mdadm - Unix, Linux Command

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mdadm - manage MD devices aka Linux Software RAID


mdadm [mode] <raiddevice> [options] <component-devices>


RAID devices are virtual devices created from two or more real block devices. This allows multiple devices (typically disk drives or partitions thereof) 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, RAID10, MULTIPATH, and FAULTY.

MULTIPATH is not a Software RAID mechanism, but does involve multiple devices: each device is a path to one common physical storage device.

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.


mdadm has several major modes of operation:
  Assemble the components 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 an array that doesn’t have per-device superblocks. For these sorts of arrays, mdadm cannot differentiate between initial creation and subsequent assembly of an array. It also cannot perform any checks that appropriate components have been requested. Because of this, the Build mode should only be used together with a complete understanding of what you are doing.

Create Create a new array with per-device superblocks.

Follow or Monitor
  Monitor one or more md devices and act on any state changes. This is only meaningful for raid1, 4, 5, 6, 10 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 include changing the active size of component devices and changing the number of active devices in RAID levels 1/4/5/6, as well as adding or removing a write-intent bitmap.

Incremental Assembly
  Add a single device to an appropriate array. If the addition of the device makes the array runnable, the array will be started. This provides a convenient interface to a hot-plug system. As each device is detected, mdadm has a chance to include it in some array as appropriate.

Manage This is for doing things to specific components of an array such as adding new spares and removing faulty devices.

Misc This is an ’everything else’ mode that supports operations on active arrays, operations on component devices such as erasing old superblocks, and information gathering operations.

  This mode does not act on a specific device or array, but rather it requests the Linux Kernel to activate any auto-detected arrays.


Options for selecting a mode are:

-A, --assemble
  Assemble a pre-existing array.

-B, --build
  Build a legacy array without superblocks.

-C, --create
  Create a new array.

-F, --follow, --monitor
  Select Monitor mode.

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

-I, --incremental
  Add a single device into an appropriate array, and possibly start the array.

  Request that the kernel starts any auto-detected arrays. This can only work if md is compiled into the kernel — not if it is a module. Arrays can be auto-detected by the kernel if all the components are in primary MS-DOS partitions with partition type FD. In-kernel autodetect is not recommended for new installations. Using mdadm to detect and assemble arrays — possibly in an initrd — is substantially more flexible and should be preferred.

If a device is given before any options, or if the first option is --add, --fail, or --remove, then the MANAGE mode is assumed. Anything other than these will cause the Misc mode to be assumed.

Options that are not mode-specific are:

-h, --help
  Display general help message or, after one of the above options, a mode-specific help message.

  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. This can be used twice to be extra-verbose. The extra verbosity currently only affects --detail --scan and --examine --scan.

-q, --quiet
  Avoid printing purely informative messages. With this, mdadm will be silent unless there is something really important to report.

-b, --brief
  Be less verbose. This is used with --detail and --examine. Using --brief with --verbose gives an intermediate level of verbosity.

-f, --force
  Be more forceful about certain operations. See the various modes for the exact meaning of this option in different contexts.

-c, --config=
  Specify the config file. Default is to use /etc/mdadm.conf, or if that is missing then /etc/mdadm/mdadm.conf. If the config file given is partitions then nothing will be read, but mdadm will act as though the config file contained exactly DEVICE partitions and will read /proc/partitions to find a list of devices to scan. If the word none is given for the config file, then mdadm will act as though the config file were empty.

-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 (see previous option); 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.

-e , --metadata=
  Declare the style of superblock (raid metadata) to be used. The default is 0.90 for --create, and to guess for other operations. The default can be overridden by setting the metadata value for the CREATE keyword in mdadm.conf.

Options are:

0, 0.90, default Use the original 0.90 format superblock. This format limits arrays to 28 component devices and limits component devices of levels 1 and greater to 2 terabytes.
1, 1.0, 1.1, 1.2 Use the new version-1 format superblock. This has few restrictions. The different sub-versions store the superblock at different locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from the start (for 1.2).

  This will override any HOMEHOST setting in the config file and provides the identity of the host which should be considered the home for any arrays.

When creating an array, the homehost will be recorded in the superblock. 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 assembled.

For create, build, or grow:

-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, RAID5 and RAID6 arrays, and only on kernels which provide necessary support.

-x, --spare-devices=
  Specify the number of spare (eXtra) devices in the initial array. The number of component devices listed on the command line must equal the number of raid devices plus the number of spare devices.

