58 lines
2.6 KiB
ReStructuredText
58 lines
2.6 KiB
ReStructuredText
=====================
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Device-Mapper Logging
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=====================
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The device-mapper logging code is used by some of the device-mapper
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RAID targets to track regions of the disk that are not consistent.
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A region (or portion of the address space) of the disk may be
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inconsistent because a RAID stripe is currently being operated on or
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a machine died while the region was being altered. In the case of
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mirrors, a region would be considered dirty/inconsistent while you
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are writing to it because the writes need to be replicated for all
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the legs of the mirror and may not reach the legs at the same time.
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Once all writes are complete, the region is considered clean again.
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There is a generic logging interface that the device-mapper RAID
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implementations use to perform logging operations (see
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dm_dirty_log_type in include/linux/dm-dirty-log.h). Various different
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logging implementations are available and provide different
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capabilities. The list includes:
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============== ==============================================================
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Type Files
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============== ==============================================================
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disk drivers/md/dm-log.c
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core drivers/md/dm-log.c
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userspace drivers/md/dm-log-userspace* include/linux/dm-log-userspace.h
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============== ==============================================================
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The "disk" log type
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-------------------
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This log implementation commits the log state to disk. This way, the
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logging state survives reboots/crashes.
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The "core" log type
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-------------------
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This log implementation keeps the log state in memory. The log state
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will not survive a reboot or crash, but there may be a small boost in
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performance. This method can also be used if no storage device is
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available for storing log state.
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The "userspace" log type
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------------------------
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This log type simply provides a way to export the log API to userspace,
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so log implementations can be done there. This is done by forwarding most
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logging requests to userspace, where a daemon receives and processes the
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request.
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The structure used for communication between kernel and userspace are
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located in include/linux/dm-log-userspace.h. Due to the frequency,
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diversity, and 2-way communication nature of the exchanges between
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kernel and userspace, 'connector' is used as the interface for
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communication.
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There are currently two userspace log implementations that leverage this
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framework - "clustered-disk" and "clustered-core". These implementations
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provide a cluster-coherent log for shared-storage. Device-mapper mirroring
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can be used in a shared-storage environment when the cluster log implementations
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are employed.
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