Re: CPU 0.1% IOWAIT 99% for decisonnal queries

On Thu, 2005-03-24 at 10:48 +0100, Patrick Vedrines wrote:
> > You've got 1.5Gb of shared_buffers and > 2Gb data. In 8.0, the scan
> will
> > hardly use the cache at all, nor will it ever, since the data is
> bigger
> > than the cache. Notably, the scan of B should NOT spoil the cache
> for A
> Are you sure of that ? Is Postgres able to say to OS: "don't use the
> cache for this query"?

PostgreSQL 8.0 has the ARC algorithm which prevents cache spoiling of
the shared_buffers, but has no direct influence over the OS cache.

> > Priming the cache is quite hard...but not impossible.
> > What will kill you on a shared_buffers that big is the bgwriter,
> which
> > you should turn off by setting bgwriter_maxpages = 0
> Is bgwriter concerned as my application applyes only SELECT ?

With very large shared_buffers the bgwriter's default settings are a
problem. You don't need it, so I suggest turning it off.

Best Regards, Simon Riggs