use O_SYNC when available

include/libtorrent/settings_pack.hpp

@@ -941,11 +941,7 @@ namespace libtorrent
 			//   disabled, enabling this may reduce performance.
 			// 
 			// One reason to disable caching is that it may help the operating
-			// system from growing its file cache indefinitely. Since some OSes
-			// only allow aligned files to be opened in unbuffered mode, It is
-			// recommended to make the largest file in a torrent the first file
-			// (with offset 0) or use pad files to align all files to piece
-			// boundries.
+			// system from growing its file cache indefinitely.
 			disk_io_write_mode,
 			disk_io_read_mode,
 

src/file.cpp

@@ -1308,6 +1308,9 @@ namespace libtorrent
 #endif
 #ifdef O_DIRECT
 			| ((mode & direct_io) ? O_DIRECT : 0)
+#endif
+#ifdef O_SYNC
+			| ((mode & no_cache) ? O_SYNC: 0)
 #endif
 			, permissions);