Re: Standby accepts recovery_target_timeline setting?

On Sat, Sep 28, 2019 at 12:14 AM David Steele <david(at)pgmasters(dot)net> wrote:
>
> Hi Peter,
>
> On 9/27/19 10:36 AM, Peter Eisentraut wrote:
> > On 2019-09-26 23:02, David Steele wrote:
> >> On 9/26/19 4:48 PM, Peter Eisentraut wrote:
> >>
> >>> I don't know if recovery_target_timeline is actually useful to change in
> >>> standby mode.
> >
> > OK, I have committed your original documentation patch.
>
> Thanks, that's a good start.
>
> As Fujii noticed and I have demonstrated upthread, just about any target
> setting can be used in a standby restore. This matches the behavior of
> prior versions so it's not exactly a regression, but the old docs made
> no claim that standby_mode disabled targeted restore.
>
> If fact, for both PG12 and before, setting a recovery target in standby
> mode causes the cluster to drop out of standby mode.
>
> Also, the presence or absence of recovery.signal does not seem to have
> any effect on how targeted recovery proceeds, except as Fujii has
> demonstrated in [1].
>
> I'm not sure what the best thing to do is. The docs are certainly
> incorrect, but fixing them would be weird. What do we say, setting
> targets will exit standby mode? That certainly what happens, though.
>
> Also, the fact that target settings are being used when recovery.signal
> is missing is contrary to the docs, and this is a new behavior in PG12.
> Prior to 12 you could not have target settings without recovery.conf
> being present by definition.
>
> I think, at the very least, the fact that targeted recovery proceeds in
> the absence of recovery.signal represents a bug.

Yes, recovery target settings are used even when neither backup_label
nor recovery.signal exist, i.e., just a crash recovery, in v12. This is
completely different behavior from prior versions.

IMO, since v12 is RC1 now, it's not good idea to change the logic to new.
So at least for v12, we basically should change the recovery logic so that
it behaves in the same way as prior versions. That is,

- Stop the recovery with an error if any recovery target is set in
crash recovery
- Use recovery target settings if set even when standby mode
- Do not enter an archive recovery mode if recovery.signal is missing

Thought?

If we want new behavior in recovery, we can change the logic for v13.

Regards,

--
Fujii Masao