dm-log.rst 2.6 KB

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  1. =====================
  2. Device-Mapper Logging
  3. =====================
  4. The device-mapper logging code is used by some of the device-mapper
  5. RAID targets to track regions of the disk that are not consistent.
  6. A region (or portion of the address space) of the disk may be
  7. inconsistent because a RAID stripe is currently being operated on or
  8. a machine died while the region was being altered. In the case of
  9. mirrors, a region would be considered dirty/inconsistent while you
  10. are writing to it because the writes need to be replicated for all
  11. the legs of the mirror and may not reach the legs at the same time.
  12. Once all writes are complete, the region is considered clean again.
  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. The "core" log type
  30. -------------------
  31. This log implementation keeps the log state in memory. The log state
  32. will not survive a reboot or crash, but there may be a small boost in
  33. performance. This method can also be used if no storage device is
  34. available for storing log state.
  35. The "userspace" log type
  36. ------------------------
  37. This log type simply provides a way to export the log API to userspace,
  38. so log implementations can be done there. This is done by forwarding most
  39. logging requests to userspace, where a daemon receives and processes the
  40. request.
  41. The structure used for communication between kernel and userspace are
  42. located in include/linux/dm-log-userspace.h. Due to the frequency,
  43. diversity, and 2-way communication nature of the exchanges between
  44. kernel and userspace, 'connector' is used as the interface for
  45. communication.
  46. There are currently two userspace log implementations that leverage this
  47. framework - "clustered-disk" and "clustered-core". These implementations
  48. provide a cluster-coherent log for shared-storage. Device-mapper mirroring
  49. can be used in a shared-storage environment when the cluster log implementations
  50. are employed.