=================Writecache target================= The writecache target caches writes on persistent memory or on SSD. Itdoesn't cache reads because reads are supposed to be cached in page cachein normal RAM. When the device is constructed, the first sector should be zeroed or thefirst sector should contain valid superblock from previous invocation. Constructor parameters: 1. type of the cache device - "p" or "s" 	- p - persistent memory	- s - SSD2. the underlying device that will be cached3. the cache device4. block size (4096 is recommended; the maximum block size is the page   size)5. the number of optional parameters (the parameters with an argument   count as two) 	start_sector n		(default: 0)		offset from the start of cache device in 512-byte sectors	high_watermark n	(default: 50)		start writeback when the number of used blocks reach this		watermark	low_watermark x		(default: 45)		stop writeback when the number of used blocks drops below		this watermark	writeback_jobs n	(default: unlimited)		limit the number of blocks that are in flight during		writeback. Setting this value reduces writeback		throughput, but it may improve latency of read requests	autocommit_blocks n	(default: 64 for pmem, 65536 for ssd)		when the application writes this amount of blocks without		issuing the FLUSH request, the blocks are automatically		committed	autocommit_time ms	(default: 1000)		autocommit time in milliseconds. The data is automatically		committed if this time passes and no FLUSH request is		received	fua			(by default on)		applicable only to persistent memory - use the FUA flag		when writing data from persistent memory back to the		underlying device	nofua		applicable only to persistent memory - don't use the FUA		flag when writing back data and send the FLUSH request		afterwards 		- some underlying devices perform better with fua, some		  with nofua. The user should test it Status:1. error indicator - 0 if there was no error, otherwise error number2. the number of blocks3. the number of free blocks4. the number of blocks under writeback Messages:	flush		flush the cache device. The message returns successfully		if the cache device was flushed without an error	flush_on_suspend		flush the cache device on next suspend. Use this message		when you are going to remove the cache device. The proper		sequence for removing the cache device is: 		1. send the "flush_on_suspend" message		2. load an inactive table with a linear target that maps		   to the underlying device		3. suspend the device		4. ask for status and verify that there are no errors		5. resume the device, so that it will use the linear		   target		6. the cache device is now inactive and it can be deleted