neulab/tldr huggingface dataset:
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, --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.  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. If the word clustered is given, the array is created for a clustered environment. One bitmap is created for each node 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 serious problems.
When creating an array on devices which are 100G or larger, mdadm automatically adds an internal bitmap as it will usually be beneficial.  This can be suppressed with --bitmap=none or by selecting a different consistency policy with --consistency-policy.
--bitmap-chunk= 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 defaults to 64Meg, or larger if necessary to fit the bitmap into the available space.
-W, --write-mostly subsequent devices listed 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.
--write-behind= 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.
--failfast subsequent devices listed in a --create or --add command will be flagged as  'failfast'.  This is valid for RAID1 and RAID10 only. 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 ignored and extended delays will be preferred to complete failure.
The 'failfast' flag is appropriate for storage arrays which have a low probability of true failure, but which may sometimes cause unacceptable delays due to internal maintenance functions.
--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 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.
When the devices that will be part of a new array were filled with zeros before creation the operator knows the array is actually clean. If that is the case, such as after running badblocks, this argument can be used to tell mdadm the facts the operator knows.
When an array is resized to a larger size with --grow --size= the new space is normally resynced in that same way that the whole array is resynced at creation.  From Linux version 3.0, --assume-clean can be used with that command to avoid the automatic 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 devices available, or to shrink, change RAID level or layout.  See the GROW MODE section below on RAID-DEVICES CHANGES.  The file must be 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 used for various metadata.  The start of data is known as the data-offset. Normally an appropriate data offset is computed automatically. However it can be useful to set it explicitly such as when re- creating an array which was originally created using a 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' is used to explicitly indicate Kilobytes, Megabytes or Gigabytes respectively.
Since Linux 3.4, --data-offset can also be used with --grow for some RAID levels (initially on RAID10).  This allows the data-offset to be changed as part of the reshape process.  When the data offset is changed, no backup file is required as the difference in offsets is used to provide the same functionality.
When the new offset is earlier than the old offset, the number of devices in the array cannot shrink.  When it is after the old offset, the number of devices in the array cannot increase.
When creating an array, --data-offset can be specified as variable.  In the case each member device is expected to have a offset appended to the name, separated by a colon.  This makes it possible to recreate exactly an array which has varying data offsets (as can happen when different versions of mdadm are used to add different devices).
--continue This option is complementary to the --freeze-reshape option for assembly. It is needed when --grow operation is interrupted and it is not restarted automatically due to --freeze-reshape usage during array assembly.  This option is used together with -G , ( --grow ) command and device for a pending reshape to be continued. All parameters required for reshape continuation will be read from array metadata.  If initial --grow command had required --backup-file= option to be set, continuation option will require to have exactly the same backup file given as well.
Any other parameter passed together with --continue option will be ignored.
-N, --name= Set a name for the array.  This is currently only effective when creating an array with a version-1 superblock, or an array in a DDF container.  The name is a simple textual string that can be used to identify array components when assembling.  If name is 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, --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.
-o, --readonly 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, --auto{=yes,md,mdp,part,p}{NN} Instruct mdadm how to create the device file if needed, possibly allocating an unused minor number.  "md" causes a non- partitionable 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, and the type and minor number will be determined from this.  With mdadm 3.0, device creation is normally left up to udev so this option 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 command line.
For partitionable arrays, mdadm will create the device file for the whole array and for the first 4 partitions.  A different number of partitions can be specified at the end of this option (e.g.  --auto=p7).  If the device name ends with a digit, the partition names add a 'p', and a number, e.g.  /dev/md/home1p3. If there is no trailing digit, then the partition names just have a number added, e.g.  /dev/md/scratch3.
If the md device name is in a 'standard' format as described in DEVICE NAMES, then it will be created, if necessary, with the appropriate device number based on that name.  If the device name is not in one of these formats, then a unused device number will be allocated.  The device number will be considered unused if there is no active array for that number, and there is no entry in /dev for that number and with a non-standard name.  Names that are not in 'standard' format are only allowed in "/dev/md/".
This is meaningful with --create or --build.
-a, --add This option can be used in Grow mode in two cases.
