be added to a RAID0. When requested to do this, mdadm will convert the RAID0 to a RAID4, add the necessary disks and make the reshape happen, and then convert the RAID4 back to RAID0.
When decreasing the number of devices, the size of the array will also decrease. If there was data in the array, it could get destroyed and this is not reversible, so you should firstly shrink the filesystem on the array to fit within the new size. To help prevent accidents, mdadm requires that the size of the array be decreased first with mdadm --grow --array-size. This is a reversible change which simply makes the end of the array inaccessible. The integrity of any data can then be checked before the non-reversible reduction in the number of devices is request.
When relocating the first few stripes on a RAID5 or RAID6, it is not possible to keep the data on disk completely consistent and crash-proof. To provide the required safety, mdadm disables writes to the array while this "critical section" is reshaped, and takes a backup of the data that is in that section. For grows, this backup may be stored in any spare devices that the array has, however it can also be stored in a separate file specified with the --backup-file option, and is required to be specified for shrinks, RAID level changes and layout changes. If this option is used, and the system does crash during the critical period, the same file must be passed to --assemble to restore the backup and reassemble the array. When shrinking rather than growing the array, the reshape is done from the end towards the beginning, so the "critical section" is at the end of the reshape.
Changing the RAID level of any array happens instantaneously. However in the RAID5 to RAID6 case this requires a non-standard layout of the RAID6 data, and in the RAID6 to RAID5 case that non-standard layout is required before the change can be accomplished. So while the level change is instant, the accompanying layout change can take quite a long time. A --backup-file is required. If the array is not simultaneously being grown or shrunk, so that the array size will remain the same - for example, reshaping a 3-drive RAID5 into a 4-drive RAID6 - the backup file will be used not just for a "cricital section" but throughout the reshape operation, as described below under LAYOUT CHANGES.
Changing the chunk-size of layout without also changing the number of devices as the same time will involve re-writing all blocks in-place. To ensure against data loss in the case of a crash, a --backup-file must be provided for these changes. Small sections of the array will be copied to the backup file while they are being rearranged. This means that all the data is copied twice, once to the backup and once to the new layout on the array, so this type of reshape will go very slowly.
If the reshape is interrupted for any reason, this backup file must be made available to mdadm --assemble so the array can be reassembled. Consequently the file cannot be stored on the device being reshaped.
A write-intent bitmap can be added to, or removed from, an active array. Either internal bitmaps, or bitmaps stored in a separate file, can be added. Note that if you add a bitmap stored in a file which is in a filesystem that is on the RAID array being affected, the system will deadlock. The bitmap must be on a separate filesystem.
The consistency policy of an active array can be changed by using the --consistency-policy option in Grow mode. Currently this works only for the ppl and resync policies and allows to enable or disable the RAID5 Partial Parity Log (PPL).
This mode is designed to be used in conjunction with a device discovery system. As devices are found in a system, they can be passed to mdadm --incremental to be conditionally added to an appropriate array.
Conversely, it can also be used with the --fail flag to do just the opposite and find whatever array a particular device is part of and remove the device from that array.
If the device passed is a CONTAINER device created by a previous call to mdadm, then rather than trying to add that device to an array, all the arrays described by the metadata of the container will be started.
mdadm performs a number of tests to determine if the device is part of an array, and which array it should be part of. If an appropriate array is found, or can be created, mdadm adds the device to the array and conditionally starts the array.
Note that mdadm will normally only add devices to an array which were previously working (active or spare) parts of that array. The support for automatic inclusion of a new drive as a spare in some array requires a configuration through POLICY in config file.
The tests that mdadm makes are as follow:
This is the only context where the aliases are used. They are usually provided by a udev rules mentioning $env{DEVLINKS}.
mdadm keeps a list of arrays that it has partially assembled in /run/mdadm/map. If no array exists which matches the metadata on the new device, mdadm must choose a device name and unit number. It does this based on any name given in mdadm.conf or any name information stored in the metadata. If this name suggests a unit number, that number will be used, otherwise a free unit number will be chosen. Normally mdadm will prefer to create a partitionable array, however if the CREATE line in mdadm.conf suggests that a non-partitionable array is preferred, that will be honoured.
If the array is not found in the config file and its metadata does not identify it as belonging to the "homehost", then mdadm will choose a name for the array which is certain not to conflict with any array which does belong to this host. It does this be adding an underscore and a small number to the name preferred by the metadata.