After initial array creation, new devices are added to the array using the --add command. If you add devices in excess of the number needed for the array, they are automatically treated as spare devices. For grow mode, it is not possible to grow the number of spare devices, instead you need to grow (or shrink) the number of active devices in the array. Spare devices are handled automatically after initial array creation.

-z, --size=
  Amount (in Kibibytes) of space to use from each drive in RAID level 1/4/5/6. 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. The size can be given as max which means to choose the largest size that fits on all current drives.

-c, --chunk=
  Specify chunk size in kibibytes. The default is 64.

  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, raid10, 10, multipath, mp, faulty. Obviously some of these are synonymous.

When used with --build, only linear, stripe, raid0, 0, raid1, multipath, mp, and faulty are valid.

Not yet supported with --grow.

-p, --layout=
  This option configures the fine details of data layout for raid5, and raid10 arrays, and controls the failure modes for faulty.

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.

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.

To set the parity with --grow, the level of the array ("faulty") must be specified before the fault mode is specified.

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’ 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).

  same as --layout (thus explaining the p of -p).

-b, --bitmap=
  Specify a file to store a write-intent bitmap in. The file should not exist unless --force is also given. The same file should be provided when assembling the array. The file may not reside on a filesystem that is built on top of the array the bitmap file is for or else a kernel deadlock will occur. This is not a bug, it’s a feature. If the word internal is given, then the bitmap is stored with the metadata on the array, and so is replicated on all devices. If the word none is given with --grow mode, then any bitmap that is present is removed.

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.

Note: The choice of internal versus external bitmap can have a drastic impact on performance. While an internal bitmap is the most convenient as it doesn’t require a device totally separate from the array on which to store the bitmap file, it has a larger impact on performance than an external bitmap. This is because we can’t predict which device in the array might fail, so we store a copy of the bitmap on every device in the array when using an internal bitmap. This means that prior to allowing a write to a section of the array that is currently marked clean in the bitmap, we must issue a write to change the bit for that section of the array from clean to dirty, and must wait for the bitmap write to complete on all of the array devices before the pending write to the array data area can proceed. Especially if the array is under heavy load, these syncronous writes can drastically impact performance. An external bitmap file is less convenient, but there is only one copy of the bitmap, so there is only one bitmap write that must complete before the pending write to the array data can proceed. In addition, if your bitmap file device is not heavily loaded, and the array is, then you will notice a considerable performance benefit from the fact that bitmap writes are not competing with array reads/writes. The performance impact of this option can be somewhat mitigated by appropriate selection of a bitmap chunk size (next option).

  Set the chunksize of the bitmap. Each bit corresponds to that many Kilobytes of storage. When using a file based bitmap, the default is to use the smallest size that is at-least 4 and requires no more than 2^21 chunks. When using an internal bitmap, the chunksize is automatically determined to make best use of available space.

Note: This option can drastically effect performance of the array. The more granular the bitmap is, then the more frequently writes will trigger syncronous bitmap updates and be delayed until the bitmap update is complete. The trade off is that a more granular bitmap means a shorter array resync time after any event causes the array to go down unclean. Given raw drive speeds can be in excess of 100MB/s on modern SATA/SAS drives, any bitmap chunk up to 262144 (256MB) can generally be synced in a matter of just a few seconds. Smaller chunks can be synced faster, but you reach a point of diminishing returns that is quickly offset by the increased write performance degradation seen in every day operation. Considering that the smaller bitmap chunk sizes will only ever be a benefit on rare occasions (hopefully never), but that you will pay for a small bitmap chunk every single day, it is recommended that you select the largest bitmap chunk size you feel comforable with.

-W, --write-mostly
  subsequent devices lists in a --build, --create, or --add command will be flagged as ’write-mostly’. This is valid for RAID1 only and means that the ’md’ driver will avoid reading from these devices if at all possible. This can be useful if mirroring over a slow link.

  Specify that write-behind mode should be enabled (valid for RAID1 only). If an argument is specified, it will set the maximum number of outstanding writes allowed. The default value is 256. A write-intent bitmap is required in order to use write-behind mode, and write-behind is only attempted on drives marked as write-mostly.

  Tell mdadm that the array pre-existed and is known to be clean. It can be useful when trying to recover from a major failure as you can be sure that no data will be affected unless you actually write to the array. It can also be used when creating a RAID1 or RAID10 if you want to avoid the initial resync, however this practice — while normally safe — is not recommended. Use this only if you really know what you are doing.

