-A

Assemble a pre-existing array.

-B

Build a legacy array without superblocks.

-C

Create a new array.

-F

Select Monitor mode.

-G

Change the size or shape of an active array.

-I

Add/remove a single device to/from an appropriate array, and possibly start the array.

--auto-detect

Request that the kernel starts any auto-detected arrays. This can only work if md is compiled into the kernel — not if it isa module. Arrays can be auto-detected by the kernel if all the components are in primary MS-DOS partitions with partitiontype FD, and all use v0.90 metadata. In-kernel autodetect is not recommended for new installations. Using mdadm to detectand 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 one of --add, --re-add, --add-spare, --fail, --remove, or

-h

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

--help-options

Display more detailed help about command line parsing and some commonly used options.

-V

Print version information for mdadm.

-v

Be more verbose about what is happening. This can be used twice to be extra-verbose. The extra verbosity currently onlyaffects --detail --scan and --examine --scan.

-q

Avoid printing purely informative messages. With this, mdadm will be silent unless there is something really important toreport.

-f

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

-c

Specify the config file or directory. Default is to use /etc/mdadm/mdadm.conf and /etc/mdadm/mdadm.conf.d, or if those aremissing then /etc/mdadm.conf and /etc/mdadm.conf.d. If the config file given is partitions then nothing will be read, butmdadm will act as though the config file contained exactlyDEVICE partitions containersand will read /proc/partitions to find a list of devices to scan, and /proc/mdstat to find a list of containers to examine.If the word none is given for the config file, then mdadm will act as though the config file were empty.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.

-s

Scan config file or /proc/mdstat for missing information. In general, this option gives mdadm permission to get any missinginformation (like component devices, array devices, array identities, and alert destination) from the configuration file (seeprevious option); one exception is MISC mode when using --detail or --stop, in which case --scan says to get a list of arraydevices from /proc/mdstat.

-e

Declare the style of RAID metadata (superblock) to be used. The default is 1.2 for --create, and to guess for other opera‐tions. The default can be overridden by setting the metadata value for the CREATE keyword in mdadm.conf.Options are:0, 0.90Use the original 0.90 format superblock. This format limits arrays to 28 component devices and limits componentdevices of levels 1 and greater to 2 terabytes. It is also possible for there to be confusion about whether thesuperblock applies to a whole device or just the last partition, if that partition starts on a 64K boundary.1, 1.0, 1.1, 1.2 defaultUse the new version-1 format superblock. This has fewer restrictions. It can easily be moved between hosts with dif‐ferent endian-ness, and a recovery operation can be checkpointed and restarted. The different sub-versions store thesuperblock at different locations on the device, either at the end (for 1.0), at the start (for 1.1) or 4K from thestart (for 1.2). "1" is equivalent to "1.2" (the commonly preferred 1.x format). "default" is equivalent to "1.2".ddf Use the "Industry Standard" DDF (Disk Data Format) format defined by SNIA. When creating a DDF array a CONTAINER willbe created, and normal arrays can be created in that container.imsm Use the Intel(R) Matrix Storage Manager metadata format. This creates a CONTAINER which is managed in a similar mannerto DDF, and is supported by an option-rom on some platforms:http://www.intel.com/design/chipsets/matrixstorage_sb.htm

--homehost

This will override any HOMEHOST setting in the config file and provides the identity of the host which should be consideredthe home for any arrays.When creating an array, the homehost will be recorded in the metadata. For version-1 superblocks, it will be prefixed to thearray 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 bestored in the array and it can be assembled in the same way on any host. If an array is assembled with this option, then thehomehost recorded on the array will be ignored.

--prefer

When mdadm needs to print the name for a device it normally finds the name in /dev which refers to the device and is shortest.When a path component is given with --prefer mdadm will prefer a longer name if it contains that component. For example

--prefer=by-uuid

This functionality is currently only provided by --detail and --monitor.

--home-cluster

specifies the cluster name for the md device. The md device can be assembled only on the cluster which matches the name speci‐fied. If this option is not provided, mdadm tries to detect the cluster name automatically.For create, build, or grow:

-n

Specify the number of active devices in the array. This, plus the number of spare devices (see below) must equal the numberof component-devices (including "missing" devices) that are listed on the command line for --create. Setting a value of 1 isprobably 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, RAID5 or RAID6.This number can only be changed using --grow for RAID1, RAID4, RAID5 and RAID6 arrays, and only on kernels which provide thenecessary support.

-x

Specify the number of spare (eXtra) devices in the initial array. Spares can also be added and removed later. The number ofcomponent devices listed on the command line must equal the number of RAID devices plus the number of spare devices.

