Waldo Nell <pwnell@xxxxxxxxxxxx> writes: > I have PostgreSQL 8.4.8 on Ubuntu Linux x64. Server is a Core i7 950 > with 6GB of RAM. 2GB of RAM us used by Java, some small amount by the > kernel / services and the rest is available to PostgreSQL. [ and the DB is 74GB, and things get slower when raising shared_buffers from 24MB to 1500MB ] One other point here is that with the DB so much larger than available RAM, you are almost certainly doing lots of I/O (unless your test case has lots of locality of reference). With small shared_buffers, the space for kernel disk cache amounts to 3 or so GB, and that's your primary buffer against duplicate I/Os. When you crank shared_buffers up to half that, you now have two buffer pools of about the same size independently trying to cache the most-used parts of the DB. This is likely to not work too well and result in much more I/O. You save some shared-buffers-to-kernel-buffers transfers with more shared_buffers, but if the amount of disk I/O goes up a lot in consequence, you'll come out way behind. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
