Date: Fri, 23 Apr 2021 17:29:21 -0700
From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@bowt.ie>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Testing autovacuum wraparound (including failsafe)
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Hi,

On 2021-04-23 16:12:33 -0700, Peter Geoghegan wrote:
> The only reason that I chose 4GB for FAILSAFE_MIN_PAGES is because the
> related VACUUM_FSM_EVERY_PAGES constant was 8GB -- the latter limits
> how often we'll consider the failsafe in the single-pass/no-indexes
> case.

I don't really understand why it makes sense to tie FAILSAFE_MIN_PAGES
and VACUUM_FSM_EVERY_PAGES together? They seem pretty independent to me?


> I see no reason why it cannot be changed now. VACUUM_FSM_EVERY_PAGES
> also frustrates FSM testing in the single-pass case in about the same
> way, so maybe that should be considered as well? Note that the FSM
> handling for the single pass case is actually a bit different to the
> two pass/has-indexes case, since the single pass case calls
> lazy_vacuum_heap_page() directly in its first and only pass over the
> heap (that's the whole point of having it of course).

I'm not opposed to lowering VACUUM_FSM_EVERY_PAGES (the costs don't seem
all that high compared to vacuuming?), but I don't think there's as
clear a need for testing around that as there is around wraparound.


The failsafe mode affects the table scan itself by disabling cost
limiting. As far as I can see the ways it triggers for the table scan (vs
truncation or index processing) are:

1) Before vacuuming starts, for heap phases and indexes, if already
   necessary at that point
2) For a table with indexes, before/after each index vacuum, if now
   necessary
3) On a table without indexes, every 8GB, iff there are dead tuples, if now necessary

Why would we want to trigger the failsafe mode during a scan of a table
with dead tuples and no indexes, but not on a table without dead tuples
or with indexes but fewer than m_w_m dead tuples? That makes little
sense to me.


It seems that for the no-index case the warning message is quite off?

		ereport(WARNING,
				(errmsg("abandoned index vacuuming of table \"%s.%s.%s\" as a failsafe after %d index scans",

Doesn't exactly make one understand that vacuum cost limiting now is
disabled? And is confusing because there would never be index vacuuming?

And even in the cases indexes exist, it's odd to talk about abandoning
index vacuuming that hasn't even started yet?


> > For 2), I don't really have a better idea than making that configurable
> > somehow?
> 
> That could make sense as a developer/testing option, I suppose. I just
> doubt that it makes sense as anything else.

Yea, I only was thinking of making it configurable to be able to test
it. If we change the limit to something considerably lower I wouldn't
see a need for that anymore.

Greetings,

Andres Freund