mdmon [--all] [--takeover] [--offroot] CONTAINER
The 2.6.27 kernel brings the ability to support external metadata
arrays. External metadata implies that user space handles all updates
to the metadata. The kernel's responsibility is to notify user space
when a "metadata event" occurs, like disk failures and clean-to-dirty
transitions. The kernel, in important cases, waits for user space to
take action on these notifications.
To service metadata update requests a daemon, mdmon, is introduced.
Mdmon is tasked with polling the sysfs namespace looking for changes in
array_state, sync_action, and per disk state attributes. When a change
is detected it calls a per metadata type handler to make modifications
to the metadata. The following actions are taken:
array_state - inactive
Clear the dirty bit for the volume and let the array be
array_state - write pending
Set the dirty bit for the array and then set array_state
to active. Writes are blocked until userspace writes
array_state - active-idle
The safe mode timer has expired so set array state to
clean to block writes to the array
array_state - clean
Clear the dirty bit for the volume
array_state - read-only
This is the initial state that all arrays start at.
mdmon takes one of the three actions:
1/ Transition the array to read-auto keeping the
dirty bit clear if the metadata handler determines
that the array does not need resyncing or other
2/ Transition the array to active if the metadata
handler determines a resync or some other manipu-
lation is necessary
3/ Leave the array read-only if the volume is marked
to not be monitored; for example, the metadata
metadata handler's opportunity to clear any "out-of-sync"
bits and clear the volume's degraded status. If a recov-
ery process is idled before it completes this event
allows the metadata handler to checkpoint recovery.
<disk>/state - faulty
A disk failure kicks off a series of events. First,
notify the metadata handler that a disk has failed, and
then notify the kernel that it can unblock writes that
were dependent on this disk. After unblocking the kernel
this disk is set to be removed+ from the member array.
Finally the disk is marked failed in all other member
arrays in the container.
+ Note This behavior differs slightly from native MD
arrays where removal is reserved for a mdadm --remove
event. In the external metadata case the container holds
the final reference on a block device and a mdadm
--remove <container> <victim> call is still required.
External metadata formats, like DDF, differ from the native MD metadata
formats in that they define a set of disks and a series of sub-arrays
within those disks. MD metadata in comparison defines a 1:1 relation-
ship between a set of block devices and a raid array. For example to
create 2 arrays at different raid levels on a single set of disks, MD
metadata requires the disks be partitioned and then each array can be
created with a subset of those partitions. The supported external for-
mats perform this disk carving internally.
Container devices simply hold references to all member disks and allow
tools like mdmon to determine which active arrays belong to which con-
tainer. Some array management commands like disk removal and disk add
are now only valid at the container level. Attempts to perform these
actions on member arrays are blocked with error messages like:
"mdadm: Cannot remove disks from a 'member' array, perform this
operation on the parent container"
Containers are identified in /proc/mdstat with a metadata version
string "external:<metadata name>". Member devices are identified by
"external:/<container device>/<member index>", or "external:-<container
device>/<member index>" if the array is to remain readonly.
The container device to monitor. It can be a full path like
/dev/md/container, or a simple md device name like md127.
This instructs mdmon to replace any active mdmon which is cur-
rently monitoring the array. This is primarily used late in the
trarily extended, e.g. to --all-active-arrays.
Set first character of argv to @ to indicate mdmon was
launched from initrd/initramfs and should not be shutdown by
systemd as part of the regular shutdown process. This option is
normally only used by the system's initscripts. Please see here
for more details on how systemd handled argv:
Note that mdmon is automatically started by mdadm when needed and so
does not need to be considered when working with RAID arrays. The only
times it is run other than by mdadm is when the boot scripts need to
restart it after mounting the new root filesystem.
START UP AND SHUTDOWN
As mdmon needs to be running whenever any filesystem on the monitored
device is mounted there are special considerations when the root
filesystem is mounted from an mdmon monitored device. Note that in
general mdmon is needed even if the filesystem is mounted read-only as
some filesystems can still write to the device in those circumstances,
for example to replay a journal after an unclean shutdown.
When the array is assembled by the initramfs code, mdadm will automati-
cally start mdmon as required. This means that mdmon must be installed
on the initramfs and there must be a writable filesystem (typically
tmpfs) in which mdmon can create a .pid and .sock file. The particular
filesystem to use is given to mdmon at compile time and defaults to
This filesystem must persist through to shutdown time.
After the final root filesystem has be instantiated (usually with
pivot_root) mdmon should be run with --all --takeover so that the mdmon
running from the initramfs can be replaced with one running in the main
root, and so the memory used by the initramfs can be released.
At shutdown time, mdmon should not be killed along with other pro-
cesses. Also as it holds a file (socket actually) open in /dev (by
default) it will not be possible to unmount /dev if it is a separate
mdmon --all-active-arrays --takeover
Any mdmon which is currently running is killed and a new instance is
started. This should be run during in the boot sequence if an
initramfs was used, so that any mdmon running from the initramfs will
not hold the initramfs active.
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