Re: Add recovery to pg_control and remove backup_label

Hi,

On 2023-11-21 07:42:42 -0400, David Steele wrote:
> On 11/20/23 19:58, Andres Freund wrote:
> > On 2023-11-21 08:52:08 +0900, Michael Paquier wrote:
> > > On Mon, Nov 20, 2023 at 12:37:46PM -0800, Andres Freund wrote:
> > > > Given that, I wonder if what we should do is to just add a new field to
> > > > pg_control that says "error out if backup_label does not exist", that we set
> > > > when creating a streaming base backup
> > >
> > > That would mean that one still needs to take an extra step to update a
> > > control file with this byte set, which is something you had a concern
> > > with in terms of compatibility when it comes to external backup
> > > solutions because more steps are necessary to take a backup, no?
> >
> > I was thinking we'd just set it in the pg_basebackup style path, and we'd
> > error out if it's set and backup_label is present. But we'd still use
> > backup_label without the pg_control flag set.
> >
> > So it'd just provide a cross-check that backup_label was not removed for
> > pg_basebackup style backup, but wouldn't do anything for external backups. But
> > imo the proposal to just us pg_control doesn't actually do anything for
> > external backups either - which is why I think my proposal would achieve as
> > much, for a much lower price.
>
> I'm not sure why you think the patch under discussion doesn't do anything
> for external backups. It provides the same benefits to both pg_basebackup
> and external backups, i.e. they both receive the updated version of
> pg_control.

Sure. They also receive a backup_label today. If an external solution forgets
to replace pg_control copied as part of the filesystem copy, they won't get an
error after the remove of backup_label, just like they don't get one today if
they don't put backup_label in the data directory. Given that users don't do
the right thing with backup_label today, why can we rely on them doing the
right thing with pg_control?

Greetings,

Andres Freund