Re: Possible causes of high_replay lag, given replication settings?

On Fri, Jul 25, 2025 at 7:13 PM Greg Sabino Mullane <htamfids(at)gmail(dot)com> wrote:
>
> On Fri, Jul 25, 2025 at 9:57 AM Jon Zeppieri <zeppieri(at)gmail(dot)com> wrote:
>>
>> Thanks for the response, Nick. I'm curious why the situation you describe wouldn't also lead to the write_lag and flush_lag also being
>> high. If the problem is simply keeping up with the primary, wouldn't you expect all three lag times to be elevated?
>
>
> No - write and flush are pretty quick and simple, it's just putting the WAL onto the local disk. Replay involves a lot more work as we have to parse the WAL and apply the changes, which means doing a lot of I/O across many files. Still, *hours* to me indicates more than just a lot of extra traffic. Check that recovery_min_apply_delay is still 0, then log onto the replica and see what's going on with regards to open transactions and locks.

Thanks Greg. `recovery_min_apply_delay` is 0, just checked. Also, I
didn't mention in my initial post that it seemed the cause of the
delay was long-running queries on the replica, rather than the
primary. It's possible, of course, that I'm wrong, but I was able to
get the replica moving again when I killed off old queries on the
replica. If those were the problem, though, then I don't understand
why the max_standby_streaming_delay didn't prevent that situation.

- Jon