  This is needed when --grow is used to increase the number of raid-devices in a RAID5 if there are no spare devices available. See the section below on RAID_DEVICE CHANGES. The file should be stored on a separate device, not on the raid array being reshaped.

-N, --name=
  Set a name for the array. This is currently only effective when creating an array with a version-1 superblock. The name is a simple textual string that can be used to identify array components when assembling.

-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, possibly allocating an unused minor number. "md" causes a non-partitionable array to be used. "mdp", "part" or "p" causes a partitionable array (2.6 and later) to be used. "yes" requires the named md device to have a ’standard’ format, and the type and minor number will be determined from this. See DEVICE NAMES below.

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/home1p3". If there is no trailing digit, then the partition names just have a number added, e.g. "/dev/scratch3".

If the md device name is in a ’standard’ format as described in DEVICE NAMES, then it will be created, if necessary, with the appropriate number based on that name. If the device name is not in one of these formats, then a unused minor number will be allocated. The minor number will be considered unused if there is no active array for that number, and there is no entry in /dev for that number and with a non-standard name.

  Normally when --auto causes mdadm to create devices in /dev/md/ it will also create symlinks from /dev/ with names starting with md or md_. Use --symlink=no to suppress this, or --symlink=yes to enforce this even if it is suppressing mdadm.conf.

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, --super-minor=dev will look for super blocks with a minor number of 0.

-N, --name=
  Specify the name of the array to assemble. This must be the name that was specified when creating the array. It must either match the name stored in the superblock exactly, or it must match with the current homehost prefixed to the start of the given name.

-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 were present last time the array was active. Normally if not all the expected 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.

  This is the reverse of --run in that it inhibits the startup of array unless all expected drives are present. This is only needed with --scan, and can be used if the physical connections to devices are not as reliable as you would like.

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

-b, --bitmap=
  Specify the bitmap file that was given when the array was created. If an array has an internal bitmap, there is no need to specify this when assembling the array.

  If --backup-file was used to grow the number of raid-devices in a RAID5, and the system crashed during the critical section, then the same --backup-file must be presented to --assemble to allow possibly corrupted data to be restored.

-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, uuid, name, homehost, resync, byteorder, devicesize, 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 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 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 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. 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 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.

  This flag is only meaningful with auto-assembly (see discussion below). In that situation, if no suitable arrays are found for this homehost, mdadm will rescan for any arrays at all and will assemble them and update the homehost to match the current host.

For Manage mode:

-a, --add
  add listed devices to a live array. When the array is in a degraded state and you add a device, the device will be added as a spare device and reconstruction on to the spare device will commence. Upon completion of the reconstruction, the device will be transitioned to an active device. If you add more devices than the array’s normal capacity of active devices, then they are automatically added as hot spare devices. In order to utilize the spare devices, use the Grow mode of mdadm to increase the number of active devices in the array.

--re-add re-add a device that was recently removed from an array. This only applies to devices that were part of an array built without a persistent superblock, and for which a write intent bitmap exists. In this isolated case, the kernel will treat this device as a previous member of the array even though there is no superblock to tell it to do so. For all add operations involving arrays with persistent superblocks, use the --add command above and the kernel will automatically determine whether a full resync or partial resync is needed based upon the superblock state and the write intent bitmap state (if it exists).

-r, --remove
  remove listed devices. They must not be active. i.e. they should be failed or spare devices. As well as the name of a device file (e.g. /dev/sda1) the words failed and detached 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. This will only succeed for devices that are spares or have already been marked as failed.

-f, --fail
  mark listed devices as faulty. As well as the name of a device file, the word detached can be given. This will cause any device that has been detached from the system to be marked as failed. It can then be removed.

  same as --fail.

  Subsequent devices that are added or re-added will have the ’write-mostly’ flag set. This is only valid for RAID1 and means that the ’md’ driver will avoid reading from these devices if possible.
  Subsequent devices that are added or re-added will have the ’write-mostly’ flag cleared.

Each of these options require that the first device listed is the array to be acted upon, and the remainder are component devices to be added, removed, or marked as faulty. 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 was removed recently by using --re-add instead of --add command (see above).

Devices can only be removed from an array if they are not in active use, i.e. they must be spares or failed devices. To remove an active device, it must first be marked as faulty.

For Misc mode:

-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.

-Y, --export
  When used with --detail or --examine, output will be formatted as key=value pairs for easy import into the environment.

-E, --examine
  Print content of md superblock on device(s).
  If 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.

