From: André Volpato <andre.volpato@ecomtecnologia.com.br>
To: PGSQL-Performance <pgsql-performance@postgresql.org>
Subject: AIX slow buffer reads
Date: Mon, 25 Oct 2010 16:21:34 -0200 (BRST)
Message-ID: <1416587032.9578.1288030894805.JavaMail.root@zimbra01a> (raw)
In-Reply-To: <1290167597.9546.1288029134827.JavaMail.root@zimbra01a>
Hi all,
We are tuning a PostgreSQL box with AIX 5.3 and got stucked in a very odd situation.
When a query got ran for the second time, the system seems to deliver the results to slow.
Here´s some background info:
AIX Box:
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 performance.
Of course, postgres got tunned as well. I can post all changes made until now 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 Debian.
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´t touching the disks anymore.
I´ve 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
[]´s, André Volpato
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To: pgsql-performance@postgresql.org
Cc: andre.volpato@ecomtecnologia.com.br
Subject: Re: AIX slow buffer reads
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