Re: New strategies for freezing, advancing relfrozenxid early

Hi,

On 2023-01-24 14:49:38 -0800, Peter Geoghegan wrote:
> From e41d3f45fcd6f639b768c22139006ad11422575f Mon Sep 17 00:00:00 2001
> From: Peter Geoghegan <pg@bowt.ie>
> Date: Thu, 24 Nov 2022 18:20:36 -0800
> Subject: [PATCH v17 1/3] Add eager and lazy freezing strategies to VACUUM.
> 
> Eager freezing strategy avoids large build-ups of all-visible pages.  It
> makes VACUUM trigger page-level freezing whenever doing so will enable
> the page to become all-frozen in the visibility map.  This is useful for
> tables that experience continual growth, particularly strict append-only
> tables such as pgbench's history table.  Eager freezing significantly
> improves performance stability by spreading out the cost of freezing
> over time, rather than doing most freezing during aggressive VACUUMs.
> It complements the insert autovacuum mechanism added by commit b07642db.

However, it significantly increases the overall work when rows have a somewhat
limited lifetime. The documented reason why vacuum_freeze_min_age exist -
although I think it doesn't really achieve its documented goal anymore, after
the recent changes page-level freezing changes.


> VACUUM determines its freezing strategy based on the value of the new
> vacuum_freeze_strategy_threshold GUC (or reloption) with logged tables;
> tables that exceed the size threshold use the eager freezing strategy.

I think that's not a sufficient guard at all. The size of a table doesn't say
much about how a table is used.


> Unlogged tables and temp tables will always use eager freezing strategy,
> since there is essentially no downside.

I somewhat doubt that that is true, but certainly the cost is lower.


> Eager freezing is strictly more aggressive than lazy freezing.  Settings
> like vacuum_freeze_min_age still get applied in just the same way in
> every VACUUM, independent of the strategy in use.  The only mechanical
> difference between eager and lazy freezing strategies is that only the
> former applies its own additional criteria to trigger freezing pages.

That's only true because vacuum_freeze_min_age being has been fairly radically
redefined recently.

Greetings,

Andres Freund