Message-ID: <4DC88AB5.1030003@2ndquadrant.com>
Date: Mon, 09 May 2011 20:45:41 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Craig James <craig_james@emolecules.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Benchmarking a large server
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Craig James wrote:
> Maybe this is a dumb question, but why do you care?  If you have 1TB 
> RAM and just a little more actual disk space, it seems like your 
> database will always be cached in memory anyway.  If you "eliminate 
> the cach effect," won't the benchmark actually give you the wrong 
> real-life results?

If you'd just spent what two FusionIO drives cost, you'd want to make 
damn sure they worked as expected too.  Also, if you look carefully, 
there is more disk space than this on the server, just not on the SSDs.  
It's possible this setup could end up with most of RAM filled with data 
that's stored on the regular drives.  In that case the random 
performance of the busy SSD would be critical.  It would likely take a 
very bad set of disk layout choices for that to happen, but I could see 
heavy sequential scans of tables in a data warehouse pushing in that 
direction.

Isolating out the SSD performance without using the larger capacity of 
the regular drives on the server is an excellent idea here, it's just 
tricky to do.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books