Hi Maxim,
> Secondly, shouldn't we introduce a new WAL record type in order to
> make the code backward compatible with previous PG versions? I'm not
> 100% sure how the upgrade procedure works in all the details. If it
> requires the DBMS to be gracefully shut down before the upgrade then
> we are probably fine here.
After reading [1] carefully it looks like we shouldn't worry about
this. The upgrade procedure explicitly requires to run `pg_ctl stop`
during step 8 of the upgrade procedure, i.e. not in the immediate mode
[2]. It also has explicit instructions regarding the replicas. From
what I can tell there is no way they will see WAL records they
wouldn't understand.
[1]: https://www.postgresql.org/docs/current/pgupgrade.html
[2]: https://www.postgresql.org/docs/current/app-pg-ctl.html
--
Best regards,
Aleksander Alekseev