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 meta‐
       data implies that user space handles all updates to the metadata.  The kernel's  responsi‐
       bility  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  moni‐
                            tored;  for  example,  the  metadata  version has been set to "exter‐
                            nal:-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.

              /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  ker‐
                     nel 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 con‐
                     tainer holds the final reference on a block  device  and  a  mdadm  --remove
                       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 meta‐
       data 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:".  Member  devices  are  identified  by  "external://", or "external:-/" if the array is to remain  read‐

              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 foreground.

              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-

              Note that
              mdmon is automatically started by mdadm when needed and so does not need to be con‐
              sidered  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  moni‐
       tored  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

       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.

         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).

v3.3                                                                                     MDMON(8)


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