mariadb-service-convert during migration can create a file containing
ExecStartPre=/usr/sbin/sysctl -q -w vm.drop_caches=3 if the users my.cnf
contains [mysqld_safe] flush_caches. This sysctl entry change requires root
access. No existing ExecStartPre requires execution requires execution
as another user.

There is a comment in the mariadb{,@}.service.in that indicates
mysqld_install which would require -u mysql to explicity change user to
mysql from root since PermissionsStartOnly=true.

Otherwise the following error would be generated:
Oct 14 07:38:38 spaceman systemd[1]: Starting MariaDB database server...
-- Subject: Unit mariadb.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--
-- Unit mariadb.service has begun starting up.
Oct 14 07:38:38 spaceman sysctl[10089]: sysctl: permission denied on key 'vm.drop_caches'
Oct 14 07:38:38 spaceman systemd[1]: mariadb.service: control process exited, code=exited status=255
Oct 14 07:38:38 spaceman systemd[1]: Failed to start MariaDB database server.