From: Greg Smith <greg@2ndquadrant.com>
To: David Kerr <dmk@mr-paradox.net>
Cc: Scott Marlowe <scott.marlowe@gmail.com>
Cc: Nikolas Everett <nik9000@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Very high effective_cache_size == worse performance?
Date: Tue, 20 Apr 2010 16:26:52 -0400
Message-ID: <4BCE0E0C.1020903@2ndquadrant.com> (raw)
In-Reply-To: <20100420182832.GB53489@mr-paradox.net>
References: <20100420173936.GA50886@mr-paradox.net>
<h2w603c8f071004201044tbc8ce8fenb846b7643a2b9376@mail.gmail.com>
<20100420180351.GA51940@mr-paradox.net>
<w2ld4e11e981004201112v3571f350zaaa0112d1e035097@mail.gmail.com>
<20100420182030.GA53489@mr-paradox.net>
<j2qdcc563d11004201123u21219445v4a9129ee7d7b520b@mail.gmail.com>
<20100420182832.GB53489@mr-paradox.net>
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.
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.
--
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, dmk@mr-paradox.net, scott.marlowe@gmail.com, nik9000@gmail.com, robertmhaas@gmail.com
Subject: Re: Very high effective_cache_size == worse performance?
In-Reply-To: <4BCE0E0C.1020903@2ndquadrant.com>
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