-z

Amount (in Kilobytes) of space to use from each drive in RAID levels 1/4/5/6. This must be a multiple of the chunk size, andmust leave about 128Kb of space at the end of the drive for the RAID superblock. If this is not specified (as it normally isnot) the smallest drive (or partition) sets the size, though if there is a variance among the drives of greater than 1%, awarning is issued.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 stan‐dards. Such a replacement drive will be rejected by md. To guard against this it can be useful to set the initial sizeslightly 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 maynot be able to support this. If the array was created with a size smaller than the currently active drives, the extra spacecan be accessed using --grow. The size can be given as max which means to choose the largest size that fits on all currentdrives.Before reducing the size of the array (with --grow --size=) you should make sure that space isn't needed. If the device holdsa 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 afilesystem, then an 'fsck' of the filesystem is a minimum requirement. If there are problems the array can be made biggeragain 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 creat‐ing an array inside a container.

-Z

This is only meaningful with --grow and its effect is not persistent: when the array is stopped and restarted the defaultarray size will be restored.Setting the array-size causes the array to appear smaller to programs that access the data. This is particularly neededbefore 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, youwould 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 afilesystem, then an 'fsck' of the filesystem is a minimum requirement. If there are problems the array can be made biggeragain 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 restoresthe apparent size of the array to be whatever the real amount of available space is.Clustered arrays do not support this parameter yet.

--rounding

Specify rounding factor for a Linear array. The size of each component will be rounded down to a multiple of this size. Thisis a synonym for --chunk but highlights the different meaning for Linear as compared to other RAID levels. The default is 64Kif a kernel earlier than 2.6.16 is in use, and is 0K (i.e. no rounding) in later kernels.

-l

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, container. Obviously some of these are synonymous.When a CONTAINER metadata type is requested, only the container level is permitted, and it does not need to be explicitlygiven.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.

-p

This option configures the fine details of data layout for RAID5, RAID6, and RAID10 arrays, and controls the failure modes forfaulty.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 convertingbetween RAID5 and RAID6. These provide a layout which is identical to the corresponding RAID5 layout on the first N-1devices, and has the 'Q' syndrome (the second 'parity' block used by RAID6) on the last device. These layouts are: left-sym‐metric-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, thefault 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 sup‐ported 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 arerotated 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 thenumber 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-strip‐ing) 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.

--parity

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

-b

Specify a file to store a write-intent bitmap in. The file should not exist unless --force is also given. The same fileshould be provided when assembling the array. If the word internal is given, then the bitmap is stored with the metadata onthe array, and so is replicated on all devices. If the word none is given with --grow mode, then any bitmap that is presentis removed. If the word clustered is given, the array is created for a clustered environment. One bitmap is created for eachnode as defined by the --nodes parameter and are stored internally.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 seri‐ous problems.When creating an array on devices which are 100G or larger, mdadm automatically adds an internal bitmap as it will usually bebeneficial. This can be suppressed with --bitmap=none or by selecting a different consistency policy with --consistency-pol‐icy.

--bitmap-chunk

Set the chunksize of the bitmap. Each bit corresponds to that many Kilobytes of storage. When using a file based bitmap, thedefault 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 defaults to 64Meg, or larger if necessary to fit the bitmap into the available space.A suffix of 'K', 'M' or 'G' can be given to indicate Kilobytes, Megabytes or Gigabytes respectively.

-W

subsequent devices listed in a --build, --create, or --add command will be flagged as 'write-mostly'. This is valid for RAID1only and means that the 'md' driver will avoid reading from these devices if at all possible. This can be useful if mirroringover a slow link.

--write-behind

Specify that write-behind mode should be enabled (valid for RAID1 only). If an argument is specified, it will set the maximumnumber 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.

--failfast

subsequent devices listed in a --create or --add command will be flagged as 'failfast'. This is valid for RAID1 and RAID10only. IO requests to these devices will be encouraged to fail quickly rather than cause long delays due to error handling.Also no attempt is made to repair a read error on these devices.If an array becomes degraded so that the 'failfast' device is the only usable device, the 'failfast' flag will then be ignoredand 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 sometimescause unacceptable delays due to internal maintenance functions.

--assume-clean

Tell mdadm that the array pre-existed and is known to be clean. It can be useful when trying to recover from a major failureas you can be sure that no data will be affected unless you actually write to the array. It can also be used when creating aRAID1 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.When the devices that will be part of a new array were filled with zeros before creation the operator knows the array is actu‐ally clean. If that is the case, such as after running badblocks, this argument can be used to tell mdadm the facts the opera‐tor knows.When an array is resized to a larger size with --grow --size= the new space is normally resynced in that same way that thewhole array is resynced at creation. From Linux version 3.0, --assume-clean can be used with that command to avoid the auto‐matic resync.

--backup-file

This is needed when --grow is used to increase the number of raid-devices in a RAID5 or RAID6 if there are no spare devicesavailable, or to shrink, change RAID level or layout. See the GROW MODE section below on RAID-DEVICES CHANGES. The file mustbe stored on a separate device, not on the RAID array being reshaped.