If the target array is a Linear array, then --add can be used to add one or more devices to the array.  They are simply catenated on to the end of the array.  Once added, the devices cannot be removed.
If the --raid-disks option is being used to increase the number of devices in an array, then --add can be used to add some extra devices to be included in the array.  In most cases this is not needed as the extra devices can be added as spares first, and then the number of raid-disks can be changed.  However for RAID0, it is not possible to add spares.  So to increase the number of devices in a RAID0, it is necessary to set the new number of devices, and to add the new devices, in the same command.
--nodes Only works when the array is for clustered environment. It specifies the maximum number of nodes in the cluster that will use this 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, --consistency-policy= 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.
journal For RAID levels 4/5/6, journal device is used to log transactions and replay after unclean shutdown. Implicitly selected 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 metadata region 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 CHANGES below.
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.
--super-minor is only relevant for v0.90 metadata, and should not normally be used.  Using --uuid is much safer.
-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 the metadata on some devices appears to be out-of-date.  If mdadm cannot find enough working devices to start the array, but can find some devices that are recorded as having failed, then it will mark those devices as working so that the array can be started.  An array which requires --force to be started may contain data corruption.  Use it carefully.
-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.
--no-degraded 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.
--backup-file= If --backup-file was used while reshaping an array (e.g. changing number of devices or chunk size) 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, and the reshape to be completed.
--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 option to indicate that the backup file is invalid.  In this case the data that was being rearranged at the time of the crash could be irrecoverably lost, but the rest of the array may still be recoverable.  This option should only be used as a last resort if there is no way to recover the backup file.
-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, nodes, homehost, home-cluster, resync, byteorder, devicesize, no-bitmap, bbl, no-bbl, ppl, no-ppl, metadata, 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 nodes option will change the nodes of the array as stored in the bitmap superblock. This option only works for a clustered environment.
The homehost option will change the homehost as recorded in the superblock.  For version-0 superblocks, this is the same as updating the UUID.  For version-1 superblocks, this involves updating the name.
The home-cluster option will change the cluster name as recorded in the superblock and bitmap. This option only works for clustered environment.
The resync option will cause the array to be marked dirty meaning that any redundancy in the array (e.g. parity for RAID5, copies for RAID1) may be incorrect.  This will cause the RAID system to perform a "resync" pass to make sure that all redundant information is correct.
The byteorder option allows arrays to be moved between machines with different byte-order, such as from a big-endian machine like a Sparc or some MIPS machines, to a little-endian x86_64 machine. When assembling such an array for the first time after a move, giving --update=byteorder will cause mdadm to expect superblocks to have their byteorder reversed, and will correct that order before assembling the array.  This is only valid with original (Version 0.90) superblocks.
The summaries option will correct the summaries in the superblock. That is the counts of total, working, active, failed, and spare devices.
The devicesize option will rarely be of use.  It applies to version 1.1 and 1.2 metadata only (where the metadata is at the start of the device) and is only useful when the component device has changed size (typically become larger).  The version 1 metadata records the amount of the device that can be used to store data, so if a device in a version 1.1 or 1.2 array becomes larger, the metadata will still be visible, but the extra space will not.  In this case it might be useful to assemble the array with --update=devicesize.  This will cause mdadm to determine the maximum usable amount of space on each device and update the relevant field in the metadata.
The metadata option only works on v0.90 metadata arrays and will convert them to v1.0 metadata.  The array must not be dirty (i.e. it must not need a sync) and it must not have a write-intent bitmap.
The old metadata will remain on the devices, but will appear older than the new metadata and so will usually be ignored. The old metadata (or indeed the new metadata) can be removed by giving the appropriate --metadata= option to --zero-superblock.
The no-bitmap option can be used when an array has an internal bitmap which is corrupt in some way so that assembling the array normally fails.  It will cause any internal bitmap to be ignored.
The bbl option will reserve space in each device for a bad block list.  This will be 4K in size and positioned near the end of any free space between the superblock and the data.
The no-bbl option will cause any reservation of space for a bad block list to be removed.  If the bad block list contains entries, this will fail, as removing the list could cause data corruption.
