Re: Remove Deprecated Exclusive Backup Mode

On Wed, Jul 1, 2020 at 4:27 PM David Steele <david(at)pgmasters(dot)net> wrote:
> Well, the only thing pg_stop_backup() *really* needs to know is the
> starting WAL position. pg_start_backup() gets that info so if it passes
> it back to pg_stop_backup() that could be enough. Or as was proposed
> above, it just passes the backup_label back to pg_stop_backup() for
> parsing. To write the .backup file to WAL (which I personally think is
> pretty useless) you'd need backup_label.
>
> The issue I have not been able to work around is that full page writes
> need to be on for backups and they won't get turned back off if you
> don't end all the backups that get started.
>
> Here's a thought. What if we just stored the oldest starting LSN and a
> count of how many backups have been requested. When the backup ends it
> checks that backup count is > 0 and starting LSN is <= its starting LSN.
> If not, it throws an error. When backups go to 0 FPWs are turned off if
> they were off before the first backup.
>
> That way, we could have a single function to cancel all backups in
> progress. Even if a new one started, one ending that was started prior
> to the cancel would know that it could not end successfully and error.
>
> For the vast majority of people who have full page writes on, it
> wouldn't really matter how many backups were running so we could
> probably just skip all of that.

Yeah, I like this line of thinking. Not sure about the details.

> If we wanted to really make it more foolproof we could add a checksum to
> the backup label to make sure the user doesn't damage it before passing
> it back.

I don't see a whole lot of point in that, honestly. It doesn't make it
impossible to damage the backup label; it just makes it harder. And
that doesn't seem worth much.

--
Robert Haas
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