From: | André Volpato <andre(dot)volpato(at)ecomtecnologia(dot)com(dot)br> |
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To: | PGSQL-Performance <pgsql-performance(at)postgresql(dot)org> |
Subject: | AIX slow buffer reads |
Date: | 2010-10-25 18:21:34 |
Message-ID: | 1416587032.9578.1288030894805.JavaMail.root@zimbra01a |
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Lists: | pgsql-performance |
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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