The ppl option will enable PPL for a RAID5 array and reserve space for PPL on each device. There must be enough free space between the data and superblock and a write-intent bitmap or journal must not be used.
The no-ppl option will disable PPL in the superblock.
--freeze-reshape Option is intended to be used in start-up scripts during initrd boot phase.  When array under reshape is assembled during initrd phase, this option stops reshape after reshape critical section is being restored. This happens before file system pivot operation 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.
--symlinks See this option under Create and Build options.
For Manage mode: -t, --test 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 at least one change was made.  This can be useful when an indirect specifier such as missing, detached or faulty is used in requesting an operation on the array.  --test will report failure if these specifiers didn't find any match.
-a, --add hot-add listed devices.  If a device appears to have recently been part of the array (possibly it failed or was removed) the device is re-added as described in the next point.  If that fails or the device was never part of the array, the device is added as a hot- spare.  If the array is degraded, it will immediately start to rebuild data onto that spare.
Note that this and the following options are only meaningful on array with redundancy.  They don't apply to RAID0 or Linear.
--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 the array, and the slot that it used is still vacant, then the device will be added back to the array in the same position.  This will normally cause the data for that device to be recovered.  However based on the event count on the device, the recovery may 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 is enough to make the device fully consistent with the array.
When used with v1.x metadata, --re-add can be accompanied by --update=devicesize, --update=bbl, or --update=no-bbl.  See the description of these option when used in Assemble mode for an explanation of their use.
If the device name given is missing then mdadm will try to find any device that looks like it should be part of the array but isn't and will try to re-add all such devices.
If the device name given is faulty then mdadm will find all devices in the array that are marked faulty, remove them and attempt to immediately re-add them.  This can be useful if you are certain that the reason for failure has been resolved.
--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 added as a spare even if it looks like it could be an recent member of the array.
-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, detached and names like set-A can be given to --remove. The first causes all failed device to be removed.  The second causes any device which is no longer connected to the system (i.e an 'open' returns ENXIO) to be removed.  The third will remove a set as describe below under --fail.
-f, --fail Mark listed devices as faulty.  As well as the name of a device file, the word detached or a set name like set-A can be given. The former will cause any device that has been detached from the system to be marked as failed.  It can then be removed.
For RAID10 arrays where the number of copies evenly divides the number of devices, the devices can be conceptually divided into sets where each set contains a single complete copy of the data on the array.  Sometimes a RAID10 array will be configured so that these sets are on separate controllers.  In this case all the devices in one set can be failed by giving a name like set-A or set-B to --fail.  The appropriate set names are reported by --detail.
--set-faulty same as --fail.
--replace Mark listed devices as requiring replacement.  As soon as a spare is available, it will be rebuilt and will replace the marked device.  This is similar to marking a device as faulty, but the device remains in service during the recovery process to increase resilience against multiple failures.  When the replacement process finishes, the replaced device will be marked as faulty.
--with This can follow a list of --replace devices.  The devices listed after --with will be preferentially used to replace the devices listed after --replace.  These device must already be spare devices in the array.
--write-mostly 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.
--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 a device it sends a message to all nodes in the cluster to look for a device with a UUID. This translates to a udev notification with the UUID of the device to be added and the slot number. The receiving node must acknowledge this message with --cluster-confirm. Valid arguments are <slot>:<devicename> in case the device is found or <slot>:missing in case the device is not found.
--add-journal Add journal to an existing array, or recreate journal for RAID-4/5/6 array that lost a journal device. To avoid interrupting on-going write opertions, --add-journal only works for array in Read-Only state.
--failfast Subsequent devices that are added or re-added will have the 'failfast' flag set.  This is only valid for RAID1 and RAID10 and means that the 'md' driver will avoid long timeouts on error handling where possible.
--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 to be added, removed, marked as faulty, etc.  Several different operations can be specified for different devices, e.g. mdadm /dev/md0 --add /dev/sda1 --fail /dev/sdb1 --remove /dev/sdb1 Each operation applies to all devices listed until the next operation.
