added largest_contiguous cache flush algorithm

docs/manual.rst

@@ -3443,6 +3443,11 @@ session_settings
 		bool optimize_hashing_for_speed;
 
 		int file_checks_delay_per_block;
+
+		enum disk_cache_algo_t
+		{ lru, largest_contiguous };
+
+		disk_cache_algo_t disk_cache_algorithm;
 	};
 
 ``user_agent`` this is the client identification to the tracker.
@@ -3826,6 +3831,15 @@ data is read from the disk while checking. This may be useful for
 background tasks that doesn't matter if they take a bit longer, as long
 as they leave disk I/O time for other processes.
 
+``disk_cache_algorithm`` tells the disk I/O thread which cache flush
+algorithm to use. The default (and original) algorithm is LRU. This
+flushes the entire piece, in the write cache, that was least recently
+written to. This is specified by the ``session_settings::lru`` enum
+value. ``session_settings::largest_contiguous`` will flush the largest
+sequences of contiguous blocks from the write cache, regarless of the
+piece's last use time.
+
+
 pe_settings
 ===========