Re: Berserk Autovacuum (let's save next Mandrill)

On Tue, 2020-03-17 at 16:07 -0500, Justin Pryzby wrote:
> > Assume a scale factor >= 1, for example 2, and n live tuples.
> > The table has just been vacuumed.
> >
> > Now we insert m number tuples (which are live).
> >
> > Then the condition
> >
> > threshold + scale_factor * live_tuples < newly_inserted_tuples
> >
> > becomes
> >
> > 10000000 + 2 * (n + m) < m
> >
> > which can never be true for non-negative n and m.
> >
> > So a scale factor >= 1 disables the feature.
>
> No, this is what we mailed about privately yesterday, and I demonstrated that
> autovac can still run with factor=100. I said:

I remember.
Can you point out where exactly the flaw in my reasoning is?

> > It's a multiplier, not a percent out of 100 (fraction is not a great choice of
> > words).
> >
> > &autovacuum_vac_scale,
> > 0.2, 0.0, 100.0,
> >
> > The default is 0.2 (20%), so 100 means after updating/deleting 100*reltuples.

Yes, exactly.

> If 1.0 disabled the feature, it wouldn't make much sense to allow factor up to
> 100.

True, we could set the upper limit to 2, but it doesn't matter much.

Note that this is different from autovacuum_vacuum_scale_factor,
because inserted tuples are live, while dead tuples are not.

Yours,
Laurenz Albe