If an array is using a write-intent bitmap, then devices which have been removed can be re-added in a way that avoids a full reconstruction but instead just updates the blocks that have changed since the device was removed.  For arrays with persistent metadata (superblocks) this is done automatically.  For arrays created with --build mdadm needs to be told that this device we removed recently with --re-add.
Devices can only be removed from an array if they are not in active use, i.e. that must be spares or failed devices.  To remove an active device, it must first be marked as faulty.
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 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 without argument, mdadm will scan all controllers looking for their capabilities. Otherwise, mdadm will only look at the controller specified by the argument in form of an absolute filepath or a link, e.g.  /sys/devices/pci0000:00/0000:00:1f.2.
-Y, --export When used with --detail, --detail-platform, --examine, or --incremental output will be formatted as key=value pairs for easy import 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 from elsewhere (yes).
-E, --examine Print contents of the metadata stored on the named device(s). Note the contrast between --examine and --detail.  --examine applies 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 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.  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 supports bad- blocks lists.
--dump=directory
--restore=directory Save metadata from lists devices, or restore metadata to listed devices.
-R, --run start a partially assembled array.  If --assemble did not find enough devices to fully start the array, it might leaving it partially assembled.  If you wish, you can then use --run to start the array in degraded mode.
-S, --stop deactivate array, releasing all resources.
-o, --readonly mark array as readonly.
-w, --readwrite mark array as readwrite.
--zero-superblock If the device contains a valid md superblock, the block is overwritten with zeros.  With --force the block where the superblock 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 node before execute it.
--kill-subarray= If the device is a container and the argument to --kill-subarray specifies an inactive subarray in the container, then the subarray is deleted.  Deleting all subarrays will leave an 'empty- container' or spare superblock on the drives.  See --zero-superblock for completely removing a superblock.  Note that some formats depend on the subarray index for generating a UUID, this command will fail if it would change the UUID of an active subarray.
--update-subarray= If the device is a container and the argument to --update-subarray specifies a subarray in the container, then attempt to update the given superblock field in the subarray. See below in MISC MODE for details.
-t, --test When used with --detail, the exit status of mdadm is set to reflect the status of the device.  See below in MISC MODE for details.
-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.
--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 soon as possible.  mdadm will return with success if the array uses external metadata and we successfully waited.  For native arrays this returns immediately as the kernel handles dirty-clean transitions at shutdown.  No action is taken if safe-mode handling 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 currently running action though some actions will automatically restart. Setting to frozen will abort any current action and ensure 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, -r Rebuild the map file (/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.
--fail, -f This allows the hot-plug system to remove devices that have fully disappeared from the kernel.  It will first fail and then remove the device from any array it belongs to.  The device name given should be a kernel device name such as "sda", not a name 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 automatically added to the same array.  This allows the failed device to be automatically replaced by a new device without metadata if it appears at specified path.   This option is normally only set by a udev script.
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.  Since 2.6.16, there is no need to reduce this as the kernel alerts mdadm immediately when there is any change.
-r, --increment Give a percentage increment.  mdadm will generate RebuildNN events with the given percentage increment.
-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 from 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.
--no-sharing This inhibits the functionality for moving spares between arrays. Only one monitoring process started with --scan but without this flag is allowed, otherwise the two could interfere with each other.
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 no arrays are described by the configuration file, then any arrays that can be found on unused devices will be assembled.
If precisely one device is listed, but --scan is not given, then mdadm acts as though --scan was given and identity information is extracted from the configuration file.
The identity can be given with the --uuid option, the --name option, or the --super-minor option, will be taken from the md-device record in the config file, or will be taken from the super block of the first component-device listed on the command line.
Devices can be given on the --assemble command line or in the config file.  Only devices which have an md superblock which contains the right identity will be considered for any array.
The config file is only used if explicitly named with --config or requested with (a possibly implicit) --scan.  In the later case, /etc/mdadm.conf or /etc/mdadm/mdadm.conf is used.
If --scan is not given, then the config file will only be used to find the identity of md arrays.
Normally the array will be started after it is assembled.  However if --scan is not given and not all expected drives were listed, then the array is not started (to guard against usage errors).  To insist that the array be started in this case (as may work for RAID1, 4, 5, 6, or 10), give the --run flag.
