Date: Tue, 20 Jun 2006 11:16:35 +1200
From: Mark Kirkwood <markir@paradise.net.nz>
Subject: Re: SAN performance mystery
In-reply-to: <20060619102430.GD3303@mathom.us>
To: pgsql-performance@postgresql.org
Cc: tim@proximity.com.au, mstone+postgres@mathom.us
Message-id: <44973053.4080906@paradise.net.nz>
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References: <4491D61B.9040507@proximity.com.au>
	<1150408614.26538.95.camel@state.g2switchworks.com>
	<449677EB.50609@proximity.com.au> <20060619102430.GD3303@mathom.us>

Michael Stone wrote:
> On Mon, Jun 19, 2006 at 08:09:47PM +1000, Tim Allen wrote:
>> Certainly, the read performance of the SATA disk still beats the SAN, 
>> and there is no way to lie about read performance.
> 
> Sure there is: you have the data cached in system RAM. I find it real  
> hard to believe that you can sustain 161MB/s off a single SATA disk.
> 

Agreed - approx 60-70Mb/s seems to be the ballpark for modern SATA 
drives, so get get 161Mb/s you would need about 3 of them striped 
together (or a partially cached file as indicated).

What is interesting is that (presumably) the same test is getting such 
uninspiring results on the SAN...

Having said that, I've been there too, about 4 years ago with a SAN that 
had several 6 disk RAID5 arrays, and the best sequential *read* 
performance we ever saw from them was about 50Mb/s. I recall trying to 
get performance data from the vendor - only to be told that if we were 
doing benchmarks - could they have our results when we were finished!

regards

Mark