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   mdadm    ( 8 )

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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.