If udev is active, mdadm does not create any entries in /dev but leaves that to udev.  It does record information in /run/mdadm/map which will allow udev to choose the correct name.
If mdadm detects that udev is not configured, it will create the devices in /dev itself.
In Linux kernels prior to version 2.6.28 there were two distinctly different types of md devices that could be created: one that could be partitioned using standard partitioning tools and one that could not. Since 2.6.28 that distinction is no longer relevant as both type of devices can be partitioned.  mdadm will normally create the type that originally could not be partitioned as it has a well defined major number (9).
Prior to 2.6.28, it is important that mdadm chooses the correct type of array device to use.  This can be controlled with the --auto option.  In particular, a value of "mdp" or "part" or "p" tells mdadm to use a partitionable device rather than the default.
In the no-udev case, the value given to --auto can be suffixed by a number.  This tells mdadm to create that number of partition devices rather than the default of 4.
The value given to --auto can also be given in the configuration file as a word starting auto= on the ARRAY line for the relevant array.
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 no arrays are listed in the config (other than those marked <ignore>) it will look through the available devices for possible arrays and will try to assemble anything that it finds.  Arrays which are tagged as belonging to the given homehost will be assembled and started normally. Arrays which do not obviously belong to this host are given names that are expected not to conflict with anything local, and are started "read- auto" so that nothing is written to any device until the array is written to. i.e.  automatic resync etc is delayed.
If mdadm finds a consistent set of devices that look like they should comprise an array, and if the superblock is tagged as belonging to the given home host, it will automatically choose a device name and try to assemble the array.  If the array uses version-0.90 metadata, then the minor number as recorded in the superblock is used to create a name in /dev/md/ so for example /dev/md/3.  If the array uses version-1 metadata, then the name from the superblock is used to similarly create a name in /dev/md/ (the name will have any 'host' prefix stripped first).
This behaviour can be modified by the AUTO line in the mdadm.conf configuration file.  This line can indicate that specific metadata type should, or should not, be automatically assembled.  If an array is found which is not listed in mdadm.conf and has a metadata format that is denied by the AUTO line, then it will not be assembled.  The AUTO line can also request that all arrays identified as being for this homehost should be assembled regardless of their metadata type.  See mdadm.conf(5) for further details.
Note: Auto assembly cannot be used for assembling and activating some arrays which are undergoing reshape.  In particular as the backup-file cannot be given, any reshape which requires a backup-file to continue cannot be started by auto assembly.  An array which is growing to more devices and has passed the critical section can be assembled using auto- assembly.
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, raid1, raid10, multipath, or faulty, or one of their synonyms.  All devices must be listed and the array will be started once complete.  It will often be appropriate to use --assume-clean with levels raid1 or raid10.
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.
The named device will normally not exist when mdadm --create is run, but will be created by udev once the array becomes active.
As devices are added, they are checked to see if they contain RAID superblocks or filesystems.  They are also checked to see if the variance in device size exceeds 1%.
If any discrepancy is found, the array will not automatically be run, though the presence of a --run can override this caution.
To create a "degraded" array in which some devices are missing, simply give the word "missing" in place of a device name.  This will cause mdadm to leave the corresponding slot in the array empty.  For a RAID4 or RAID5 array at most one slot can be "missing"; for a RAID6 array at most two slots.  For a RAID1 array, only one real device needs to be given.  All of the others can be "missing".
When creating a RAID5 array, mdadm will automatically create a degraded array with an extra spare drive.  This is because building the spare into a degraded array is in general faster than resyncing the parity on a non- degraded, but not clean, array.  This feature can be overridden with the --force option.
When creating an array with version-1 metadata a name for the array is required.  If this is not given with the --name option, mdadm will choose a name based on the last component of the name of the device being created.  So if /dev/md3 is being created, then the name 3 will be chosen.  If /dev/md/home is being created, then the name home will be used.
