Re: How about a option to disable autovacuum cancellation on lock conflict?

On 2014-12-02 11:12:40 -0800, Josh Berkus wrote:
> On 12/02/2014 11:08 AM, Andres Freund wrote:
> > On 2014-12-02 11:02:07 -0800, Josh Berkus wrote:
> >> On 12/02/2014 10:35 AM, Alvaro Herrera wrote:
> >>> If the table is large, the time window for this to happen is large also;
> >>> there might never be a time window large enough between two lock
> >>> acquisitions for one autovacuum run to complete in a table. This
> >>> starves the table from vacuuming completely, until things are bad enough
> >>> that an emergency vacuum is forced. By then, the bloat is disastrous.
> >>>
> >>> I think it's that suicide that Andres wants to disable.
> >
> > Correct.
> >
> >> A much better solution for this ... and one which would solve a *lot* of
> >> other issues with vacuum and autovacuum ... would be to give vacuum a
> >> way to track which blocks an incomplete vacuum had already visited.
> >> This would be even more valuable for freeze.
> >
> > That's pretty much a different problem. Yes, some more persistent would
> > be helpful - although it'd need to be *much* more than which pages it
> > has visited - but you'd still be vulnerable to the same issue.
>
> If we're trying to solve the problem that vacuums of large, high-update
> tables never complete, it's solving the same problem.

Which isn't what I'm talking about.

The problem is that vacuum is cancelled if a conflicting lock request is
acquired. Plain updates don't do that. But there's workloads where you
need more heavyweight updates, and then it can easily happen.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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