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Message-ID: <469DD5EB.3010904@enterprisedb.com>
Date: Wed, 18 Jul 2007 09:57:15 +0100
From: Heikki Linnakangas <heikki@enterprisedb.com>
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To: Thomas Finneid <tfinneid@student.matnat.uio.no>
CC: pgsql-performance@postgresql.org
Subject: Re: importance of fast disks with pg
References: <469D382E.6030603@ifi.uio.no>
In-Reply-To: <469D382E.6030603@ifi.uio.no>
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Thomas Finneid wrote:
> During the somes I did I noticed that it does not necessarily seem to be
> true that one needs the fastest disks to have a pg system that is fast.
> 
> It seems to me that its more important to:
> - choose the correct methods to use for the operation
> - tune the pg memory settings
> - tune/disable pg xlog/wal etc
> 
> It also seems to me that fast disks are more important for db systems of
> the OLTP type applications with real concurrency of both readers and
> writes across many, possibly larger, tables etc.
> 
> Are the above statements close to having any truth in them?

It depends.

The key to performance is to identify the bottleneck. If your CPU is
running at 50%, and spends 50% of the time waiting for I/O, a faster
disk will help. But only up to a point. After you add enough I/O
capability that the CPU is running at 100%, getting faster disks doesn't
help anymore. At that point you need to get more CPU power.

Here's the algorithm for increasing application throughput:

while throughput is not high enough
{
  identify bottleneck
  resolve bottleneck, by faster/more hardware, or by optimizing application
}

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com