Update C comment about O_DIRECT and fsync().

Author: bmomjian

src/include/access/xlogdefs.h

@@ -85,13 +85,12 @@ typedef uint32 TimeLineID;
 
 /*
  *	Because O_DIRECT bypasses the kernel buffers, and because we never
- *	read those buffers except during crash recovery, it is a win to use
- *	it in all cases where we sync on each write().	We could allow O_DIRECT
- *	with fsync(), but because skipping the kernel buffer forces writes out
- *	quickly, it seems best just to use it for O_SYNC.  It is hard to imagine
- *	how fsync() could be a win for O_DIRECT compared to O_SYNC and O_DIRECT.
- *	Also, O_DIRECT is never enough to force data to the drives, it merely
- *	tries to bypass the kernel cache, so we still need O_SYNC or fsync().
+ *	read those buffers except during crash recovery or if wal_level != minimal,
+ *	it is a win to use it in all cases where we sync on each write().  We could
+ *	allow O_DIRECT with fsync(), but it is unclear if fsync() could process
+ *	writes not buffered in the kernel.  Also, O_DIRECT is never enough to force
+ *	data to the drives, it merely tries to bypass the kernel cache, so we still
+ *	need O_SYNC/O_DSYNC.
  */
 #ifdef O_DIRECT
 #define PG_O_DIRECT				O_DIRECT