On Fri, 18 Jul 2025 at 21:29, Jon Zeppieri <zeppi...@gmail.com> wrote: > > I just had a situation where physical replication fell far behind > (hours). The write and flush lag times were 0, but replay_lag was > high. The replica has hot_standby_feedback on, and both > max_standby_streaming_delay and max_standby_archive_delay are set to > 30s. > > What could cause a situation like this? If the network were a problem, > I'd expect the other _lag times to be high. So it appears that the > replica was getting the WAL but was unable to apply it. Are there > situations where the replica cannot apply WAL other than the kinds of > conflicts that would be addressed by the _delay settings? > > I checked pg_stat_database_conflicts, but there was nothing in it -- all > zeros.
This can happen when there are several busy writing processes on the primary. The single replay process on the replica can't keep up with the writes.