--data-offset

Arrays with 1.x metadata can leave a gap between the start of the device and the start of array data. This gap can be usedfor various metadata. The start of data is known as the data-offset. Normally an appropriate data offset is computed auto‐matically. However it can be useful to set it explicitly such as when re-creating an array which was originally created usinga different version of mdadm which computed a different offset.Setting the offset explicitly over-rides the default. The value given is in Kilobytes unless a suffix of 'K', 'M' or 'G' isused 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 thedata-offset to be changed as part of the reshape process. When the data offset is changed, no backup file is required as thedifference 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 theold 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 off‐set appended to the name, separated by a colon. This makes it possible to recreate exactly an array which has varying dataoffsets (as can happen when different versions of mdadm are used to add different devices).

--continue

This option is complementary to the --freeze-reshape option for assembly. It is needed when --grow operation is interruptedand it is not restarted automatically due to --freeze-reshape usage during array assembly. This option is used together with

-N

Set a name for the array. This is currently only effective when creating an array with a version-1 superblock, or an array ina DDF container. The name is a simple textual string that can be used to identify array components when assembling. If nameis needed but not specified, it is taken from the basename of the device that is being created. e.g. when creating/dev/md/home the name will default to home.

-R

Insist that mdadm run the array, even if some of the components appear to be active in another array or filesystem. Normallymdadm will ask for confirmation before including such components in an array. This option causes that question to be sup‐pressed.

-o

Start the array read only rather than read-write as normal. No writes will be allowed to the array, and no resync, recovery,or reshape will be started. It works with Create, Assemble, Manage and Misc mode.

-a

Instruct mdadm how to create the device file if needed, possibly allocating an unused minor number. "md" causes a non-parti‐tionable array to be used (though since Linux 2.6.28, these array devices are in fact partitionable). "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, andthe type and minor number will be determined from this. With mdadm 3.0, device creation is normally left up to udev so thisoption is unlikely to be needed. 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 commandline.For partitionable arrays, mdadm will create the device file for the whole array and for the first 4 partitions. A differentnumber of partitions can be specified at the end of this option (e.g. --auto=p7). If the device name ends with a digit, thepartition names add a 'p', and a number, e.g. /dev/md/home1p3. If there is no trailing digit, then the partition names justhave 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 theappropriate device number based on that name. If the device name is not in one of these formats, then a unused device numberwill be allocated. The device number will be considered unused if there is no active array for that number, and there is noentry 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.

--nodes

Only works when the array is for clustered environment. It specifies the maximum number of nodes in the cluster that will usethis device simultaneously. If not specified, this defaults to 4.

--write-journal

Specify journal device for the RAID-4/5/6 array. The journal device should be a SSD with reasonable lifetime.

--symlinks

Auto creation of symlinks in /dev to /dev/md, option --symlinks must be 'no' or 'yes' and work with --create and --build.

-k

Specify how the array maintains consistency in case of unexpected shutdown. Only relevant for RAID levels with redundancy.Currently supported options are:resync Full resync is performed and all redundancy is regenerated when the array is started after unclean shutdown.bitmap Resync assisted by a write-intent bitmap. Implicitly selected when using --bitmap.journalFor RAID levels 4/5/6, journal device is used to log transactions and replay after unclean shutdown. Implicitlyselected when using --write-journal.ppl For RAID5 only, Partial Parity Log is used to close the write hole and eliminate resync. PPL is stored in the metadataregion of RAID member drives, no additional journal drive is needed.Can be used with --grow to change the consistency policy of an active array in some cases. See CONSISTENCY POLICY CHANGESbelow.For assemble:

-u

uuid of array to assemble. Devices which don't have this uuid are excluded

-m

Minor number of device that array was created for. Devices which don't have this minor number are excluded. If you create anarray 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 assem‐bled. 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.

--no-degraded

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

--invalid-backup

If the file needed for the above option is not available for any reason an empty file can be given together with this optionto indicate that the backup file is invalid. In this case the data that was being rearranged at the time of the crash couldbe irrecoverably lost, but the rest of the array may still be recoverable. This option should only be used as a last resortif there is no way to recover the backup file.

