Re: Benchmarking a large server

On 05/09/2011 04:32 PM, Chris Hoover wrote:
> So, does anyone have any suggestions/experiences in benchmarking
> storage when the storage is smaller then 2x memory?

If you do the Linux trick to drop its caches already mentioned, you can
start a database test with zero information in memory. In that
situation, whether or not everything could fit in RAM doesn't matter as
much; you're starting with none of it in there. In that case, you can
benchmark things without having twice as much disk space. You just have
to recognize that the test become less useful the longer you run it, and
measure the results accordingly.

A test starting from that state will start out showing you random I/O
speed on the device, slowing moving toward in-memory cached speeds as
the benchmark runs for a while. You really need to capture the latency
data for every transaction and graph it over time to make any sense of
it. If you look at "Using and Abusing pgbench" at
http://projects.2ndquadrant.com/talks , starting on P33 I have several
slides showing such a test, done with pgbench and pgbench-tools. I
added a quick hack to pgbench-tools around then to make it easier to run
this specific type of test, but to my knowledge no one else has ever
used it. (I've had talks about PostgreSQL in my yard that were better
attended than that session, for which I blame Jonah Harris for doing a
great talk in the room next door concurrent with it.)

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books