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NAME | SYNOPSIS | OVERVIEW | DESCRIPTION | OPTIONS | START UP AND SHUTDOWN | EXAMPLES | SEE ALSO | COLOPHON |
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MDMON(8) System Manager's Manual MDMON(8)
mdmon - monitor MD external metadata arrays
mdmon [--all] [--takeover] [--foreground] 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.
Metadata updates:
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 stopped
array_state - write pending
Set the dirty bit for the array and then set
array_state to active. Writes are blocked until
userspace writes active.
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 modification
2/ Transition the array to active if the
metadata handler determines a resync or some
other manipulation is necessary
3/ Leave the array read-only if the volume is
marked to not be monitored; for example, the
metadata version has been set to
"external:-dev/md127" instead of
"external:/dev/md127"
sync_action - resync-to-idle
Notify the metadata handler that a resync may have
completed. If a resync process is idled before it
completes this event allows the metadata handler to
checkpoint resync.
sync_action - recover-to-idle
A spare may have completed rebuilding so tell the
metadata handler about the state of each disk.
This is the metadata handler's opportunity to clear
any "out-of-sync" bits and clear the volume's
degraded status. If a recovery 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.
Containers:
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 relationship 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 formats 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 container. 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.
CONTAINER
The container device to monitor. It can be a full path
like /dev/md/container, or a simple md device name like
md127.
--foreground
Normally, mdmon will fork and continue in the background.
Adding this option will skip that step and run mdmon in
the foreground.
--takeover
This instructs mdmon to replace any active mdmon which is
currently monitoring the array. This is primarily used
late in the boot process to replace any mdmon which was
started from an initramfs before the root filesystem was
mounted. This avoids holding a reference on that
initramfs indefinitely and ensures that the pid and sock
files used to communicate with mdmon are in a standard
place.
--all This tells mdmon to find any active containers and start
monitoring each of them if appropriate. This is normally
used with --takeover late in the boot sequence. A
separate mdmon process is started for each container as
the --all argument is over-written with the name of the
container. To allow for containers with names longer than
5 characters, this argument can be arbitrarily extended,
e.g. to --all-active-arrays.
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.
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
automatically 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 /run/mdadm.
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
processes. 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 filesystem.
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.
mdadm(8), md(4).
This page is part of the mdadm (Tool for managing md arrays in
Linux) project. Information about the project can be found at
⟨http://neil.brown.name/blog/mdadm⟩. If you have a bug report for
this manual page, send it to [email protected]. This
page was obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/utils/mdadm/mdadm.git/⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-13.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
v4.3 MDMON(8)
Pages that refer to this page: md(4), mdadm(8), raid6check(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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