Re: Testing Sandforce SSD

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