Date: Mon, 25 Oct 2010 16:21:34 -0200 (BRST)
From: =?utf-8?Q?Andr=C3=A9_Volpato?= <andre.volpato@ecomtecnologia.com.br>
To: PGSQL-Performance <pgsql-performance@postgresql.org>
Message-ID: <1416587032.9578.1288030894805.JavaMail.root@zimbra01a>
In-Reply-To: <1290167597.9546.1288029134827.JavaMail.root@zimbra01a>
Subject: AIX slow buffer reads
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Hi all,=20

We are tuning a PostgreSQL box with AIX 5.3 and got stucked in a very odd s=
ituation.
When a query got ran for the second time, the system seems to deliver the r=
esults to slow.

Here=C2=B4s some background info:

AIX Box:=20
PostgreSQL 8.4.4, AIX 5.3-9 64bits, SAN IBM DS3400, 8x450GB SAS 15K Raid-5
8GB RAM, 2.3GB Shared buffers

Debian Box:
PostgreSQL 8.4.4, Debian 4.3.2 64bits, SAN IBM DS3400, 5x300GB SAS 15K Raid=
-0
7GB RAM, 2.1GB Shared buffers

Right now, we changed lots of AIX tunables to increase disk and SO performa=
nce.=20
Of course, postgres got tunned as well. I can post all changes made until n=
ow if needed.

To keep it simple, I will try to explain only the buffer read issue.
This query [1] took like 14s to run at AIX, and almost the same time at Deb=
ian.
The issue is when I run it for the second time:
AIX - 8s
Debian - 0.3s

These times keep repeating after the second run, and I can ensure AIX isn=
=C2=B4t touching the disks anymore.
I=C2=B4ve never seen this behaviour before. I heard about Direct I/O and I =
was thinking about givng it a shot.
Any ideas?


1 - http://explain.depesz.com/s/5oz


[]=C2=B4s, Andr=C3=A9 Volpato =20