Once an appropriate array is found or created and the device is added, mdadm must decide if the array is ready to be started. It will normally compare the number of available (non-spare) devices to the number of devices that the metadata suggests need to be active. If there are at least that many, the array will be started. This means that if any devices are missing the array will not be restarted.
As an alternative, --run may be passed to mdadm in which case the array will be run as soon as there are enough devices present for the data to be accessible. For a RAID1, that means one device will start the array. For a clean RAID5, the array will be started as soon as all but one drive is present.
Note that neither of these approaches is really ideal. If it can
be known that all device discovery has completed, then
mdadm -IRs
can be run which will try to start all arrays that are being
incrementally assembled. They are started in "read-auto" mode in
which they are read-only until the first write request. This means
that no metadata updates are made and no attempt at resync or recovery
happens. Further devices that are found before the first write can
still be added safely.
These checks can be suppressed by setting IMSM_NO_PLATFORM=1 in the environment. This can be useful for testing or for disaster recovery. You should be aware that interoperability may be compromised by setting this value.
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 config
file. This command will typically go in a system startup file.
mdadm --stop --scan
This will shut down all arrays that can be shut down (i.e. are not
currently 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
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 arrays which could be assembled from existing IDE and
SCSI whole drives (not partitions), and store the information in 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 were 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 > /run/mdadm/mon.pid
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
/run/mdadm/mon.pid.
mdadm -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as
appropriate.
mdadm --incremental --rebuild-map --run --scan
Rebuild the array map from any current arrays, and then start any that
can be started.
mdadm /dev/md4 --fail detached --remove detached
Any devices which are components of /dev/md4 will be marked as faulty
and then remove from the array.
mdadm --grow /dev/md4 --level=6 --backup-file=/root/backup-md4
The array
/dev/md4
which is currently a RAID5 array will be converted to RAID6. There
should normally already be a spare drive attached to the array as a
RAID6 needs one more drive than a matching RAID5.
mdadm --create /dev/md/ddf --metadata=ddf --raid-disks 6 /dev/sd[a-f]
Create a DDF array over 6 devices.
mdadm --create /dev/md/home -n3 -l5 -z 30000000 /dev/md/ddf
Create a RAID5 array over any 3 devices in the given DDF set. Use
only 30 gigabytes of each device.
mdadm -A /dev/md/ddf1 /dev/sd[a-f]
Assemble a pre-exist ddf array.
mdadm -I /dev/md/ddf1
Assemble all arrays contained in the ddf array, assigning names as
appropriate.
mdadm --create --help
Provide help about the Create mode.
mdadm --config --help
Provide help about the format of the config file.
mdadm --help
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.
A directory containing configuration files which are read in lexical order.
mdadm understand two sorts of names for array devices.
The first is the so-called 'standard' format name, which matches the names used by the kernel and which appear in /proc/mdstat.
The second sort can be freely chosen, but must reside in /dev/md/. When giving a device name to mdadm to create or assemble an array, either full path name such as /dev/md0 or /dev/md/home can be given, or just the suffix of the second sort of name, such as home can be given.
When mdadm chooses device names during auto-assembly or incremental assembly, it will sometimes add a small sequence number to the end of the name to avoid conflicted between multiple arrays that have the same name. If mdadm can reasonably determine that the array really is meant for this host, either by a hostname in the metadata, or by the presence of the array in mdadm.conf, then it will leave off the suffix if possible. Also if the homehost is specified as <ignore> mdadm will only use a suffix if a different array of the same name already exists or is listed in the config file.
The standard names for non-partitioned arrays (the only sort of md array available in 2.4 and earlier) are of the form
where NN is a number. The standard names for partitionable arrays (as available from 2.6 onwards) are of the form:
Partition numbers should be indicated by adding "pMM" to these, thus "/dev/md/d1p2".
From kernel version 2.6.28 the "non-partitioned array" can actually be partitioned. So the "md_dNN" names are no longer needed, and partitions such as "/dev/mdNNpXX" are possible.
From kernel version 2.6.29 standard names can be non-numeric following the form:
where XXX is any string. These names are supported by mdadm since version 3.3 provided they are enabled in mdadm.conf.
(based upon Jakob Østergaard's Software-RAID.HOWTO)
The latest version of mdadm should always be available from
Related man pages:
mdmon(8), mdadm.conf(5), md(4).