Re: MySQL versus Postgres

On Fri, 13 Aug 2010 14:17:17 +0800
Craig Ringer <craig@postnewspapers.com.au> wrote:

> On 13/08/10 08:38, Tatsuo Ishii wrote:
> >> It's slower than smaller numbers, and if you actually dirty a
> >> significant portion of it you can have a checkpoint that takes
> >> hours to sync, completely trashing system responsiveness for a
> >> good portion of it.
> >
> > So how much is the reasonal upper limit of shared_buffers at this
> > point? If it's obvious, should we disable or warn to use more
> > than that number?
>
> Trouble is, there won't be a "reasonable upper limit" ... because
> it depends so much on the ratio of memory to I/O throughput, the
> system's writeback aggressiveness, etc etc etc.
>
> Personally I've had two Pg machines where one seems to suffer with
> shared_buffers > 250MB out of 4GB and the other, which has 8GB of
> RAM, wants shared_buffers to be around 4GB! The main difference:
> disk subsystems.

What about the ratio of R/W? If it is a mostly read system is the
memory/IO throughput still a limiting factor for increasing
shared_buffers?

--
Ivan Sergio Borgonovo
http://www.webthatworks.it