Re: lastOverflowedXid does not handle transaction ID wraparound

On Thu, Oct 21, 2021 at 07:21 Stan Hu <stanhu@gmail.com> wrote:

> On Wed, Oct 20, 2021 at 9:01 PM Kyotaro Horiguchi
> <horikyota.ntt@gmail.com> wrote:
> >
> > lastOverflowedXid is the smallest subxid that possibly exists but
> > possiblly not known to the standby. So if all top-level transactions
> > older than lastOverflowedXid end, that means that all the
> > subtransactions in doubt are known to have been ended.
>
> Thanks for the patch! I verified that it appears to reset
> lastOverflowedXid properly.
>

Is it right time to register the patch in the current commit fest, right?
(How to do that?)

On a separate note, I think it would be really good to improve
observability for SLRUs -- particularly for Subtrans SLRU and this
overflow-related aspects.  pg_stat_slru added in PG13 is really helpful,
but not enough to troubleshoot, analyze and tune issues like this, and the
patches related to SLRU. Basic ideas:
- expose to the user how many pages are currently used (especially useful
if SLRU sizes will be configurable, see
https://commitfest.postgresql.org/34/2627/)
- Andrew Borodin also expressed the idea to extend pageinspect to allow
seeing the content of SLRUs
- a more specific thing: allow seeing lastOverflowedXid somehow (via SQL or
in logs) - we see how important it for standbys health, but we cannot see
it now.

Any ideas in the direction of observability?

Nik

>