> > > but how to find out what is this "cluster-name" for pg_ctlcluster,
> > > pg_dropcluster and pg_upgradecluster ?
> >
> > OK from the conf files assume cluster-name is '10/main' and '11/main'
> > respectively ?
>
> default cluster name for both clusters should be "main".
yes I got there eventually
You could also try reading the README.Debian.gz file in
/usr/share/doc/postgresql-*/
In stretch, this is /usr/share/doc/postgresql-9.6/README.Debian.gz
and it contains the exact steps needed to upgrade from 9.1 to 9.4.
Just replace the version numbers with whatever you have.