Re: BUG #14326: Unexpected status after crash during exclusive backup

On 21/09/16 08:50, Michael Paquier wrote:
> On Fri, Sep 16, 2016 at 6:54 PM, <marco(dot)nenciarini(at)2ndquadrant(dot)it> wrote:
>> The workaround I found to reset this state is to force the instance through
>> another backup recovery, by starting an exclusive backup, saving the
>> backup_label, stopping the backup and restarting the instance with the saved
>> backup_label in place.
>
> That's not user-friendly.
>

I agree, it isn't. But it's the only way to reset that state with the
current available tools. Probably, the pg_resetxlog tool could be
modified to allow the user to reset that value only.

>> I don't know the best way to handle this situation, but at least, I'd like a
>> warning message when the instance exits from the crash recovery while
>> backupStartPoint is still set.
>
> So you would get such a warning even when you restore from a backup
> willingly, no? That may confuse users. Now, the case you are referring
> to is unfortunately a known problem with exclusive backups... There is
> no way to make the difference between a node restored from a backup
> and a node that crashed while a backup is taken.
> And that may be a
> reason to make non-exclusive backups more popular because they are
> more reliable.

You are right, an eventual solution to this issue must not interfere
with normal recovery from a backup.

To mitigate the effect we could to reset the state of the
backupStartPoint field during the pg_start_backup invocation. So, if a
backup will be interrupted by a reboot, the instance state will be
cleaned during the next backup.

Another possibility could be to emit a warning (and maybe reset
backupStartPoint value) during the shutdown of an instance that is fully
"in production".

Regards,
Marco

--
Marco Nenciarini - 2ndQuadrant Italy
PostgreSQL Training, Services and Support
marco(dot)nenciarini(at)2ndQuadrant(dot)it | www.2ndQuadrant.it