On Tue, 2007-04-10 at 11:40 +0100, Heikki Linnakangas wrote:
> Tom Lane wrote:
> > Heikki Linnakangas <heikki@enterprisedb.com> writes:
> >> I've been working on the patch to enhance our group commit behavior. The
> >> patch is a dirty hack at the moment, but I'm settled on the algorithm
> >> I'm going to use and I know the issues involved.
> >
> > One question that just came to mind is whether Simon's no-commit-wait
> > patch doesn't fundamentally alter the context of discussion for this.
I was certainly intending that it would.
> > Aside from the prospect that people won't really care about group commit
> > if they can just use the periodic-WAL-sync approach, ISTM that one way
> > to get group commit is to just make everybody wait for the dedicated
> > WAL writer to write their commit record. With a sufficiently short
> > delay between write/fsync attempts in the background process, won't
> > that net out at about the same place as a complicated group-commit
> > patch?
>
> Possibly. To get efficient group commit there would need to be some kind
> of signaling between the WAL writer and normal backends. I think there
> is some in the patch, but I'm not sure if it gives efficient group
> commit. A constant delay will just give us something similar to
> commit_delay.
Agreed.
> I've refrained from spending time on group commit until the
> commit-no-wait patch lands, because it's going to conflict anyway. I'm
> starting to feel we should not try to rush group commit into 8.3, unless
> it somehow falls out of the commit-no-wait patch by accident, given that
> we're past feature freeze and coming up with a proper group commit
> algorithm would need a lot of research and testing. Better do it for 8.4
> with more time, we've got enough features on plate for 8.3 anyway.
My feeling was that I couldn't get both done for 8.3, and that including
the WAL Writer in 8.3 would make the dev path clearer for a later
attempt upon group commit.
I think it was worth exploring whether it would be easy, but I think we
can see it'll take a lot of work to make it "fly right".
-- Simon Riggs EnterpriseDB http://www.enterprisedb.com