On Mon, Oct 23, 2006 at 03:08:03PM -0400, Tom Lane wrote:
> "Jim C. Nasby" <jim@nasby.net> writes:
> > The only case I can think of where autovac might not work as well as
> > smartvacuum would be if you had a lot of databases in the cluster, since
> > autovacuum will only vacuum one database at a time.
>
> It's conceivable that it'd make sense to allow multiple autovac
> processes running in parallel. (The infrastructure part of this is easy
> enough, the hard part is keeping them from all deciding to vacuum the
> same table.)
It might be worth creating a generic framework that prevents multiple
vacuums from hitting a table at once, autovac or not.
> One reason we have not done that already is the thought that multiple
> vacuum processes would suck too much I/O to be reasonable. Now you
> could dial back their resource demands with the cost-delay settings,
> but it's not clear that ten autovacs running at one-tenth speed are
> better than one autovac using all the cycles you can spare. Usually
> I think it's best if a vacuum transaction finishes as fast as it can.
There's other things that would benefit from having some idea on what IO
resources are available. For example, having a separate bgwriter (or
reader) for each set of physical volumes. So a means of grouping
tablespaces wouldn't hurt.
> In any case, these exact same concerns would apply to manual vacuums
> or a combination of manual and auto vacuum.
Well, the advantage to manual vacuums is that you can tune things to
utilize multiple arrays...
--
Jim Nasby jim@nasby.net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
