Re: Very high effective_cache_size == worse performance?

On Tue, Apr 20, 2010 at 04:26:52PM -0400, Greg Smith wrote:
- David Kerr wrote:
- >the db, xlog and logs are all on separate areas of the SAN.
- >separate I/O controllers, etc on the SAN. it's setup well, I wouldn't
- >expect
- >contention there.
- >
-
- Just because you don't expect it doesn't mean it's not there.
- Particularly something as complicated as a SAN setup, presuming anything
- without actually benchmarking it is a recipe for fuzzy diagnostics when
- problems pop up. If you took anyone's word that your SAN has good
- performance without confirming it yourself, that's a path that's lead
- many to trouble.

that's actually what I'm doing, performance testing this environment.
everything's on the table for me at this point.

- Anyway, as Robert already stated, effective_cache_size only impacts how
- some very specific types of queries are executed; that's it. If there's
- some sort of query behavior involved in your load, maybe that has
- something to do with your slowdown, but it doesn't explain general slow
- performance. Other possibilities include that something else changed
- when you reloaded the server as part of that, or it's a complete
- coincidence--perhaps autoanalyze happened to finish at around the same
- time and it lead to new plans.

Ok that's good to know. I didn't think it would have any impact, and was
surprised when it appeared to.

I just finished running the test on another machine and wasn't able to
reproduce the problem, so that's good news in some ways. But now i'm back
to the drawing board.

I don't think it's anything in the Db that's causing it. ( drop and re-create
the db between tests) I actually suspect a hardware issue somewhere.

Dave