Re: use CREATE DATABASE STRATEGY = FILE_COPY in pg_upgrade

On Wed, 19 Jun 2024 at 15:17, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Fri, Jun 14, 2024 at 5:29 PM Nathan Bossart <nathandbossart(at)gmail(dot)com> wrote:
> > On Wed, Jun 12, 2024 at 09:41:01AM -0500, Nathan Bossart wrote:
> > > Actually, I think you are right that we need a manual checkpoint, except I
> > > think we need it to be after prepare_new_globals(). set_frozenxids()
> > > connects to each database (including template0) and updates a bunch of
> > > pg_class rows, and we probably want those on disk before we start copying
> > > the files to create all the user's databases.
> >
> > Here is an updated patch.
>
> OK, I have a (probably) stupid question. The comment says:
>
> + * In binary upgrade mode, we can skip this checkpoint because neither of
> + * these problems applies: we don't ever replay the WAL generated during
> + * pg_upgrade, and we don't concurrently modify template0 (not to mention
> + * that trying to take a backup during pg_upgrade is pointless).
>
> But what happens if the system crashes during pg_upgrade? Does this
> patch make things worse than they are today? And should we care?

As Nathan just said, AFAIK we don't have a way to resume progress from
a crashed pg_upgrade, which implies you currently have to start over
with a fresh dbinit.
This patch wouldn't change that for the worse, it'd only add one more
process that depends on that behaviour.

Maybe in the future that'll change if pg_upgrade and pg_dump are made
smart enough to resume their restore progress across forceful
disconnects, but for now this patch seems like a nice performance
improvement.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)