Message-ID: <4B86FE2A.1030205@2ndquadrant.com>
Date: Thu, 25 Feb 2010 17:48:10 -0500
From: Greg Smith <greg@2ndquadrant.com>
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To: Yu-Ju Hong <yuru.hong@gmail.com>
CC: pgsql-performance@postgresql.org
Subject: Re: dbt2 performance
References: <516a4a601002251323udcc2b41tf5a09b968bef3a7c@mail.gmail.com>
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Yu-Ju Hong wrote:
> 2. Moreover, the disk utilization was high and the "await" time from 
> iostat is around 500 ms. Could disk I/O limit the overall throughput? 
> The server has 2 SATA disks, one for system and postgresql and the 
> other is dedicated to logging (pg_xlog). As far as I understand, 
> modern database systems should be CPU-bound rather than I/O-bound, is 
> it because I did not perform adequate performance tuning?

dbt2 is almost exclusively disk I/O bound once the data set gets big 
enough.  There are some applications where most of the data fits in RAM 
and therefore CPU performance is the limiter.  dbt2 is exactly the 
opposite of such an application though, and the idea that "modern 
database systems should be CPU bound" is not really true at all.  That's 
only the case if the data you're operating on fits in RAM.  Otherwise, 
databases are just as I/O bound as they've always been.  Main thing 
that's changed is there's a lot more RAM in systems nowadays.

By the way:  a large increase in checkpoint_segments is the first thing 
you should do.  If you check the database logs, they're probably filled 
with complaints about it being too low.  32 would be a useful starting 
value, going much higher for a test that's only 10 minutes long is 
probably cheating.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us