From: Greg Smith <greg@2ndquadrant.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: Yeb Havinga <yebhavinga@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Testing Sandforce SSD
Date: Mon, 26 Jul 2010 14:34:39 -0400
Message-ID: <4C4DD53F.4070901@2ndquadrant.com> (raw)
In-Reply-To: <alpine.DEB.2.00.1007261632380.20471@aragorn.flymine.org>
References: <4C4A9452.9070100@gmail.com>
<4C4B1F84.5030306@2ndquadrant.com>
<4C4C83DF.3010703@gmail.com>
<4C4C8CCC.7080406@gmail.com>
<alpine.DEB.2.00.1007261045470.20471@aragorn.flymine.org>
<4C4DA982.5090308@2ndquadrant.com>
<alpine.DEB.2.00.1007261632380.20471@aragorn.flymine.org>
Matthew Wakeling wrote:
> Yeb also made the point - there are far too many points on that graph
> to really tell what the average latency is. It'd be instructive to
> have a few figures, like "only x% of requests took longer than y".
Average latency is the inverse of TPS. So if the result is, say, 1200
TPS, that means the average latency is 1 / (1200 transactions/second) =
0.83 milliseconds/transaction. The average TPS figure is normally on a
more useful scale as far as being able to compare them in ways that make
sense to people.
pgbench-tools derives average, worst-case, and 90th percentile figures
for latency from the logs. I have 37MB worth of graphs from a system
showing how all this typically works for regular hard drives I've been
given permission to publish; just need to find a place to host it at
internally and I'll make the whole stack available to the world. So far
Yeb's data is showing that a single SSD is competitive with a small
array on average, but with better worst-case behavior than I'm used to
seeing.
--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com www.2ndQuadrant.us
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To: pgsql-performance@postgresql.org
Cc: greg@2ndquadrant.com, matthew@flymine.org, yebhavinga@gmail.com
Subject: Re: Testing Sandforce SSD
In-Reply-To: <4C4DD53F.4070901@2ndquadrant.com>
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