-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.
