Writing WAL files

I am moving a database from PSQL 9 (!) on CentOS 6 to PSQL 12 on CentOS 7

I have a pair of servers -- one live, one standby.
The live server defines an archive_command as "rsync...." to shift WAL
files to the standby server,
The standby server uses "pg_standby" to monitor and process incoming WAL
files.
I believe this is all very vanilla, and indeed changes made in the live
database are duly shipped to the standby.

BUT...

One of the things I like about the old PGSQL 9 setup is that it generates
and ships a WAL file every few minutes, even if nothing has happened in the
database.
I find it re-assuring to be able to see WAL files arriving and being
processed without problem even when the live system was idle.

But I cannot get PGSQL 12 to do this. It only writes (and thus ships) WAL
files when something happens in the database.
If the database is idle, it simply does not write any WAL files.

I thought I would get WAL files written from an idle database if, in
postgresql.conf, I set "archive_timeout" to 120.

And I've tried setting "checkpoint_timeout" to 90s,

But to no avail. No WAL files are written unless the database changes.

So what am I missing? How CAN I get postgresql 12 to write "unnecessary"
WAL files every couple of minutes?

Robert.

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