RE: Synchronizing slots from primary to standby

On Friday, February 16, 2024 8:33 AM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com> wrote:
> On Thursday, February 15, 2024 8:29 PM Amit Kapila
> <amit.kapila16@gmail.com> wrote:
> 
> >
> > On Thu, Feb 15, 2024 at 5:46 PM Bertrand Drouvot
> > <bertranddrouvot.pg@gmail.com> wrote:
> > >
> > > On Thu, Feb 15, 2024 at 05:00:18PM +0530, Amit Kapila wrote:
> > > > On Thu, Feb 15, 2024 at 4:29 PM Zhijie Hou (Fujitsu)
> > > > <houzj.fnst@fujitsu.com> wrote:
> > > > > Attach the v2 patch here.
> > > > >
> > > > > Apart from the new log message. I think we can add one more
> > > > > debug message in reserve_wal_for_local_slot, this could be
> > > > > useful to analyze the
> > failure.
> > > >
> > > > Yeah, that can also be helpful, but the added message looks naive to me.
> > > > + elog(DEBUG1, "segno: %ld oldest_segno: %ld", oldest_segno,
> > > > + segno);
> > > >
> > > > Instead of the above, how about something like: "segno: %ld of
> > > > purposed restart_lsn for the synced slot, oldest_segno: %ld
> > > > available"?
> > >
> > > Looks good to me. I'm not sure if it would make more sense to elog
> > > only if segno < oldest_segno means just before the
> > XLogSegNoOffsetToRecPtr() call?
> > >
> > > But I'm fine with the proposed location too.
> > >
> >
> > I am also fine either way but the current location gives required
> > information in more number of cases and could be helpful in debugging this
> new facility.
> >
> > > >
> > > > > And we
> > > > > can also enable the DEBUG log in the 040 tap-test, I see we have
> > > > > similar setting in 010_logical_decoding_timline and logging
> > > > > debug1 message doesn't increase noticable time on my machine.
> > > > > These are done
> > in 0002.
> > > > >
> > > >
> > > > I haven't tested it but I think this can help in debugging BF
> > > > failures, if any. I am not sure if to keep it always like that but
> > > > till the time these tests are stabilized, this sounds like a good
> > > > idea. So, how, about just making test changes as a separate patch
> > > > so that later if required we can revert/remove it easily?
> > > > Bertrand, do you have any thoughts on this?
> > >
> > > +1 on having DEBUG log in the 040 tap-test until it's stabilized (I
> > > +think we
> > > took the same approach for 035_standby_logical_decoding.pl IIRC) and
> > > then revert it back.
> > >
> >
> > Good to know!
> >
> > > Also I was thinking: what about adding an output to
> > pg_sync_replication_slots()?
> > > The output could be the number of sync slots that have been created
> > > and are not considered as sync-ready during the execution.
> > >
> >
> > Yeah, we can consider outputting some information via this function
> > like how many slots are synced and persisted but not sure what would
> > be appropriate here. Because one can anyway find that or more
> > information by querying pg_replication_slots. I think we can keep
> > discussing what makes more sense as a return value but let's commit
> > the debug/log patches as they will be helpful to analyze BF failures or any
> other issues reported.
> 
> Agreed. Here is new patch set as suggested. I used debug2 in the 040 as it could
> provide more information about communication between primary and standby.
> This also doesn't increase noticeable testing time on my machine for debug
> build.

Sorry, there was a miss in the DEBUG message, I should have used
UINT64_FORMAT for XLogSegNo(uint64) instead of %ld. Here is a small patch
to fix this.

Best Regards,
Hou zj