Re: Improve handling of parameter differences in physical replication

At Mon, 9 Mar 2020 21:13:38 +0900, Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote in
> On Mon, 9 Mar 2020 at 18:45, Peter Eisentraut
> <peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:
> >
> > On 2020-03-09 09:11, Masahiko Sawada wrote:
> > > I think after recovery is paused users will be better to restart the
> > > server rather than resume the recovery. I agree with this idea but I'm
> > > slightly concerned that users might not realize that recovery is
> > > paused until they look at that line in server log or at
> > > pg_stat_replication because the standby server is still functional. So
> > > I think we can periodically send WARNING to inform user that we're
> > > still waiting for parameter change and restart.
> >
> > I think that would be annoying, unless you create a system for
> > configuring those periodic warnings.
> >
> > I imagine in a case like having set max_prepared_transactions but never
> > actually using prepared transactions, people will just ignore the
> > warning until they have their next restart, so it could be months of
> > periodic warnings.
>
> Well I meant to periodically send warning messages while waiting for
> parameter change, that is after exhausting resources and stopping
> recovery. In this situation user need to notice that as soon as
> possible.

If we lose connection, standby continues to complain about lost
connection every 5 seconds. This is a situation of that kind.

By the way, when I reduced max_connection only on master then take
exclusive locks until standby complains on lock exchaustion, I see a
WARNING that is saying max_locks_per_transaction instead of
max_connection.

WARNING: insufficient setting for parameter max_connections
DETAIL: max_connections = 2 is a lower setting than on the master server (where its value was 3).
HINT: Change parameters and restart the server, or there may be resource exhaustion errors sooner or later.
CONTEXT: WAL redo at 0/60000A0 for XLOG/PARAMETER_CHANGE: max_connections=3 max_worker_processes=8 max_wal_senders=2 max_prepared_xacts=0 max_locks_per_xact=10 wal_level=replica wal_log_hints=off track_commit_timestamp=off
WARNING: recovery paused because of insufficient setting of parameter max_locks_per_transaction (currently 10)
DETAIL: The value must be at least as high as on the primary server.
HINT: Recovery cannot continue unless the parameter is changed and the server restarted.
CONTEXT: WAL redo at 0/6004A80 for Standb

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center