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

On Wed, Jul 17, 2024 at 12:11 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Wed, Jul 17, 2024 at 12:07 PM Melanie Plageman
> <melanieplageman(at)gmail(dot)com> wrote:
> > We didn't end up doing two index vacuum passes. Because it doesn't
> > repro locally for me, I can only assume that the conditions for
> > forcing two index vacuuming passes in master just weren't met in this
> > case. I'm unsurprised, as it is much harder since 17 to force two
> > passes of index vacuuming. It seems like this might be as unstable as
> > I feared. I could add more dead data. Or, I could just commit the test
> > to the back branches before 17. What do you think?
>
> How much margin of error do you have, in terms of total number of
> dead_items? That is, have you whittled it down to the minimum possible
> threshold for 2 passes?

When I run it on my machine with some added logging, the space taken
by dead items is about 330 kB more than maintenance_work_mem (which is
set to 1 MB). I could roughly double the excess by increasing the
number of inserted tuples from 400000 to 600000. I'll do this.

> Some logging with VACUUM VERBOSE (run on the ci instance) might be illuminating.

Vacuum verbose only will tell us the number of dead tuples and dead
item identifiers but not how much space they take up -- which is how
we decide whether or not to do index vacuuming.

- Melanie