Re: PG12 autovac issues

Hi,

On 2020-03-24 14:26:06 +0900, Michael Paquier wrote:
> > Could you share what the config of the server was?
>
> Nothing really fancy:
> - autovacuum_vacuum_cost_delay to 2ms (default of v12, but we used it
> in v11 as well).
> - autovacuum_naptime = 15s
> - autovacuum_max_workers = 6
> - log_autovacuum_min_duration = 0

Oh, so you're also involved in this? I'm starting to get a bit confused
as to who is reporting what.

> > Did you see any errors / fatals around the time autovacuum stopped
> > working?
>
> Before going rogue (we are not sure if autovacuum didn't launch any
> workers or if the workers were spawned and exited early as we did not
> capture any worker information in pg_stat_activity), we saw a bunch of
> aggressive wraparound jobs. Even after that, we have traces in the
> logs of one autovacuum analyze happening at equal interval of time (17
> minutes) on one single table, which is... Er... uncommon to say the
> least.

Well, there's no logging of autovacuum launchers that don't do anything
due to the "skipping redundant" logic, with normal log level. If somehow
the horizon logic of autovacuum workers gets out of whack with what
vacuumlazy.c does, then you'd not get any vacuums. But a usage level
triggered analyze could still happen on such a table, I think.

While looking at this issue I found a few problems, btw. That seems more
like a -hackers discussion, so I started:
https://postgr.es/m/20200323235036.6pje6usrjjx22zv3%40alap3.anarazel.de

I think I might just have figured out another one...

Greetings,

Andres Freund