Re: BUG #17103: WAL segments are not removed after exceeding max_slot_wal_keep_size

At Sat, 17 Jul 2021 10:28:09 -0400, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote in
> On 2021-Jul-16, Alvaro Herrera wrote:
>
> > The buildfarm has remained green so far, but clearly this is something
> > we need to fix. Maybe it's as simple as adding the loop we use below,
> > starting at line 219.
>
> There are a few failures of this on buildfarm now,
..
> I am running the test in a loop with the attached; if it doesn't fail in
> a few more rounds I'll push it.
>
> There are two instances of a different failure:
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=kittiwake&dt=2021-07-17%2013%3A39%3A43
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hornet&dt=2021-07-16%2021%3A14%3A14
>
> # Failed test 'check that segments have been removed'
> # at t/019_replslot_limit.pl line 213.
> # got: '000000010000000000000021'
> # expected: '000000010000000000000022'
> # Looks like you failed 1 test of 19.
> [23:55:14] t/019_replslot_limit.pl ..............
> Dubious, test returned 1 (wstat 256, 0x100)
>
> I'm afraid about this not being something we can fix with some
> additional wait points ...

Sorry for the mistake. It seems to me the cause the above is that
segment removal happens *after* invalidation. Since (at least
currently) the "slot is invalidated" warning issued only at the time
just before WAL-removal, we should expect that old segments are gone
after the checkpoint-ending log, which should be seen after
WAL-removal. If not, that shows that there's a bug.

What do you think about the attached?

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center