Message-ID: <4CC87C69.5070301@2ndquadrant.com>
Date: Wed, 27 Oct 2010 15:24:25 -0400
From: Greg Smith <greg@2ndquadrant.com>
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 <andre.volpato@ecomtecnologia.com.br>
CC: Brad Nicholson <bnichols@ca.afilias.info>,
	pgsql-performance@postgresql.org
Subject: Re: AIX slow buffer reads
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André Volpato wrote:
> | 
> | If it is being spent in the bitmap index scan, try setting
> | effective_io_concurrency to 0 for Linux, and see what effect that has.
>
> I disabled effective_io_concurrency at AIX but it made no changes on bitmap index times.
>   

Brad's point is that it probably doesn't do anything at all on AIX, and 
is already disabled accordingly.  But on Linux, it is doing something, 
and that might be contributing to why it's executing so much better on 
that platform.  If you disable that parameter on your Debian box, that 
should give you an idea whether that particular speed-up is a major 
component to the difference you're seeing or not.

Also, if the system check was done by the "vendor team" team, don't 
trust them at all.  It doesn't sound like a disk problem is involved in 
your case yet, but be sure to do your own basic disk benchmarking too 
rather than believing what you're sold.  There's a quick intro to that 
at http://www.westnet.com/~gsmith/content/postgresql/pg-disktesting.htm 
and a much longer treatment of the subject in my book if you want a lot 
more details.  I don't have any AIX-specific tuning advice in there though.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services and Support        www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books