Re: Postgres restart in the middle of exclusive backup and the presence of backup_label file

Greetings,

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Michael Paquier <michael(at)paquier(dot)xyz> writes:
> > On Thu, Nov 25, 2021 at 06:19:03PM -0800, SATYANARAYANA NARLAPURAM wrote:
> >> If we are keeping it then why not make it better?
>
> > Well, non-exclusive backups are better by design in many aspects, so I
> > don't quite see the point in spending time on something that has more
> > limitations than what's already in place.
>
> IMO the main reason for keeping it is backwards compatibility for users
> who have a satisfactory backup arrangement using it. That same argument
> implies that we shouldn't change how it works (at least, not very much).

There isn't a satisfactory backup approach using it specifically because
of this issue, hence why we should remove it to make it so users don't
run into this. Would also simplify the documentation around the low
level backup API, which would be a very good thing. Right now, making
improvements in that area is very challenging even if all you want to do
is improve the documentation around the non-exclusive API.

We dealt with this as best as one could in pgbackrest for PG versions
prior to when non-exclusive backup was added- which is to remove the
backup_label file as soon as possible and then put it back right before
you call pg_stop_backup() (since it'll complain otherwise). Not a
perfect answer though and a risk still exists there of a failed restart
happening. Of course, for versions which support non-exclusive backup,
we use that to avoid this issue.

We also extensively changed how restore works a couple releases ago and
while there was some noise about it, it certainly wasn't all that bad.
I don't find the reasons brought up to continue to support exclusive
backup to be at all compelling and the lack of huge issues with the new
way restore works to make it abundently clear that we can, in fact,
remove exclusive backup in a major version change without the world
coming down.

Thanks,

Stephen