ASR_BASE
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+=================
+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)
+
+ 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
+ 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