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