Re: pg_stop_backup(wait_for_archive := true) on standby server

On Fri, Jul 7, 2017 at 3:48 PM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> On Wed, Jul 5, 2017 at 4:57 PM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> Why not refactoring a bit do_pg_stop_backup() so as the wait phase
>> happens even if a backup is started in recovery? Now wait_for_archive
>> is ignored because no wait is happening and the stop point is directly
>> returned back to the caller. For the wait actually happening, I don't
>> have a better idea than documenting the fact that enforcing manually a
>> segment switch on the primary needs to happen. That's better than
>> having users including WAL in their base backups but not actually
>> having everything they need. And I think that documenting that
>> properly is better than restricting things that should work.
>
> While looking at that in more details, I got surprised by two things:
> 1) No backup history file is generated on a standby during a base backup.
> 2) Because of 1), those files are not archived even if archive_mode = always.
>
> This sounds to me like a missing optimization of archive_mode =
> always, and the following comment block in xlog.c is at least
> incorrect as an archiver can be invoked in recovery:
>      * XXX currently a backup history file is for informational and debug
>      * purposes only. It's not essential for an online backup. Furthermore,
>      * even if it's created, it will not be archived during recovery because
>      * an archiver is not invoked. So it doesn't seem worthwhile to write a
>      * backup history file during recovery.
>
> So I would suggest the following things to address this issue:
> 1) Generate a backup history file for backups taken at recovery as well.
> 2) Archive it if archive_mode = always.
> 3) Wait for both the segment of the stop point and the backup history
> files to be archived before returning back to the caller of
> pg_stop_backup.
>
> It would be nice to get all that addressed in 10. Thoughts?

Yeah, I agree. Attached patch makes it works and deals with the
history file issue.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center