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