Re: Bgwriter behavior

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> First, we remove the GUC bgwriter_maxpages because I don't see a good
> way to set a default for that.  A default value needs to be based on a
> percentage of the full buffer cache size.

This is nonsense.  The admin knows what he set shared_buffers to, and so
maxpages and percent of shared buffers are not really distinct ways of
specifying things.  The cases that make a percent spec useful are if
(a) it is a percent of a non-constant number (eg, percent of total dirty
pages as in the current code), or (b) it is defined in a way that lets
it limit the amount of scanning work done (which it isn't useful for in
the current code).  But a maxpages spec is useful for (b) too.  More to
the point, maxpages is useful to set a hard limit on the amount of I/O
generated by the bgwriter, and I think people will want to be able to do
that.

> Now, to control the bgwriter frequency we multiply the percent of the
> list it had to span by the bgwriter_delay value to determine when to run
> bgwriter next.

I'm less than enthused about this.  The idea of the bgwriter is to
trickle out writes in a way that doesn't affect overall performance too
much.  Not to write everything in sight at any cost.

I like the hybrid "keep the bottom of the ARC list clean, plus do a slow
clock scan on the main buffer array" approach better.  I can see that
that directly impacts both of the goals that the bgwriter has.  I don't
see how a variable I/O rate really improves life on either score; it
just makes things harder to predict.
        regards, tom lane