# stop the old cluster
systemctl stop postgresql@10-<name>
# Disable starting of the old cluster: change auto to disabled in the following file
vim /etc/postgresql/10/<name>/start.conf
# initialize an empty cluster with the proper options (LC_ALL=C.UTF-8 and data directory)
LC_ALL=C.UTF-8 pg_createcluster -d <datadir> 11 <name>
# apply potential configuration changes from the old cluster to the new
vimdiff /etc/postgresql/{10,11}/<name>/postgresql.conf
# link the postgresql config in the datadir of the old/new cluster as `pg_upgrade` needs that
ln -s /etc/postgresql/11/<name>/postgresql.conf <datadir>/postgresql.conf
# move in a directory writable by the postgres user
mkdir /tmp/upgrade-<name>; chown postgres /tmp/upgrade-<name>; cd /tmp/upgrade-<name>
# run pg_upgrade in check mode
sudo -u postgres env LC_ALL=C.UTF-8 /usr/lib/postgresql/11/bin/pg_upgrade --old-datadir <olddatadir> --new-datadir <datadir> --old-bindir /usr/lib/postgresql/10/bin --new-bindir /usr/lib/postgresql/11/bin -k --check
# if that succeeded, run pg_upgrade in the "do it" mode by dropping the --check flag
sudo -u postgres env LC_ALL=C.UTF-8 /usr/lib/postgresql/11/bin/pg_upgrade --old-datadir <olddatadir> --new-datadir <datadir> --old-bindir /usr/lib/postgresql/10/bin --new-bindir /usr/lib/postgresql/11/bin -k
# Allow systemd to start the new cluster
systemctl daemon-reload
# Start the new cluster
systemctl start postgresql@11-<name>
# Cleanup after the upgrade
sudo -u postgres "/usr/lib/postgresql/11/bin/vacuumdb" --port <port> --analyze-in-stages --all
rm -r <olddatadir>
rm -r /etc/postgresql/10/<name>