Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 Device-Mapper Logging 2 ===================== 3 The device-mapper logging code is used by some of the device-mapper 4 RAID targets to track regions of the disk that are not consistent. 5 A region (or portion of the address space) of the disk may be 6 inconsistent because a RAID stripe is currently being operated on or 7 a machine died while the region was being altered. In the case of 8 mirrors, a region would be considered dirty/inconsistent while you 9 are writing to it because the writes need to be replicated for all 10 the legs of the mirror and may not reach the legs at the same time. 11 Once all writes are complete, the region is considered clean again. 12 13 There is a generic logging interface that the device-mapper RAID 14 implementations use to perform logging operations (see 15 dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different 16 logging implementations are available and provide different 17 capabilities. The list includes: 18 19 Type Files 20 ==== ===== 21 disk drivers/md/dm-log.c 22 core drivers/md/dm-log.c 23 userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h 24 25 The "disk" log type 26 ------------------- 27 This log implementation commits the log state to disk. This way, the 28 logging state survives reboots/crashes. 29 30 The "core" log type 31 ------------------- 32 This log implementation keeps the log state in memory. The log state 33 will not survive a reboot or crash, but there may be a small boost in 34 performance. This method can also be used if no storage device is 35 available for storing log state. 36 37 The "userspace" log type 38 ------------------------ 39 This log type simply provides a way to export the log API to userspace, 40 so log implementations can be done there. This is done by forwarding most 41 logging requests to userspace, where a daemon receives and processes the 42 request. 43 44 The structure used for communication between kernel and userspace are 45 located in include/linux/dm-log-userspace.h. Due to the frequency, 46 diversity, and 2-way communication nature of the exchanges between 47 kernel and userspace, 'connector' is used as the interface for 48 communication. 49 50 There are currently two userspace log implementations that leverage this 51 framework - "clustered-disk" and "clustered-core". These implementations 52 provide a cluster-coherent log for shared-storage. Device-mapper mirroring 53 can be used in a shared-storage environment when the cluster log implementations 54 are employed.