-U

Update the superblock on each device while assembling the array. The argument given to this flag can be one of sparc2.2, sum‐maries, uuid, name, nodes, homehost, home-cluster, resync, byteorder, devicesize, no-bitmap, bbl, no-bbl, ppl, no-ppl, meta‐data, 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 Linuxkernel. This kernel got the alignment of part of the superblock wrong. You can use the --examine --sparc2.2 option to mdadmto 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 beingassembled. This can be useful if --examine reports a different "Preferred Minor" to --detail. In some cases this update willbe performed automatically by the kernel driver. In particular the update happens automatically at the first write to anarray 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 newUUID 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-1superblocks.The nodes option will change the nodes of the array as stored in the bitmap superblock. This option only works for a clusteredenvironment.The homehost option will change the homehost as recorded in the superblock. For version-0 superblocks, this is the same asupdating 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 forclustered 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 redun‐dant information is correct.The byteorder option allows arrays to be moved between machines with different byte-order, such as from a big-endian machinelike a Sparc or some MIPS machines, to a little-endian x86_64 machine. When assembling such an array for the first time aftera move, giving --update=byteorder will cause mdadm to expect superblocks to have their byteorder reversed, and will correctthat 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, andspare 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 thestart of the device) and is only useful when the component device has changed size (typically become larger). The version 1metadata 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 becomeslarger, the metadata will still be visible, but the extra space will not. In this case it might be useful to assemble thearray with --update=devicesize. This will cause mdadm to determine the maximum usable amount of space on each device andupdate 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. Theold 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 thearray 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 ofany 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 containsentries, 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 spacebetween 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.

--freeze-reshape

Option is intended to be used in start-up scripts during initrd boot phase. When array under reshape is assembled during ini‐trd phase, this option stops reshape after reshape critical section is being restored. This happens before file system pivotoperation and avoids loss of file system context. Losing file system context would cause reshape to be broken.Reshape can be continued later using the --continue option for the grow command.

-t

Unless a more serious error occurred, mdadm will exit with a status of 2 if no changes were made to the array and 0 if atleast one change was made. This can be useful when an indirect specifier such as missing, detached or faulty is used inrequesting an operation on the array. --test will report failure if these specifiers didn't find any match.

--re-add

re-add a device that was previously removed from an array. If the metadata on the device reports that it is a member of thearray, and the slot that it used is still vacant, then the device will be added back to the array in the same position. Thiswill normally cause the data for that device to be recovered. However based on the event count on the device, the recoverymay only require sections that are flagged a write-intent bitmap to be recovered or may not require any recovery at all.When used on an array that has no metadata (i.e. it was built with --build) it will be assumed that bitmap-based recovery isenough 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 thedescription 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 butisn'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 andattempt to immediately re-add them. This can be useful if you are certain that the reason for failure has been resolved.

--add-spare

Add a device as a spare. This is similar to --add except that it does not attempt --re-add first. The device will be addedas a spare even if it looks like it could be an recent member of the array.

-r

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, detached and names like set-A can be given to

--replace

Mark listed devices as requiring replacement. As soon as a spare is available, it will be rebuilt and will replace the markeddevice. This is similar to marking a device as faulty, but the device remains in service during the recovery process toincrease resilience against multiple failures. When the replacement process finishes, the replaced device will be marked asfaulty.

--write-mostly

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

--readwrite

Subsequent devices that are added or re-added will have the 'write-mostly' flag cleared.

--cluster-confirm

Confirm the existence of the device. This is issued in response to an --add request by a node in a cluster. When a node adds adevice it sends a message to all nodes in the cluster to look for a device with a UUID. This translates to a udev notificationwith the UUID of the device to be added and the slot number. The receiving node must acknowledge this message with --clus‐ter-confirm. Valid arguments are <slot>:<devicename> in case the device is found or <slot>:missing in case the device is notfound.

--add-journal

Add journal to an existing array, or recreate journal for RAID-4/5/6 array that lost a journal device. To avoid interruptingon-going write opertions, --add-journal only works for array in Read-Only state.

--nofailfast

Subsequent devices that are re-added will be re-added without the 'failfast' flag set.Each of these options requires that the first device listed is the array to be acted upon, and the remainder are component devices tobe 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/sdb1Each 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 recon‐struction 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 removedrecently 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 anactive device, it must first be marked as faulty.For Misc mode:

-Q

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 dis‐covered is presented.

-D

Print details of one or more md devices.

--detail-platform

Print details of the platform's RAID capabilities (firmware / hardware topology) for a given metadata format. If used withoutargument, mdadm will scan all controllers looking for their capabilities. Otherwise, mdadm will only look at the controllerspecified by the argument in form of an absolute filepath or a link, e.g. /sys/devices/pci0000:00/0000:00:1f.2.

-Y

When used with --detail, --detail-platform, --examine, or --incremental output will be formatted as key=value pairs for easyimport into the environment.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 fromelsewhere (yes).

-E

Print contents of the metadata stored on the named device(s). Note the contrast between --examine and --detail. --examineapplies to devices which are components of an array, while --detail applies to a whole array which is currently active.

--sparc2.2

If an array was created on a SPARC machine with a 2.2 Linux kernel patched with RAID support, the superblock will have beencreated incorrectly, or at least incompatibly with 2.4 and later kernels. Using the --sparc2.2 flag with --examine will fixthe superblock before displaying it. If this appears to do the right thing, then the array can be successfully assembledusing --assemble --update=sparc2.2.

