linux/Documentation/device-mapper/dm-log.txt
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   1Device-Mapper Logging
   2=====================
   3The device-mapper logging code is used by some of the device-mapper
   4RAID targets to track regions of the disk that are not consistent.
   5A region (or portion of the address space) of the disk may be
   6inconsistent because a RAID stripe is currently being operated on or
   7a machine died while the region was being altered.  In the case of
   8mirrors, a region would be considered dirty/inconsistent while you
   9are writing to it because the writes need to be replicated for all
  10the legs of the mirror and may not reach the legs at the same time.
  11Once all writes are complete, the region is considered clean again.
  12
  13There is a generic logging interface that the device-mapper RAID
  14implementations use to perform logging operations (see
  15dm_dirty_log_type in include/linux/dm-dirty-log.h).  Various different
  16logging implementations are available and provide different
  17capabilities.  The list includes:
  18
  19Type            Files
  20====            =====
  21disk            drivers/md/dm-log.c
  22core            drivers/md/dm-log.c
  23userspace       drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h
  24
  25The "disk" log type
  26-------------------
  27This log implementation commits the log state to disk.  This way, the
  28logging state survives reboots/crashes.
  29
  30The "core" log type
  31-------------------
  32This log implementation keeps the log state in memory.  The log state
  33will not survive a reboot or crash, but there may be a small boost in
  34performance.  This method can also be used if no storage device is
  35available for storing log state.
  36
  37The "userspace" log type
  38------------------------
  39This log type simply provides a way to export the log API to userspace,
  40so log implementations can be done there.  This is done by forwarding most
  41logging requests to userspace, where a daemon receives and processes the
  42request.
  43
  44The structure used for communication between kernel and userspace are
  45located in include/linux/dm-log-userspace.h.  Due to the frequency,
  46diversity, and 2-way communication nature of the exchanges between
  47kernel and userspace, 'connector' is used as the interface for
  48communication.
  49
  50There are currently two userspace log implementations that leverage this
  51framework - "clustered-disk" and "clustered-core".  These implementations
  52provide a cluster-coherent log for shared-storage.  Device-mapper mirroring
  53can be used in a shared-storage environment when the cluster log implementations
  54are employed.
  55
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