Re: pg_upgrade: Support for upgrading to checksums enabled

On Mon, Aug 26, 2024 at 08:23:44AM +0200, Peter Eisentraut wrote:
> The purpose of this patch is to allow using pg_upgrade between clusters that
> have different checksum settings. When upgrading between instances with
> different checksum settings, the --copy (default) mode automatically sets
> (or unsets) the checksum on the fly.
>
> This would be particularly useful if we switched to enabling checksums by
> default, as [0] proposes, but it's also useful without that.

Given enabling checksums can be rather expensive, I think it makes sense to
add a way to do it during pg_upgrade versus asking folks to run
pg_checksums separately. I'd anticipate arguments against enabling
checksums automatically, but as you noted, we can move it to a separate
option (e.g., --copy --enable-checksums). Disabling checksums with
pg_checksums is fast because it just updates pg_control, so I don't see any
need for --disable-checkums in pg_upgrade.

> - Windows has a separate code path in the --copy mode. I don't know the
> reasons or advantages of that. So it's not clear how the checksum rewriting
> mode should be handled in that case. We could switch to the non-Windows
> code path in that case, but then the performance difference between the
> normal path and the checksum-rewriting path is even more unclear.

AFAICT the separate Windows path dates back to before pg_upgrade was first
added to the Postgres tree, and unfortunately I couldn't find any
discussion about it.

--
nathan