-X

Report information about a bitmap file. The argument is either an external bitmap file or an array component in case of aninternal bitmap. Note that running this on an array device (e.g. /dev/md0) does not report the bitmap for that array.

--examine-badblocks

List the bad-blocks recorded for the device, if a bad-blocks list has been configured. Currently only 1.x metadata supportsbad-blocks lists.

--restore

Save metadata from lists devices, or restore metadata to listed devices.

-S

deactivate array, releasing all resources.

-w

mark array as readwrite.

--zero-superblock

If the device contains a valid md superblock, the block is overwritten with zeros. With --force the block where thesuperblock would be is overwritten even if it doesn't appear to be valid.Note: Be careful to call --zero-superblock with clustered raid, make sure array isn't used or assembled in other cluster nodebefore execute it.

--kill-subarray

If the device is a container and the argument to --kill-subarray specifies an inactive subarray in the container, then thesubarray is deleted. Deleting all subarrays will leave an 'empty-container' or spare superblock on the drives. See

--update-subarray

If the device is a container and the argument to --update-subarray specifies a subarray in the container, then attempt toupdate the given superblock field in the subarray. See below in MISC MODE for details.

--wait-clean

For each md device given, or each device in /proc/mdstat if --scan is given, arrange for the array to be marked clean as soonas possible. mdadm will return with success if the array uses external metadata and we successfully waited. For nativearrays this returns immediately as the kernel handles dirty-clean transitions at shutdown. No action is taken if safe-modehandling is disabled.

--action

Set the "sync_action" for all md devices given to one of idle, frozen, check, repair. Setting to idle will abort any cur‐rently running action though some actions will automatically restart. Setting to frozen will abort any current action andensure no other action starts automatically.Details of check and repair can be found it md(4) under SCRUBBING AND MISMATCHES.For Incremental Assembly mode:

--rebuild-map

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

--run

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

--scan

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

--fail

This allows the hot-plug system to remove devices that have fully disappeared from the kernel. It will first fail and thenremove the device from any array it belongs to. The device name given should be a kernel device name such as "sda", not aname in /dev.

--path

Only used with --fail. The 'path' given will be recorded so that if a new device appears at the same location it can be auto‐matically added to the same array. This allows the failed device to be automatically replaced by a new device without meta‐data if it appears at specified path. This option is normally only set by a udev script.For Monitor mode:

-y

Cause all events to be reported through 'syslog'. The messages have facility of 'daemon' and varying priorities.

-d

Give a delay in seconds. mdadm polls the md arrays and then waits this many seconds before polling again. The default is 60seconds. Since 2.6.16, there is no need to reduce this as the kernel alerts mdadm immediately when there is any change.

-i

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

-1

Check arrays only once. This will generate NewArray events and more significantly DegradedArray and SparesMissing events.Runningmdadm --monitor --scan -1from a cron script will ensure regular notification of any degraded arrays.

--no-sharing

This inhibits the functionality for moving spares between arrays. Only one monitoring process started with --scan but withoutthis flag is allowed, otherwise the two could interfere with each other.ASSEMBLE MODEUsage: 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, theidentity 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 deviceslisted are treated as md devices and assembly is attempted. In the third (where no devices are listed) all md devices that arelisted in the configuration file are assembled. If no arrays are described by the configuration file, then any arrays that can befound 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 isextracted 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-devicerecord 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 containsthe 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/mdadm.conf or /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 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 forRAID1, 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/mapwhich 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 thatcould be partitioned using standard partitioning tools and one that could not. Since 2.6.28 that distinction is no longer relevantas both type of devices can be partitioned. mdadm will normally create the type that originally could not be partitioned as it has awell 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 --autooption. 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 partitiondevices 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 relevantarray.Auto AssemblyWhen --assemble is used with --scan and no devices are listed, mdadm will first attempt to assemble all the arrays listed in the con‐fig file.If no arrays are listed in the config (other than those marked <ignore>) it will look through the available devices for possiblearrays and will try to assemble anything that it finds. Arrays which are tagged as belonging to the given homehost will be assembledand started normally. Arrays which do not obviously belong to this host are given names that are expected not to conflict with any‐thing local, and are started "read-auto" so that nothing is written to any device until the array is written to. i.e. automaticresync 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 belongingto the given home host, it will automatically choose a device name and try to assemble the array. If the array uses version-0.90metadata, 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 thearray uses version-1 metadata, then the name from the superblock is used to similarly create a name in /dev/md/ (the name will haveany '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 metadatatype should, or should not, be automatically assembled. If an array is found which is not listed in mdadm.conf and has a metadataformat that is denied by the AUTO line, then it will not be assembled. The AUTO line can also request that all arrays identified asbeing 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 thebackup-file cannot be given, any reshape which requires a backup-file to continue cannot be started by auto assembly. An array whichis growing to more devices and has passed the critical section can be assembled using auto-assembly.BUILD MODEUsage: mdadm --build md-device --chunk=X --level=Y --raid-devices=Z devicesThis usage is similar to --create. The difference is that it creates an array without a superblock. With these arrays there is nodifference between initially creating the array and subsequently assembling the array, except that hopefully there is useful datathere 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 arraywill be started once complete. It will often be appropriate to use --assume-clean with levels raid1 or raid10.CREATE MODEUsage: mdadm --create md-device --chunk=X --level=Y

