1===================== 2Device-Mapper Logging 3===================== 4The device-mapper logging code is used by some of the device-mapper 5RAID targets to track regions of the disk that are not consistent. 6A region (or portion of the address space) of the disk may be 7inconsistent because a RAID stripe is currently being operated on or 8a machine died while the region was being altered. In the case of 9mirrors, a region would be considered dirty/inconsistent while you 10are writing to it because the writes need to be replicated for all 11the 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 RAID 15implementations use to perform logging operations (see 16dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different 17logging implementations are available and provide different 18capabilities. The list includes: 19 20============== ============================================================== 21Type Files 22============== ============================================================== 23disk drivers/md/dm-log.c 24core drivers/md/dm-log.c 25userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h 26============== ============================================================== 27 28The "disk" log type 29------------------- 30This log implementation commits the log state to disk. This way, the 31logging state survives reboots/crashes. 32 33The "core" log type 34------------------- 35This log implementation keeps the log state in memory. The log state 36will not survive a reboot or crash, but there may be a small boost in 37performance. This method can also be used if no storage device is 38available for storing log state. 39 40The "userspace" log type 41------------------------ 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 most 44logging requests to userspace, where a daemon receives and processes the 45request. 46 47The structure used for communication between kernel and userspace are 48located in include/linux/dm-log-userspace.h. Due to the frequency, 49diversity, and 2-way communication nature of the exchanges between 50kernel and userspace, 'connector' is used as the interface for 51communication. 52 53There are currently two userspace log implementations that leverage this 54framework - "clustered-disk" and "clustered-core". These implementations 55provide a cluster-coherent log for shared-storage. Device-mapper mirroring 56can be used in a shared-storage environment when the cluster log implementations 57are employed. 58