Re: New strategies for freezing, advancing relfrozenxid early

On Thu, 2022-09-08 at 13:23 -0700, Peter Geoghegan wrote:
> The new patch unifies the concept of antiwraparound
> VACUUM with the concept of aggressive VACUUM. Now there is only
> antiwraparound and regular VACUUM (uh, barring VACUUM FULL). And now
> antiwraparound VACUUMs are not limited to antiwraparound autovacuums
> -- a manual VACUUM can also be antiwraparound (that's just the new
> name for "aggressive").

I like this general approach. The existing GUCs have evolved in a
confusing way.

> For the most part the
> skipping/freezing strategy stuff has a good sense of what matters
> already, and shouldn't need to be guided very often.

I'd like to know more clearly where manual VACUUM fits in here. Will it
user a more aggressive strategy than an autovacuum, and how so?

> The patch relegates vacuum_freeze_table_age to a compatibility
> option,
> making its default -1, meaning "just use autovacuum_freeze_max_age".

The purpose of vacuum_freeze_table_age seems to be that, if you
regularly issue VACUUM commands, it will prevent a surprise
antiwraparound vacuum. Is that still the case?

Maybe it would make more sense to have vacuum_freeze_table_age be a
fraction of autovacuum_freeze_max_age, and be treated as a maximum so
that other intelligence might kick in and freeze sooner?

>  This makes things less confusing for users and hackers.

It may take an adjustment period ;-)

> The details of the skipping-strategy-choice algorithm are still
> unsettled in v3 (no real change there). ISTM that the important thing
> is still the high level concepts. Jeff was slightly puzzled by the
> emphasis placed on the cost model/strategy stuff, at least at one
> point. Hopefully my intent will be made clearer by the ideas featured
> in the new patch.

Yes, it's clearing things up, but it's still a complex problem.
There's:

 a. xid age vs the actual amount of deferred work to be done
 b. advancing relfrozenxid vs skipping all-visible pages
 c. difficulty in controlling reasonable behavior (e.g.
    vacuum_freeze_min_age often being ignored, freezing
    individual tuples rather than pages)

Your first email described the motivation in terms of (a), but the
patches seem more focused on (b) and (c).

>  The skipping strategy decision making process isn't
> particularly complicated, but it now looks more like an optimization
> problem of some kind or other.

There's another important point here, which is that it gives an
opportunity to decide to freeze some all-visible pages in a given round
just to reduce the deferred work, without worrying about advancing
relfrozenxid.


-- 
Jeff Davis
PostgreSQL Contributor Team - AWS