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

On Thu, 12 Mar 2020 at 14:38, Laurenz Albe <laurenz.albe@cybertec.at> wrote:
>
> On Thu, 2020-03-12 at 17:47 +1300, David Rowley wrote:
> > I'm starting to think that we should set the scale_factor to something
> > like 0.3 and the threshold to 50. Is anyone strongly against that?  Or
> > Laurenz, are you really set on the 10 million threshold?
>
> These values are almost the same as "autovacuum_vacuum_scale_factor"
> and "autovacuum_vacuum_threshold", so you actually agree with Masahiko
> with the exception that you want it tunable separately.
>
> I don't like the high scale factor.
>
> If your insert-only table was last vacuumed when it had 500 million rows,
> the next autovacuum will freeze 150 million tuples, which is a lot.
> The impact will be less than that of an anti-wraparound vacuum because
> it is not as persistent, but if our 150 million tuple autovacuum backs
> down because it hits a lock or gets killed by the DBA, that is also not
> good, since it will just come again.
> And the bigger the vacuum run is, the more likely it is to meet an obstacle.
>
> So I think that large insert-only tables should be vacuumed more often
> than that.  If the number of tuples that have to be frozen is small,
> the vacuum run will be short and is less likely to cause problems.
> That is why I chose a scale factor of 0 here.

The reason why you want to add new GUC parameters is to use different
default values for insert-update table case and insert-only table
case? I think I understand the pros and cons of adding separate
parameters, but I still cannot understand use cases where we cannot
handle without separate parameters.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services