--raid-devices=Z

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 thevariance 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 willcause mdadm to leave the corresponding slot in the array empty. For a RAID4 or RAID5 array at most one slot can be "missing"; for aRAID6 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 buildingthe spare into a degraded array is in general faster than resyncing the parity on a non-degraded, but not clean, array. This featurecan 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, mdadmwill choose a name based on the last component of the name of the device being created. So if /dev/md3 is being created, then thename 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, youcan choose a UUID for the array by giving the --uuid= option. Be warned that creating two arrays with the same UUID is a recipe fordisaster. 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 bit‐map will automatically be added unless some other option is explicitly requested with the --bitmap option or a different consistencypolicy is selected with the --consistency-policy option. In any case space for a bitmap will be reserved so that one can be addedlater 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. Thisallows 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 auto‐matically choose which devices to use based on how much spare space is available.The General Management options that are valid with --create are:

--readonly

start the array in readonly mode.MANAGE MODEUsage: 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 operationswith on command. For example:mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1will 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. How‐ever 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 memberof 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 thearray has a write-intent bitmap which has recorded whatever changes there were, then the device will immediately become a full memberof the array and those differences recorded in the bitmap will be resolved.MISC MODEUsage: 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 discoveredis reported.

--detail

The device should be an active md device. mdadm will display a detailed description of the array. --brief or --scan willcause the output to be less detailed and the format to be suitable for inclusion in mdadm.conf. The exit status of mdadm willnormally be 0 unless mdadm failed to get useful information about the device(s); however, if the --test option is given, thenthe 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.

--metadata=

0 metadata successfully enumerated its platform components on this system1 metadata is platform independent2 metadata failed to find its platform components on this system

--examine

The device should be a component of an md array. mdadm will read the md superblock of the device and display the contents.If --brief or --scan is given, then multiple devices that are components of the one array are grouped together and reported ina single entry suitable for inclusion in mdadm.conf.Having --scan without listing any devices will cause all devices listed in the config file to be examined.

--dump

If the device contains RAID metadata, a file will be created in the directory and the metadata will be written to it. Thefile will be the same size as the device and have the metadata written in the file at the same locate that it exists in thedevice. However the file will be "sparse" so that only those blocks containing metadata will be allocated. The total spaceused will be small.The file name used in the directory will be the base name of the device. Further if any links appear in /dev/disk/by-idwhich 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.

--stop

The devices should be active md arrays which will be deactivated, as long as they are not currently in use.

