Re: Postgresql 9.5: Streaming Replication: Secondaries Fail To Start Post WAL Error

On Tue, May 28, 2024 at 05:24:56PM -0400, Ron Johnson wrote:
> On Tue, May 28, 2024 at 3:11 PM Johannes Truschnigg <
> >[...]
> > Yes, replication slots can interrupt your primary.
> >
>
> Please define "interrupt".  Using a replication slot, I thought files would
> just accumulate in pg_wal while the replica is down (or the network is
> slow, or the replica can't keep up with the primary).
>
> Disaster, of course, when that disk fills up, but that's always been the
> case.

And that is exactly the scenario I meant when I said "interrupt". If you use
replication slots, your monitoring/alerting isn't set up correctly, and you're
accumulating a lot of WAL, chances are ENOSPC on the primary is around the
corner for you.

That's why I generally prefer a WAL archive on a separate file system for
replicas to source segments from, because filling that up won't break the
primary (unless the archive_command misbehaves). That also needs proper
monitoring/alerting, of course (and a contingency plan for what to do when/if
the archive runs over) - but everyone whose workload is important enough for a
replication setup to make sense is required to have that in my book.

--
with best regards:
- Johannes Truschnigg ( johannes@truschnigg.info )

www:   https://johannes.truschnigg.info/