Section: Maintenance Commands (8)
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mdmon - monitor MD external metadata arrays






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 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:
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
Transition the array to active if the metadata handler determines a resync or some other manipulation is necessary
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 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.



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.
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 container.

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.



mdadm(8), md(4).