AIX slow buffer reads

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