[mysqld]
datadir=/home/mike/MariaDB_datadir

M17A ~ #  service mysql start
Job for mariadb.service failed because the control process exited with error code. See "systemctl status mariadb.service" and "journalctl -xe" for details.
M17A ~ #  journalctl -xe
-- Unit mariadb.service has begun starting up.
May 17 19:45:28 M17A mysqld[3958]: 2018-05-17 19:45:28 139675368278208 [Note] /usr/sbin/mysqld (mysqld 10.2.14-MariaDB-10.2.14+maria~xenial) starting as process 3958 ...
May 17 19:45:28 M17A mysqld[3958]: 2018-05-17 19:45:28 139675368278208 [Warning] Can't create test file /home/mike/MariaDB_datadir/M17A.lower-test
May 17 19:45:28 M17A mysqld[3958]: [103B blob data]
May 17 19:45:28 M17A mysqld[3958]: 2018-05-17 19:45:28 139675368278208 [ERROR] Aborting
May 17 19:45:28 M17A systemd[1]: mariadb.service: Main process exited, code=exited, status=1/FAILURE
May 17 19:45:28 M17A systemd[1]: Failed to start MariaDB 10.2.14 database server.
-- Subject: Unit mariadb.service has failed