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

On Thu, Apr 11, 2019 at 11:25:29AM +0200, Chris Travers wrote:
> On Wed, Apr 10, 2019 at 5:21 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On April 10, 2019 8:13:06 AM PDT, Alvaro Herrera
> <alvherre(at)2ndquadrant(dot)com> wrote:
> >On 2019-Mar-31, Darafei "Komяpa" Praliaskouski wrote:
> >
> >> Alternative point of "if your database is super large and actively
> >written,
> >> you may want to set autovacuum_freeze_max_age to even smaller values
> >so
> >> that autovacuum load is more evenly spread over time" may be needed.
> >
> >I don't think it's helpful to force emergency vacuuming more
> >frequently;
> >quite the contrary, it's likely to cause even more issues.  We should
> >tweak autovacuum to perform freezing more preemtively instead.
>
> I still think the fundamental issue with making vacuum less painful is
> that the all indexes have to be read entirely. Even if there's not much
> work (say millions of rows frozen, hundreds removed). Without that issue
> we could vacuum much more frequently. And do it properly in insert only
> workloads.
>
> So I see a couple of issues here and wondering what the best approach is.
> The first is to just skip lazy_cleanup_index if no rows were removed.  Is
> this the approach you have in mind?  Or is that insufficient?

I don't think that's what Andres had in mind, as he explicitly mentioned
removed rows. So just skipping lazy_cleanup_index when there were no
deleted would not help in that case.

What I think we could do is simply leave the tuple pointers in the table
(and indexes) when there are only very few of them, and only do the
expensive table/index cleanup once there's anough of them.

> The second approach would be to replace the whole idea of this patch with
> a lazy freeze worker which would basically periodically do a vacuum freeze
> on relations matching certain criteria.  This could have a lower max
> workers than autovacuum and therefore less of a threat in terms of total
> IO usage.
> Thoughts?
>

Not sure. I find it rather difficult to manage more and more different
types of cleanup workers.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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