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   mdadm    ( 8 )

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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-f, --force Assemble the array even if the metadata on some devices appears to be out-of-date.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. This works only for native. For external metadata it allows to start dirty degraded RAID 4, 5, 6. 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, layout-original, layout-alternate, layout-unspecified, 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.

The layout-original and layout-alternate options are for RAID0 arrays with non-uniform devices size that were in use before Linux 5.4. If the array was being used with Linux 3.13 or earlier, then to assemble the array on a new kernel, --update=layout-original must be given. If the array was created and used with a kernel from Linux 3.14 to Linux 5.3, then --update=layout-alternate must be given. This only needs to be given once. Subsequent assembly of the array will happen normally. For more information, see md(4).

The layout-unspecified option reverts the effect of layout-orignal or layout-alternate and allows the array to be again used on a kernel prior to Linux 5.3. This option should be used with great caution.

--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.