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

"Bruce Momjian" <momjian@svr1.postgresql.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