Thread: pgsql: Use O_DIRECT if available when using O_SYNC for wal_sync_method.
pgsql: Use O_DIRECT if available when using O_SYNC for wal_sync_method.
From
momjian@svr1.postgresql.org (Bruce Momjian)
Date:
Log Message:
-----------
Use O_DIRECT if available when using O_SYNC for wal_sync_method.
Also, write multiple WAL buffers out in one write() operation.
ITAGAKI Takahiro
---------------------------------------------------------------------------
> If we disable writeback-cache and use open_sync, the per-page writing
> behavior in WAL module will show up as bad result. O_DIRECT is similar
> to O_DSYNC (at least on linux), so that the benefit of it will disappear
> behind the slow disk revolution.
>
> In the current source, WAL is written as:
> for (i = 0; i < N; i++) { write(&buffers[i], BLCKSZ); }
> Is this intentional? Can we rewrite it as follows?
> write(&buffers[0], N * BLCKSZ);
>
> In order to achieve it, I wrote a 'gather-write' patch (xlog.gw.diff).
> Aside from this, I'll also send the fixed direct io patch (xlog.dio.diff).
> These two patches are independent, so they can be applied either or both.
>
>
> I tested them on my machine and the results as follows. It shows that
> direct-io and gather-write is the best choice when writeback-cache is off.
> Are these two patches worth trying if they are used together?
>
>
> | 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
>
> - 20runs * pgbench -s 100 -c 50 -t 200
> - with tuning (wal_buffers=64, commit_delay=500, checkpoint_segments=8)
> - using 2 ATA disks:
> - hda(reiserfs) includes system and wal.
> - hdc(jfs) includes database files. writeback-cache is always on.
>
> ---
> ITAGAKI Takahiro
Modified Files:
--------------
pgsql/src/backend/access/transam:
xlog.c (r1.210 -> r1.211)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.210&r2=1.211)
Re: pgsql: Use O_DIRECT if available when using O_SYNC for wal_sync_method.
From
"Qingqing Zhou"
Date:
"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
Re: pgsql: Use O_DIRECT if available when using O_SYNC for wal_sync_method.
From
"Qingqing Zhou"
Date:
"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