Waldo Nell <pwnell@telkomsa.net> 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