dm: add writecache target

The writecache target caches writes on persistent memory or on SSD. It

doesn't cache reads because reads are supposed to be cached in page cache

in normal RAM.

When the device is constructed, the first sector should be zeroed or the

first sector should contain valid superblock from previous invocation.

Constructor parameters:

1. type of the cache device - "p" or "s"

	p - persistent memory

	s - SSD

2. the underlying device that will be cached

3. the cache device

4. 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)

	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

		commited

	autocommit_time ms	(default: 1000)

		autocommit time in milliseconds. The data is automatically

		commited 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 number

2. the number of blocks

3. the number of free blocks

4. 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

         of less memory utilization, improved performance and increased

         adaptability in the face of changing workloads.

config DM_WRITECACHE

	tristate "Writecache target"

	depends on BLK_DEV_DM

	---help---

	   The writecache target caches writes on persistent memory or SSD.

	   It is intended for databases or other programs that need extremely

	   low commit latency.

	   The writecache target doesn't cache reads because reads are supposed

	   to be cached in standard RAM.

config DM_ERA

       tristate "Era target (EXPERIMENTAL)"

       depends on BLK_DEV_DM

obj-$(CONFIG_DM_LOG_WRITES)	+= dm-log-writes.o

obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o

obj-$(CONFIG_DM_ZONED)		+= dm-zoned.o

obj-$(CONFIG_DM_WRITECACHE)	+= dm-writecache.o

ifeq ($(CONFIG_DM_UEVENT),y)

dm-mod-objs			+= dm-uevent.o