O_DIRECT. Note that ZFS on Solaris doesn't support direct io.
On Windows the synchronous ioengines don't support direct io.
+atomic=bool If value is true, attempt to use atomic direct IO. Atomic
+ writes are guaranteed to be stable once acknowledged by
+ the operating system. Only Linux supports O_ATOMIC right
+ now.
+
buffered=bool If value is true, use buffered io. This is the opposite
of the 'direct' option. Defaults to true.