MariaDB [(none)]> SET GLOBAL log_slow_time = 1;
Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> show variables like '%slow%';
+---------------------+---------------------------------+
| Variable_name       | Value                           |
+---------------------+---------------------------------+
| log_slow_filter     |                                 |
| log_slow_queries    | ON                              |
| log_slow_rate_limit | 1000                            |
| log_slow_time       | 10.000000                       |
| log_slow_verbosity  | query_plan                      |
| slow_launch_time    | 1                               |
| slow_query_log      | ON                              |
| slow_query_log_file | /var/log/mysql/mariadb-slow.log |
+---------------------+---------------------------------+
8 rows in set (0.00 sec)