When creating a partition based array, using mdadm with version-1.x metadata, the partition type should be set to 0xDA (non fs-data).  This type selection allows for greater precision since using any other [RAID auto-detect (0xFD) or a GNU/Linux partition (0x83)], might create problems in the event of array recovery through a live cdrom.
A new array will normally get a randomly assigned 128bit UUID which is very likely to be unique.  If you have a specific need, you can choose a UUID for the array by giving the --uuid= option.  Be warned that creating two arrays with the same UUID is a recipe for disaster.  Also, using --uuid= when creating a v0.90 array will silently override any --homehost= setting.
If the array type supports a write-intent bitmap, and if the devices in the array exceed 100G is size, an internal write-intent bitmap will automatically be added unless some other option is explicitly requested with the --bitmap option or a different consistency policy is selected with the --consistency-policy option. In any case space for a bitmap will be reserved so that one can be added later with --grow --bitmap=internal.
If the metadata type supports it (currently only 1.x and IMSM metadata), space will be allocated to store a bad block list.  This allows a modest number of bad blocks to be recorded, allowing the drive to remain in service while only partially functional.
When creating an array within a CONTAINER mdadm can be given either the list of devices to use, or simply the name of the container.  The former case gives control over which devices in the container will be used for the array.  The latter case allows mdadm to automatically choose which devices to use based on how much spare space is available.
The General Management options that are valid with --create are:
--run  insist on running the array even if some devices look like they might be in use.
--readonly start the array in readonly mode.
Usage: mdadm device options... devices...
This usage will allow individual devices in an array to be failed, removed or added.  It is possible to perform multiple operations with on command.  For example: mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1 will firstly mark /dev/hda1 as faulty in /dev/md0 and will then remove it from the array and finally add it back in as a spare.  However only one md array can be affected by a single command.
When a device is added to an active array, mdadm checks to see if it has metadata on it which suggests that it was recently a member of the array. If it does, it tries to "re-add" the device.  If there have been no changes since the device was removed, or if the array has a write-intent bitmap which has recorded whatever changes there were, then the device will immediately become a full member of the array and those differences recorded in the bitmap will be resolved.
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.
--detail The device should be an active md device.  mdadm will display a detailed description of the array.  --brief or --scan will cause the output to be less detailed and the format to be suitable for inclusion in 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.
--detail-platform Print detail of the platform's RAID capabilities (firmware / hardware topology).  If the metadata is specified with -e or --metadata= then the return status will be:
0      metadata successfully enumerated its platform components on this system
1      metadata is platform independent
2      metadata failed to find its platform components on this system
--update-subarray= If the device is a container and the argument to --update-subarray specifies a subarray in the container, then attempt to update the given superblock field in the subarray.  Similar to updating an array in "assemble" mode, the field to update is selected by -U or --update= option. The supported options are name, ppl and no-ppl.
The name option updates the subarray name in the metadata, it may not affect the device node name or the device node symlink until the subarray is re-assembled.  If updating name would change the UUID of an active subarray this operation is blocked, and the command will end in an error.
The ppl and no-ppl options enable and disable PPL in the metadata. Currently supported only for IMSM subarrays.
--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 in a 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=directory If the device contains RAID metadata, a file will be created in the directory and the metadata will be written to it.  The file will be the same size as the device and have the metadata written in the file at the same locate that it exists in the device. However the file will be "sparse" so that only those blocks containing metadata will be allocated. The total space used 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-id which point to the device, then hard links to the file will be created in directory based on these by-id names.
Multiple devices can be listed and their metadata will all be stored in the one directory.
--restore=directory This is the reverse of --dump.  mdadm will locate a file in the directory that has a name appropriate for the given device and will restore metadata from it.  Names that match /dev/disk/by-id names are preferred, however if two of those refer to different files, mdadm will not choose between them but will abort the operation.
If a file name is given instead of a directory then mdadm will restore from that file to a single device, always provided the size of the file matches that of the device, and the file contains valid metadata.
--stop The devices should be active md arrays which will be deactivated, as long as they are not currently in use.
--run  This will fully activate a partially assembled md array.
--readonly This will mark an active array as read-only, providing that it is not currently being used.
--readwrite This will change a readonly array back to being read/write.