--size

Notes:· Intel's native checkpointing doesn't use --backup-file option and it is transparent for assembly feature.· Roaming between Windows(R) and Linux systems for IMSM metadata is not supported during grow process.SIZE CHANGESNormally when an array is built the "size" is taken from the smallest of the drives. If all the small drives in an arrays are, oneat a time, removed and replaced with larger drives, then you could have an array of large drives with only a small amount used. Inthis situation, changing the "size" with "GROW" mode will allow the extra space to start being used. If the size is increased inthis 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 or shrink to use orvacate the space. The filesystem will need to be explicitly told to use the extra space after growing, or to reduce its size priorto 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 thesize 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.RAID-DEVICES CHANGESA 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 toincrease or decrease the number of active devices. Note that this is different to hot-add or hot-remove which changes the number ofinactive 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. Thatis, 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 andwritten back to a new location. From 2.6.17, the Linux Kernel is able to increase the number of devices in a RAID5 safely, includingrestarting an interrupted "reshape". From 2.6.31, the Linux Kernel is able to increase or decrease the number of devices in a RAID5or 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 toadd 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 getdestroyed and this is not reversible, so you should firstly shrink the filesystem on the array to fit within the new size. To helpprevent accidents, mdadm requires that the size of the array be decreased first with mdadm --grow --array-size. This is a reversiblechange which simply makes the end of the array inaccessible. The integrity of any data can then be checked before the non-reversiblereduction 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 andcrash-proof. To provide the required safety, mdadm disables writes to the array while this "critical section" is reshaped, and takesa backup of the data that is in that section. For grows, this backup may be stored in any spare devices that the array has, howeverit can also be stored in a separate file specified with the --backup-file option, and is required to be specified for shrinks, RAIDlevel changes and layout changes. If this option is used, and the system does crash during the critical period, the same file mustbe passed to --assemble to restore the backup and reassemble the array. When shrinking rather than growing the array, the reshape isdone from the end towards the beginning, so the "critical section" is at the end of the reshape.LEVEL CHANGESChanging the RAID level of any array happens instantaneously. However in the RAID5 to RAID6 case this requires a non-standard layoutof the RAID6 data, and in the RAID6 to RAID5 case that non-standard layout is required before the change can be accomplished. Sowhile the level change is instant, the accompanying layout change can take quite a long time. A --backup-file is required. If thearray is not simultaneously being grown or shrunk, so that the array size will remain the same - for example, reshaping a 3-driveRAID5 into a 4-drive RAID6 - the backup file will be used not just for a "cricital section" but throughout the reshape operation, asdescribed below under LAYOUT CHANGES.CHUNK-SIZE AND LAYOUT CHANGESChanging 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 ofthe array will be copied to the backup file while they are being rearranged. This means that all the data is copied twice, once tothe 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 bereassembled. Consequently the file cannot be stored on the device being reshaped.BITMAP CHANGESA write-intent bitmap can be added to, or removed from, an active array. Either internal bitmaps, or bitmaps stored in a separatefile, 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 beingaffected, the system will deadlock. The bitmap must be on a separate filesystem.CONSISTENCY POLICY CHANGESThe consistency policy of an active array can be changed by using the --consistency-policy option in Grow mode. Currently this worksonly for the ppl and resync policies and allows to enable or disable the RAID5 Partial Parity Log (PPL).INCREMENTAL MODEUsage: mdadm --incremental [--run] [--quiet] component-device [optional-aliases-for-device]Usage: mdadm --incremental --fail component-deviceUsage: mdadm --incremental --rebuild-mapUsage: mdadm --incremental --run --scanThis mode is designed to be used in conjunction with a device discovery system. As devices are found in a system, they can be passedto 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 ofand 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 anarray, 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 appro‐priate 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. Thesupport 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:+ Is the device permitted by mdadm.conf? That is, is it listed in a DEVICES line in that file. If DEVICES is absent then thedefault it to allow any device. Similarly if DEVICES contains the special word partitions then any device is allowed. Other‐wise the device name given to mdadm, or one of the aliases given, or an alias found in the filesystem, must match one of thenames or patterns in a DEVICES line.This is the only context where the aliases are used. They are usually provided by a udev rules mentioning $env{DEVLINKS}.+ Does the device have a valid md superblock? If a specific metadata version is requested with --metadata or -e then only thatstyle of metadata is accepted, otherwise mdadm finds any known version of metadata. If no md metadata is found, the devicemay be still added to an array as a spare if POLICY allows.mdadm keeps a list of arrays that it has partially assembled in /run/mdadm/map. If no array exists which matches the metadata on thenew device, mdadm must choose a device name and unit number. It does this based on any name given in mdadm.conf or any name informa‐tion stored in the metadata. If this name suggests a unit number, that number will be used, otherwise a free unit number will bechosen. 