1=================
2Writecache target
3=================
4
5The writecache target caches writes on persistent memory or on SSD. It
6doesn't cache reads because reads are supposed to be cached in page cache
7in normal RAM.
8
9When the device is constructed, the first sector should be zeroed or the
10first sector should contain valid superblock from previous invocation.
11
12Constructor parameters:
13
141. type of the cache device - "p" or "s"
15
16	- p - persistent memory
17	- s - SSD
182. the underlying device that will be cached
193. the cache device
204. block size (4096 is recommended; the maximum block size is the page
21   size)
225. the number of optional parameters (the parameters with an argument
23   count as two)
24
25	start_sector n		(default: 0)
26		offset from the start of cache device in 512-byte sectors
27	high_watermark n	(default: 50)
28		start writeback when the number of used blocks reach this
29		watermark
30	low_watermark x		(default: 45)
31		stop writeback when the number of used blocks drops below
32		this watermark
33	writeback_jobs n	(default: unlimited)
34		limit the number of blocks that are in flight during
35		writeback. Setting this value reduces writeback
36		throughput, but it may improve latency of read requests
37	autocommit_blocks n	(default: 64 for pmem, 65536 for ssd)
38		when the application writes this amount of blocks without
39		issuing the FLUSH request, the blocks are automatically
40		committed
41	autocommit_time ms	(default: 1000)
42		autocommit time in milliseconds. The data is automatically
43		committed if this time passes and no FLUSH request is
44		received
45	fua			(by default on)
46		applicable only to persistent memory - use the FUA flag
47		when writing data from persistent memory back to the
48		underlying device
49	nofua
50		applicable only to persistent memory - don't use the FUA
51		flag when writing back data and send the FLUSH request
52		afterwards
53
54		- some underlying devices perform better with fua, some
55		  with nofua. The user should test it
56
57Status:
581. error indicator - 0 if there was no error, otherwise error number
592. the number of blocks
603. the number of free blocks
614. the number of blocks under writeback
62
63Messages:
64	flush
65		flush the cache device. The message returns successfully
66		if the cache device was flushed without an error
67	flush_on_suspend
68		flush the cache device on next suspend. Use this message
69		when you are going to remove the cache device. The proper
70		sequence for removing the cache device is:
71
72		1. send the "flush_on_suspend" message
73		2. load an inactive table with a linear target that maps
74		   to the underlying device
75		3. suspend the device
76		4. ask for status and verify that there are no errors
77		5. resume the device, so that it will use the linear
78		   target
79		6. the cache device is now inactive and it can be deleted
80