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Re: Testing Sandforce SSD

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Matthew Wakeling <matthew(at)flymine(dot)org>
Cc: Yeb Havinga <yebhavinga(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Testing Sandforce SSD
Date: 2010-07-26 18:34:39
Message-ID: 4C4DD53F.4070901@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)2ndQuadrant(dot)com   www.2ndQuadrant.us


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