Re: O_DIRECT for WAL writes

Neil Conway <neilc@samurai.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@postgresql.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...

---
ITAGAKI Takahiro
NTT Cyber Space Laboratories