Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin

Hi,

On 2024-06-25 12:31:11 -0400, Robert Haas wrote:
> On Tue, Jun 25, 2024 at 11:39 AM Melanie Plageman
> <melanieplageman@gmail.com> wrote:
> > On Tue, Jun 25, 2024 at 10:31 AM Robert Haas <robertmhaas@gmail.com> wrote:
> > > On Tue, Jun 25, 2024 at 9:07 AM Andres Freund <andres@anarazel.de> wrote:
> > > > It's not hard - but it has downsides. It'll mean that - outside of vacuum -
> > > > we'll much more often not react to horizons going backwards due to
> > > > hot_standby_feedback. Which means that hot_standby_feedback, when used without
> > > > slots, will prevent fewer conflicts.
> > >
> > > Can you explain this in more detail?
> >
> > If we prevent GlobalVisState from moving backward, then we would less
> > frequently be pushing the horizon backward on the primary in response
> > to hot standby feedback. Then, the primary would do more things that
> > would not be safely replayable on the standby -- so the standby could
> > end up encountering more recovery conflicts.
> 
> I don't get it. hot_standby_feedback only moves horizons backward on
> the primary, AFAIK, when it first connects, or when it reconnects.
> Which I guess could be frequent for some users with flaky networks,
> but does that really rise to the level of "much more often"?

Well, the thing is that with the "prevent it from going backwards" approach,
once the horizon is set to something recent in a backend, it's "sticky".  If a
replica is a bit behind or if there's a long-lived snapshot and disconnects,
the vistest state will advance beyond where the replica needs it to be.  Even
if the standby later reconnects, the vistest in long-lived sessions will still
have the more advanced state. So all future pruning these backends do runs
into the risk of performing pruning that removes rows the standby still deems
visible and thus causes recovery conflicts.

I.e. you don't even need frequent disconnects, you just need one disconnect
and sessions that aren't shortlived.

That said, obviously there will be plenty setups where this won't cause an
issue. I don't really have a handle on how often it'd be a problem.


Greetings,

Andres Freund