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

          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
                        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.

   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.


          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 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


     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).


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