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Re: O_DIRECT for WAL writes

From: ITAGAKI Takahiro <itagaki(dot)takahiro(at)lab(dot)ntt(dot)co(dot)jp>
To: Neil Conway <neilc(at)samurai(dot)com>
Cc: pgsql-patches(at)postgresql(dot)org
Subject: Re: O_DIRECT for WAL writes
Date: 2005-05-30 01:59:59
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Lists: pgsql-hackerspgsql-patches
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

> 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

Attachment: pgbench-result.png
Description: image/png (26.4 KB) (inlined above)

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