Re: Allow logical failover slots to wait on synchronous replication

Hi Amit,

Thanks for taking a look.

On Tue, Jun 18, 2024 at 10:34 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>

>
> The reading indicates when you set 'standby_slot_names_from_syncrep',
> the TPS reduces as compared to when it is not set. It would be better
> to see the data comparing 'standby_slot_names_from_syncrep' and the
> existing parameter 'standby_slot_names'.

I added new benchmark numbers in the reply to Bertrand, but I'll
include in this thread for posterity.

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)

> Can we make it a default
> behavior that logical slots marked with a failover option will wait
> for 'synchronous_standby_names' as per your patch's idea unless
> 'standby_slot_names' is specified? I don't know if there is any value
> in setting the 'failover' option for a slot without specifying
> 'standby_slot_names', so was wondering if we can additionally tie it
> to 'synchronous_standby_names'. Any better ideas?
>

No, I think that works pretty cleanly actually. Reserving some special
keyword isn't great
which is the only other thing I can think of. I've updated the patch
and tests to reflect that.

Attached the patch that addresses these changes.

--
John Hsu - Amazon Web Services