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Documentation / device-mapper / dm-log.txt




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Based on kernel version 3.13. Page generated on 2014-01-20 22:00 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.
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