Message-ID: <4C4DA982.5090308@2ndquadrant.com>
Date: Mon, 26 Jul 2010 11:28:02 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Matthew Wakeling <matthew@flymine.org>
CC: Yeb Havinga <yebhavinga@gmail.com>, pgsql-performance@postgresql.org
Subject: Re: Testing Sandforce SSD
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	<alpine.DEB.2.00.1007261045470.20471@aragorn.flymine.org>
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Matthew Wakeling wrote:
> Does your latency graph really have milliseconds as the y axis? If so, 
> this device is really slow - some requests have a latency of more than 
> a second!

Have you tried that yourself?  If you generate one of those with 
standard hard drives and a BBWC under Linux, I expect you'll discover 
those latencies to be >5 seconds long.  I recently saw >100 *seconds* 
running a large pgbench test due to latency flushing things to disk, on 
a system with 72GB of RAM.  Takes a long time to flush >3GB of random 
I/O out to disk when the kernel will happily cache that many writes 
until checkpoint time.

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