Feedback-ID: ia2694551:Fastmail
Date: Fri, 14 Apr 2023 18:43:29 +0200
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Greg Stark <stark@mit.edu>
Cc: "Jonathan S. Katz" <jkatz@postgresql.org>,
	Robert Haas <robertmhaas@gmail.com>,
	Laurenz Albe <laurenz.albe@cybertec.at>,
	Amit Kapila <amit.kapila16@gmail.com>,
	Andres Freund <andres@anarazel.de>,
	Justin Pryzby <pryzby@telsasoft.com>, pgsql-hackers@postgresql.org
Subject: Re: Should we remove vacuum_defer_cleanup_age?
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On 2023-Apr-14, Greg Stark wrote:

> On Fri, 14 Apr 2023 at 09:47, Jonathan S. Katz <jkatz@postgresql.org> wrote:
> >
> > Let me restate [1] in a different way.
> >
> > Using a large enough dataset, I did qualitatively look at overall usage
> > of both "vacuum_defer_cleanup_age" and compared to
> > "hot_standby_feedback", given you can use both to accomplish similar
> > outcomes.
> 
> I assume people would use hot_standby_feedback if they have streaming
> replication. 

Yes, either that or a replication slot.

vacuum_defer_cleanup_age was added in commit efc16ea52067 (2009-12-19),
for Postgres 9.0.  hot_standby_feedback is a bit newer
(bca8b7f16a3e, 2011-02-16), and replication slots are newer still
(858ec11858a9, 2014-01-31).

> The main use cases for vacuum_defer_cleanup_age would be if you're
> replaying WAL files.  That may sound archaic but there are plenty of
> circumstances where your standby may not have network access to your
> primary at all or not want to be replaying continuously.

True, those cases exist.  However, it sounds to me like in those cases
vacuum_defer_cleanup_age doesn't really help you either; you'd just want
to pause WAL replay depending on your queries or whatever.  After all,
you'd have to feed the WAL files "manually" to replay, so you're in
control anyway without having to touch the primary.

I find it very hard to believe that people are doing stuff with
vacuum_defer_cleanup_age that cannot be done with either of the other
newer mechanisms, which have also seen much wider usage and so bugs
fixed, etc.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Oh, great altar of passive entertainment, bestow upon me thy discordant images
at such speed as to render linear thought impossible" (Calvin a la TV)