MDMON(8)                   System Manager's Manual                  MDMON(8)

NAME         top

       mdmon - monitor MD external metadata arrays

SYNOPSIS         top

       mdmon [--all] [--takeover] [--foreground] CONTAINER

OVERVIEW         top

       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.

DESCRIPTION         top

   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

              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

              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

              <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 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.

OPTIONS         top

              The container device to monitor.  It can be a full path like
              /dev/md/container, or a simple md device name like md127.

              Normally, mdmon will fork and continue in the background.
              Adding this option will skip that step and run mdmon in the

              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


       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

       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

       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.

EXAMPLES         top

         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.

SEE ALSO         top

       mdadm(8), md(4).

COLOPHON         top

       This page is part of the mdadm (Tool for managing md arrays in Linux)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for
       this manual page, send it to  This page
       was obtained from the project's upstream Git repository
       ⟨⟩ on
       2020-02-08.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-01-21.)  If you discover any
       rendering problems in this HTML version of the page, or you believe
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       (which is not part of the original manual page), send a mail to

v4.1-rc2                                                            MDMON(8)

Pages that refer to this page: md(4)mdadm(8)raid6check(8)