Re: Add 64-bit XIDs into PostgreSQL 15 - Mailing list pgsql-hackers

Hi hackers,

> > I have a very serious concern about the current patch set. as someone who has faced transaction id wraparound in the past.
>
> [...]
>
> I had a similar stance when I started working on this patch. Of
> course, it seemed horrible just to postpone the consequences of
> inadequate monitoring, too long running transactions that prevent
> aggressive autovacuum etc. So I can understand your point.
>
> With time I've got to a little bit of another view of this feature i.e.
>
> 1. It's important to correctly set monitoring, the cut-off of long
> transactions, etc. anyway. It's not the responsibility of vacuum
> before wraparound to report inadequate monitoring etc. Furthermore, in
> real life, this will be already too late if it prevents 32-bit
> wraparound and invokes much downtime in an unexpected moment of time
> if it occurs already. (The rough analogy for that is the machine
> running at 120mph turns every control off and applies full brakes just
> because the cooling liquid is low (of course there might be a warning
> previously, but anyway))
>
> 2. The checks and handlers for the event that is never expected in the
> cluster lifetime (~200 years at constant rate of 1e6 TPS) can be just
> dropped. Of course we still need to do automatic routine maintenance
> like cutting SLRU buffers (but with a much bigger interval if we have
> much disk space e.g.). But I considered that we either can not care
> what will be with cluster after > 200 years (it will be migrated many
> times before this, on many reasons not related to Postgres even for
> the most conservative owners). So the radical proposal is to drop
> 64-bit wraparound at all. The most moderate one is just not taking
> very much care that after 200 years we have more hassle than next
> month if we haven't set up everything correctly. Next month's pain
> will be more significant even if it teaches dba something.
>
> Big thanks for your view on the general implementation of this feature, anyway.

I'm inclined to agree with Pavel on this one. Keeping 32-bit XIDs in
order to intentionally trigger XID wraparound to indicate the ending
disk space and/or misconfigured system (by the time when it's usually
too late anyway) is a somewhat arguable perspective. It would be great
to notify the user about the potential issues with the configuration
and/or the fact that VACUUM doesn't catch up. But it doesn't mean we
should keep 32-bit XIDs in order to achive this.

--
Best regards,
Aleksander Alekseev