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 willchoose a name for the array which is certain not to conflict with any array which does belong to this host. It does this be addingan 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. Itwill normally compare the number of available (non-spare) devices to the number of devices that the metadata suggests need to beactive. If there are at least that many, the array will be started. This means that if any devices are missing the array will notbe 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 forthe data to be accessible. For a RAID1, that means one device will start the array. For a clean RAID5, the array will be started assoon 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, thenmdadm -IRscan be run which will try to start all arrays that are being incrementally assembled. They are started in "read-auto" mode in whichthey are read-only until the first write request. This means that no metadata updates are made and no attempt at resync or recoveryhappens. Further devices that are found before the first write can still be added safely.ENVIRONMENTThis section describes environment variables that affect how mdadm operates.MDADM_NO_MDMONSetting this value to 1 will prevent mdadm from automatically launching mdmon. This variable is intended primarily for debug‐ging mdadm/mdmon.MDADM_NO_UDEVNormally, mdadm does not create any device nodes in /dev, but leaves that task to udev. If udev appears not to be configured,or if this environment variable is set to '1', the mdadm will create and devices that are needed.MDADM_NO_SYSTEMCTLIf mdadm detects that systemd is in use it will normally request systemd to start various background tasks (particularlymdmon) rather than forking and running them in the background. This can be suppressed by setting MDADM_NO_SYSTEMCTL=1.IMSM_NO_PLATFORMA key value of IMSM metadata is that it allows interoperability with boot ROMs on Intel platforms, and with other major oper‐ating systems. Consequently, mdadm will only allow an IMSM array to be created or modified if detects that it is running onan Intel platform which supports IMSM, and supports the particular configuration of IMSM that is being requested (some func‐tionality requires newer OROM support).These checks can be suppressed by setting IMSM_NO_PLATFORM=1 in the environment. This can be useful for testing or for disas‐ter recovery. You should be aware that interoperability may be compromised by setting this value.MDADM_GROW_ALLOW_OLDIf an array is stopped while it is performing a reshape and that reshape was making use of a backup file, then when the arrayis re-assembled mdadm will sometimes complain that the backup file is too old. If this happens and you are certain it is theright backup file, you can over-ride this check by setting MDADM_GROW_ALLOW_OLD=1 in the environment.MDADM_CONF_AUTOAny string given in this variable is added to the start of the AUTO line in the config file, or treated as the whole AUTO lineif none is given. It can be used to disable certain metadata types when mdadm is called from a boot script. For exampleexport MDADM_CONF_AUTO='-ddf -imsm'will make sure that mdadm does not automatically assemble any DDF or IMSM arrays that are found. This can be useful on sys‐tems configured to manage such arrays with dmraid.EXAMPLESmdadm --query /dev/name-of-deviceThis 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 --scanThis will assemble and start all arrays listed in the standard config file. This command will typically go in a system startup file.mdadm --stop --scanThis will shut down all arrays that can be shut down (i.e. are not currently in use). This will typically go in a system shutdownscript.mdadm --follow --scan --delay=120If (and only if) there is an Email address or program given in the standard config file, then monitor the status of all arrays listedin that file by polling them ever 2 minutes.mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hd[ac]1Create /dev/md0 as a RAID1 array consisting of /dev/hda1 and /dev/hdc1.echo 'DEVICE /dev/hd*[0-9] /dev/sd*[0-9]' > mdadm.confmdadm --detail --scan >> mdadm.confThis will create a prototype config file that describes currently active arrays that are known to be made from partitions of IDE orSCSI 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.confmdadm --examine --scan --config=mdadm.conf >> mdadm.confThis will find arrays which could be assembled from existing IDE and SCSI whole drives (not partitions), and store the information inthe format of a config file. This file is very likely to contain unwanted detail, particularly the devices= entries. It should bereviewed and edited before being used as an actual config file.mdadm --examine --brief --scan --config=partitionsmdadm -Ebsc partitionsCreate a list of devices by reading /proc/partitions, scan these for RAID superblocks, and printout a brief listing of all that werefound.mdadm -Ac partitions -m 0 /dev/md0Scan all partitions and devices listed in /proc/partitions and assemble /dev/md0 out of all such devices with a RAID superblock witha minor number of 0.mdadm --monitor --scan --daemonise > /run/mdadm/mon.pidIf config file contains a mail address or alert program, run mdadm in the background in monitor mode monitoring all md devices. Alsowrite pid of mdadm daemon to /run/mdadm/mon.pid.mdadm -Iq /dev/somedeviceTry to incorporate newly discovered device into some array as appropriate.mdadm --incremental --rebuild-map --run --scanRebuild the array map from any current arrays, and then start any that can be started.mdadm /dev/md4 --fail detached --remove detachedAny 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-md4The array /dev/md4 which is currently a RAID5 array will be converted to RAID6. There should normally already be a spare driveattached 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/ddfCreate 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/ddf1Assemble all arrays contained in the ddf array, assigning names as appropriate.mdadm --create --helpProvide help about the Create mode.mdadm --config --helpProvide help about the format of the config file.mdadm --helpProvide general help.FILES/proc/mdstatIf you're using the /proc filesystem, /proc/mdstat lists all active md devices with information about them. mdadm uses this to findarrays when --scan is given in Misc mode, and to monitor array reconstruction on Monitor mode./etc/mdadm/mdadm.conf (or /etc/mdadm.conf)The config file lists which devices may be scanned to see if they contain MD super block, and gives identifying information (e.g.UUID) about known MD arrays. See mdadm.conf(5) for more details./etc/mdadm/mdadm.conf.d (or /etc/mdadm.conf.d)A directory containing configuration files which are read in lexical order./run/mdadm/mapWhen --incremental mode is used, this file gets a list of arrays currently being created.DEVICE NAMESmdadm 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 anarray, 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 ashome can be given.When mdadm chooses device names during auto-assembly or incremental assembly, it will sometimes add a small sequence number to theend of the name to avoid conflicted between multiple arrays that have the same name. If mdadm can reasonably determine that thearray really is meant for this host, either by a hostname in the metadata, or by the presence of the array in mdadm.conf, then itwill leave off the suffix if possible. Also if the homehost is specified as <ignore> mdadm will only use a suffix if a differentarray 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/dev/mdNNwhere NN is a number. The standard names for partitionable arrays (as available from 2.6 onwards) are of the form:/dev/md_dNNPartition 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, andpartitions such as "/dev/mdNNpXX" are possible.From kernel version 2.6.29 standard names can be non-numeric following the form:/dev/md_XXXwhere XXX is any string. These names are supported by mdadm since version 3.3 provided they are enabled in mdadm.conf.NOTEmdadm was previously known as mdctl.