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

NAME

       mdmon - monitor MD external metadata arrays

SYNOPSIS

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

OVERVIEW

       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

   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 modifi-
       cations 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  han-
                     dler'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.

OPTIONS

       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 ref-
              erence 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.

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 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.

EXAMPLES

         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

       mdadm(8), md(4).

v4.1-rc1                                                                                                                                  MDMON(8)