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1=====================2Device-Mapper Logging3=====================4The device-mapper logging code is used by some of the device-mapper5RAID targets to track regions of the disk that are not consistent.6A region (or portion of the address space) of the disk may be7inconsistent because a RAID stripe is currently being operated on or8a machine died while the region was being altered. In the case of9mirrors, a region would be considered dirty/inconsistent while you10are writing to it because the writes need to be replicated for all11the legs of the mirror and may not reach the legs at the same time.12Once all writes are complete, the region is considered clean again.13 14There is a generic logging interface that the device-mapper RAID15implementations use to perform logging operations (see16dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different17logging implementations are available and provide different18capabilities. The list includes:19 20============== ==============================================================21Type Files22============== ==============================================================23disk drivers/md/dm-log.c24core drivers/md/dm-log.c25userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h26============== ==============================================================27 28The "disk" log type29-------------------30This log implementation commits the log state to disk. This way, the31logging state survives reboots/crashes.32 33The "core" log type34-------------------35This log implementation keeps the log state in memory. The log state36will not survive a reboot or crash, but there may be a small boost in37performance. This method can also be used if no storage device is38available for storing log state.39 40The "userspace" log type41------------------------42This log type simply provides a way to export the log API to userspace,43so log implementations can be done there. This is done by forwarding most44logging requests to userspace, where a daemon receives and processes the45request.46 47The structure used for communication between kernel and userspace are48located in include/linux/dm-log-userspace.h. Due to the frequency,49diversity, and 2-way communication nature of the exchanges between50kernel and userspace, 'connector' is used as the interface for51communication.52 53There are currently two userspace log implementations that leverage this54framework - "clustered-disk" and "clustered-core". These implementations55provide a cluster-coherent log for shared-storage. Device-mapper mirroring56can be used in a shared-storage environment when the cluster log implementations57are employed.58