Difference between fsync and open_sync

Hi, have a nice day everybody. I'm reading about WAL and i'm trying to
understand the theory about it, i mean, all the stuff related to disk
caches, controller caches and so on, and now in my lectures there is a
section about the different methods to force WAL updates, wal_sync_method

And googling more i found a very good article (old but useful)

http://www.westnet.com/~gsmith/content/postgresql/TuningPGWAL.htm

And i understood the difference between, for example, fsync and fdatasync,
and between open_datasync and open_sync, but i cannot understand the
difference between open_sync and fsync or between open_datasync and
fdatasync , As far as i could understand, the open_* methods call the
open() method of linux, and f*sync methods calls the fsync() method of
linux?

My doubt is about what's the difference between them, or a link where this
is explained. In the previous link it talks about the 4 methods but i
cannot distinguish the difference between them.

Regards.

***************************
Oscar Calderon
Analista de Sistemas
Soluciones Aplicativas S.A. de C.V.
www.solucionesaplicativas.com
Cel. (503) 7741 7850 Tel. (503) 2522-2834