Re: Allow logical failover slots to wait on synchronous replication

On Tue, Jul 9, 2024 at 12:39 AM John H <johnhyvr(at)gmail(dot)com> wrote:
>
> > Out of curiosity, did you compare with standby_slot_names_from_syncrep set to off
> > and standby_slot_names not empty?
>
> I didn't think 'standby_slot_names' would impact TPS as much since
> it's not grabbing the SyncRepLock but here's a quick test.
> Writer with 5 synchronous replicas, 10 pg_recvlogical clients and
> pgbench all running from the same server.
>
> Command: pgbench -c 4 -j 4 -T 600 -U "ec2-user" -d postgres -r -P 5
>
> Result with: standby_slot_names =
> 'replica_1,replica_2,replica_3,replica_4,replica_5'
>
> latency average = 5.600 ms
> latency stddev = 2.854 ms
> initial connection time = 5.503 ms
> tps = 714.148263 (without initial connection time)
>
> Result with: standby_slot_names_from_syncrep = 'true',
> synchronous_standby_names = 'ANY 3 (A,B,C,D,E)'
>
> latency average = 5.740 ms
> latency stddev = 2.543 ms
> initial connection time = 4.093 ms
> tps = 696.776249 (without initial connection time)
>
> Result with nothing set:
>
> latency average = 5.090 ms
> latency stddev = 3.467 ms
> initial connection time = 4.989 ms
> tps = 785.665963 (without initial connection time)
>
> Again I think it's possible to improve the synchronous numbers if we
> cache but I'll try that out in a bit.
>

Okay, so the tests done till now conclude that we won't get the
benefit by using 'standby_slot_names_from_syncrep'. Now, if we
increase the number of standby's in both lists and still keep ANY 3 in
synchronous_standby_names then the results may vary. We should try to
find out if there is a performance benefit with the use of
synchronous_standby_names in the normal configurations like the one
you used in the above tests to prove the value of this patch.

--
With Regards,
Amit Kapila.