Re: O_DIRECT for WAL writes

Neil Conway <neilc(at)samurai(dot)com> wrote:

> > The patch adds a new choice "open_direct" to wal_sync_method.
> Have you looked at what the performance difference of this option is?

Yes, I've tested pgbench and dbt2 and their performances have improved.
The two results are as follows:

1. pgbench -s 100 on one Pentium4, 1GB mem, 2 ATA disks, Linux 2.6.8
(attached image)
tps | wal_sync_method
-------+-------------------------------------------------------
147.0 | open_direct + write multipage (previous patch)
147.2 | open_direct (this patch)
109.9 | open_sync

2. dbt2 100WH on two opterons, 8GB mem, 12 SATA-RAID disks, Linux 2.4.20
tpm | wal_sync_method
--------+------------------------------------------------------
1183.9 | open_direct (this patch)
911.3 | fsync

> http://www.mail-archive.com/pgsql-patches(at)postgresql(dot)org/msg07186.html
> Is this data still applicable to the revised patch?

Direct-IO might be good on some machines, and bad on others.
This data is another reason that I revised the patch;
If you don't use open_direct, WAL writer behaves quite similarly to former.

However, the performances did not go down at least on my benchmarks.
I have no idea why the above data was bad...

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ITAGAKI Takahiro
NTT Cyber Space Laboratories