Re: 2nd Level Buffer Cache

On Fri, Mar 25, 2011 at 8:07 AM, Gurjeet Singh <singh.gurjeet@gmail.com> wrote:
> On Tue, Mar 22, 2011 at 3:53 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>>
>> On Tue, Mar 22, 2011 at 11:24 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
>> > On Fri, Mar 18, 2011 at 9:19 AM, Robert Haas <robertmhaas@gmail.com>
>> > wrote:
>> >>
>> >> A related area that could use some looking at is why performance tops
>> >> out at shared_buffers ~8GB and starts to fall thereafter.
>> >
>> > Under what circumstances does this happen?  Can a simple pgbench -S
>> > with a large scaling factor elicit this behavior?
>>
>> To be honest, I'm mostly just reporting what I've heard Greg Smith say
>> on this topic.   I don't have any machine with that kind of RAM.
>
> I can sponsor a few hours (say 10) of one High-memory on-demand Quadruple
> Extra Large instance (26 EC2 Compute Units (8 virtual cores with 3.25 EC2
> Compute Units each), 1690 GB of local instance storage, 64-bit platform).
> That's the largest memory AWS has.

Does AWS have machines with battery-backed write cache?  I think
people running servers with 192G probably have BBWC, so it may be hard
to do realistic tests without also having one on the test machine.

But probably a bigger problem is that (to the best of my knowledge) we
don't seem to have a non-proprietary, generally implementable
benchmark system or load-generator which is known to demonstrate the
problem.

Cheers,

Jeff