-X, --examine-bitmap
  Report information about a bitmap file. The argument is either an external bitmap file or an array component in case of an internal bitmap.

-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.

  If the device contains a valid md superblock, the block is overwritten with zeros. With --force the block where the superblock would be is overwritten 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.

-W, --wait
  For each md device given, wait for any resync, recovery, or reshape activity to finish before returning. mdadm will return with success if it actually waited for every device listed, otherwise it will return failure.

For Incremental Assembly mode:

--rebuild-map, -r
  Rebuild the map file (/var/run/mdadm/map) that mdadm uses to help track which arrays are currently being assembled.

--run, -R
  Run any array assembled as soon as a minimal number of devices are available, rather than waiting until all expected devices are present.

--scan, -s
  Only meaningful with -R this will scan the map file for arrays that are being incrementally assembled and will try to start any that are not already started. If any such array is listed in mdadm.conf as requiring an external bitmap, that bitmap will be attached first.

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.

-y, --syslog
  Cause all events to be reported through ’syslog’. The messages have facility of ’daemon’ and varying priorities.

-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.

-i, --pid-file
  When mdadm is running in daemon mode, write the pid of the daemon process to the specified file, instead of printing it on standard output.

-1, --oneshot
  Check arrays only once. This will generate NewArray events and more significantly DegradedArray and SparesMissing 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 do get through successfully.


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 identity information is extracted from the configuration file.

The identity can be given with the --uuid option, with 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 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, 6, or 10), give the --run flag.

If the md device does not exist, then it will be created providing the intent is clear. i.e. the name must be in a standard form, or the --auto option must be given to clarify how and whether the device should be created. 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 in use 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 option "auto" 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.

Auto Assembly

When --assemble is used with --scan and no devices are listed, mdadm will first attempt to assemble all the arrays listed in the config file.

If a homehost has been specified (either in the config file or on the command line), mdadm will look further for possible arrays and will try to assemble anything that it finds which is tagged as belonging to the given homehost. This is the only situation where mdadm will assemble arrays without being given specific device name or identity information for the array.

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).

If mdadm cannot find any array for the given host at all, and if --auto-update-homehost is given, then mdadm will search again for any array (not just an array created for this host) and will assemble each assuming --update=homehost. This will change the host tag in the superblock so that on the next run, these arrays will be found without the second pass. The intention of this feature is to support transitioning a set of md arrays to using homehost tagging.

The reason for requiring arrays to be tagged with the homehost for auto assembly is to guard against problems that can arise when moving devices from one host to another.


Usage: mdadm --build md-device --chunk=X --level=Y --raid-devices=Z devices

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, multipath, or faulty, or one of their synonyms. All devices must be listed and the array will be started once complete.


Usage: mdadm --create md-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.

If 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 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.

The General Management options that are valid with --create are:

--run insist on running the array even if some devices look like they might be in use.

  start the array readonly — not supported yet.


Usage: mdadm device options... devices...

This usage will allow individual devices in an array to be failed, removed or added. It is possible to perform multiple operations with on command. For example:
mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1
will firstly mark /dev/hda1 as faulty in /dev/md0 and will then remove it from the array and finally add it back in as a spare. However only one md array can be affected by a single command.


Usage: mdadm options ... devices ...

MISC mode includes a number of distinct operations that operate on 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.

  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 such that it is unusable.
4 There was an error while trying to get information about the device.

  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 or --scan is given, 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.

  This will mark an active array as read-only, providing that it is not currently being used.

  This will change a readonly array back to being read/write.

--scan For all operations except --examine, --scan will cause the 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.


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 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:

  An md array which previously was configured appears to no longer be configured. (syslog priority: Critical)

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.

  An md array started reconstruction. (syslog priority: Warning)

  Where NN is 20, 40, 60, or 80, this indicates that rebuild has passed that many percentage of the total. (syslog priority: Warning)

  An md array that was rebuilding, isn’t any more, either because it finished normally or was aborted. (syslog priority: Warning)

Fail An active component device of an array has been marked as faulty. (syslog priority: Critical)

  A spare component device which was being rebuilt to replace a faulty device has failed. (syslog priority: Critical)

  A spare component device which was being rebuilt to replace a faulty device has been successfully rebuilt and has been made active. (syslog priority: Info)

  A new md array has been detected in the /proc/mdstat file. (syslog priority: Info)

  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. (syslog priority: Critical)

  A spare drive has been moved from one array in a spare-group to another to allow a failed drive to be replaced. (syslog priority: Info)

  If mdadm has been told, via the config file, that an array should have a certain number of spare devices, and mdadm detects that it has fewer than this number when it first sees the array, it will report a SparesMissing message. (syslog priority: Warning)

  An array was found at startup, and the --test flag was given. (syslog priority: Info)

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 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.


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 are being added during 2.6 development, including restructuring a raid5 array to have more active devices.

Currently the only support available is to

o change the "size" attribute for RAID1, RAID5 and RAID6.
o increase the "raid-devices" attribute of RAID1, RAID5, and RAID6.
o add a write-intent bitmap to any array which supports these bitmaps, or remove a write-intent bitmap from such an array.


Normally when an array is built 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 when you want to increase or decrease the number of active devices. Note that this is different than 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.

Increasing the number of active devices in a RAID5 is much more work. Every block in the array will need to be moved to a new location. From 2.6.17, the Linux Kernel is able to do this safely, including restarting an interrupted "reshape".

When relocating the first few stripes on a raid5, 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 makes a backup of the data that is in that section. This backup is normally 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. 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.


A write-intent bitmap can be added to, or removed from, an active array. Either internal bitmaps or an external bitmap stored in a file can be added. In the case of internal bitmaps, there is one copy of the bitmap per device (since you never know what device might fail, you need a copy on every device). The bitmap is stored between the array data and the superblock, which limits the total number of bits available. For a bitmap in an external file, only one copy is needed, but this assumes that the bitmap file is not on an array device or else failure of that device would take the only copy of the bitmap with it. For this reason, the fact that the kernel will deadlock if you attempt to use a file that resides on the array it is the bitmap for is considered a safety feature.


Usage: mdadm --incremental [--run] [--quiet] component-device
Usage: mdadm --incremental --rebuild
Usage: mdadm --incremental --run --scan

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.

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 only add devices to an array which were previously working (active or spare) parts of that array. It does not currently support automatic inclusion of a new drive as a spare in some array.

mdadm --incremental requires a bug-fix in all kernels through 2.6.19. Hopefully, this will be fixed in 2.6.20; alternately, apply the patch which is included with the mdadm source distribution. If mdadm detects that this bug is present, it will abort any attempt to use --incremental.

The tests that mdadm makes are as follow:

+ Is the device permitted by mdadm.conf? That is, is it listed in a DEVICES line in that file. If DEVICES is absent then the default it to allow any device. Similar if DEVICES contains the special word partitions then any device is allowed. Otherwise the device name given to mdadm must match one of the names or patterns in a DEVICES line.

+ Does the device have a valid md superblock. If a specific metadata version is request with --metadata or -e then only that style of metadata is accepted, otherwise mdadm finds any known version of metadata. If no md metadata is found, the device is rejected.

+ Does the metadata match an expected array? The metadata can match in two ways. Either there is an array listed in mdadm.conf which identifies the array (either by UUID, by name, by device list, or by minor-number), or the array was created with a homehost specified and that homehost matches the one in mdadm.conf or on the command line. If mdadm is not able to positively identify the array as belonging to the current host, the device will be rejected.

+ mdadm keeps a list of arrays that it has partially assembled in /var/run/mdadm/map (or /var/run/ if the directory doesn’t exist). 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.

+ 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.


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 > /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 -Iq /dev/somedevice
Try to incorporate newly discovered device into some array as appropriate.

mdadm --incremental --rebuild --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 --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.


When --incremental mode is used, this file gets a list of arrays currently being created. If /var/run/mdadm does not exist as a directory, then /var/run/ is used instead.


While entries in the /dev directory can have any format you like, mdadm has an understanding of ’standard’ formats which it uses to guide its behaviour when creating device files via the --auto option.

The standard names for non-partitioned arrays (the only sort of md array available in 2.4 and earlier) are either of

where NN is a number. The standard names for partitionable arrays (as available from 2.6 onwards) are either of
Partition numbers should be indicated by added "pMM" to these, thus "/dev/md/d1p2".


mdadm was previously known as mdctl.

mdadm is completely separate from the raidtools package, and does not use the /etc/raidtab configuration file at all.


RAID, see:
(based upon Jakob \(/Ostergaard’s Software-RAID.HOWTO)
The latest version of mdadm should always be available from
Related man pages:

mdadm.conf(5), md(4).

raidtab(5), raid0run(8), raidstop(8), mkraid(8).
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