Re: pgsql: Use O_DIRECT if available when using O_SYNC for wal_sync_method.

"Bruce Momjian" <momjian(at)svr1(dot)postgresql(dot)org> writes
> > | writeback | fsync= | fdata | open_ | fsync_ | open_
> > patch | cache | false | sync | sync | direct | direct
> > ------------+-----------+--------+-------+-------+--------+---------
> > direct io | off | 124.2 | 105.7 | 48.3 | 48.3 | 48.2
> > direct io | on | 129.1 | 112.3 | 114.1 | 142.9 | 144.5
> > gather-write| off | 124.3 | 108.7 | 105.4 | (N/A) | (N/A)
> > both | off | 131.5 | 115.5 | 114.4 | 145.4 | 145.2
> >

Can you explain the huge difference here:

> | writeback | | | open_ | |
> patch | cache | | | sync | |
> ------------+-----------+--------+-------+-------+--------+---------
> direct io | off | | | 48.3 | |
> gather-write| off | | | 105.4 | |
> both | off | | | 114.4 | |

Why both direct_io and gather_write is turned on but the open_sync
performance degrades?

Regards,
Qingqing