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

NAME

       mdmon - monitor MD external metadata arrays

SYNOPSIS

       mdmon CONTAINER [NEWROOT]

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 created 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, a simple md device name like md127, or  /proc/mdstat
	      which tells mdmon to scan for containers and launch an mdmon instance for each one found.

       [NEWROOT]
	      In  order to support an external metadata raid array as the rootfs mdmon needs to be started in the initramfs environment.  Once the
	      initramfs environment mounts the final rootfs mdmon needs to be restarted in the new namespace.  When  NEWROOT  is  specified  mdmon
	      will terminate any mdmon instances that are running in the current namespace, chroot(2) to NEWROOT, and continue monitoring the con-
	      tainer.

       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 that by mdadm is when the boot scripts need to restart it after mounting the new root filesystem.

SEE ALSO

       mdadm(8), md(4).

v3.0.3																	  MDMON(8)