NB : changing the _datadir_ location can be done multiple times, as long as it is not restored to the default initial location, (because a symlink is created and can't be switched back, yet).
## Misc
We use the `mysql-apt-config` package from Oracle to configure APT sources. It is used right from the role since there is no apparent stable URL to download it from Oracle servers. We should verify from time to time if a new version is available to update the .deb in the role.
The MySQL debian package made by Oracle doesn't work with systemd out of the box. We've used the `mysql-systemd-start` script and the systemd unit made by Debian for the "mysql-5.7" package (currently only available for Sid).
On Stretch, "mytop" is not packages anymore (only provided by "mariadb-client-10.1"). We're using the version from the "mytop" package (currently only available for Sid).