Re: Help tuning a large table off disk and into RAM

On Wed, 26 Sep 2007, James Williams wrote:

> The box has 4 x Opterons, 4Gb RAM & five 15k rpm disks, RAID 5.  We
> wanted fast query/lookup.  We know we can get fast disk IO.

You might want to benchmark to prove that if you haven't already.  You
would not be the first person to presume you have fast disk I/O on RAID 5
only to discover that's not actually true when tested.
http://www.westnet.com/~gsmith/content/postgresql/pg-disktesting.htm gives
some details here.

> shared_buffers  = 128MB
> temp_buffers    = 160MB
> work_mem        = 200MB
> max_stack_depth = 7MB

The one you're missing is effective_cache_size, and I'd expect you'd need
to more than double shared_buffers to have that impact things given what
you've described of your tasks.  Take a look at
http://www.westnet.com/~gsmith/content/postgresql/pg-5minute.htm to get a
better idea the right range for those two you should be considering; 128MB
for shared_buffers is way low for your system, something >1GB is probably
right, and effective_cache_size should probably be in the multiple GB
range.

If you actually want to see what's inside the shared_buffers memory, take
a look at the contrib/pg_buffercache module.  Installing that for your
database will let you see how the memory is being used, to get a better
idea how much of your indexes are staying in that part of memory.  The
hint you already got from Bill Moran about using pg_relation_size() will
give you some basis for figuring out what % of the index is being held
there.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD