Re: Syncrep and improving latency due to WAL throttling

On Thu, Jan 26, 2023 at 9:21 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> > 7. I think we need to not let backends throttle too frequently even
> > though they have crossed wal_throttle_threshold bytes. The best way is
> > to rely on replication lag, after all the goal of this feature is to
> > keep replication lag under check - say, throttle only when
> > wal_distance > wal_throttle_threshold && replication_lag >
> > wal_throttle_replication_lag_threshold.
>
> I think my idea of only forcing to flush/wait an LSN some distance in the past
> would automatically achieve that?

I'm sorry, I couldn't get your point, can you please explain it a bit more?

Looking at the patch, the feature, in its current shape, focuses on
improving replication lag (by throttling WAL on the primary) only when
synchronous replication is enabled. Why is that? Why can't we design
it for replication in general (async, sync, and logical replication)?

Keeping replication lag under check enables one to provide a better
RPO guarantee as discussed in the other thread
https://www.postgresql.org/message-id/CAHg%2BQDcO_zhgBCMn5SosvhuuCoJ1vKmLjnVuqUEOd4S73B1urw%40mail.gmail.com.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com