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-Device-Mapper Logging
-The device-mapper logging code is used by some of the device-mapper
-RAID targets to track regions of the disk that are not consistent.
-A region (or portion of the address space) of the disk may be
-inconsistent because a RAID stripe is currently being operated on or
-a machine died while the region was being altered. In the case of
-mirrors, a region would be considered dirty/inconsistent while you
-are writing to it because the writes need to be replicated for all
-the legs of the mirror and may not reach the legs at the same time.
-Once all writes are complete, the region is considered clean again.
-There is a generic logging interface that the device-mapper RAID
-implementations use to perform logging operations (see
-dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different
-logging implementations are available and provide different
-capabilities. The list includes:
-Type Files
-==== =====
-disk drivers/md/dm-log.c
-core drivers/md/dm-log.c
-userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h
-The "disk" log type
-This log implementation commits the log state to disk. This way, the
-logging state survives reboots/crashes.
-The "core" log type
-This log implementation keeps the log state in memory. The log state
-will not survive a reboot or crash, but there may be a small boost in
-performance. This method can also be used if no storage device is
-available for storing log state.
-The "userspace" log type
-This log type simply provides a way to export the log API to userspace,
-so log implementations can be done there. This is done by forwarding most
-logging requests to userspace, where a daemon receives and processes the
-The structure used for communication between kernel and userspace are
-located in include/linux/dm-log-userspace.h. Due to the frequency,
-diversity, and 2-way communication nature of the exchanges between
-kernel and userspace, 'connector' is used as the interface for
-There are currently two userspace log implementations that leverage this
-framework - "clustered-disk" and "clustered-core". These implementations
-provide a cluster-coherent log for shared-storage. Device-mapper mirroring
-can be used in a shared-storage environment when the cluster log implementations
-are employed.