MariaDB Understanding "Event" Interval

Question

I've created my first EVENT:

ALTER EVENT flush_logs_event 
ON SCHEDULE EVERY 6 HOUR 
COMMENT 'Rotate /var/log/mysql/master-bin*' 
DO FLUSH LOGS;

I changed it from "12" to "6" because of how fast the log is growing for replication. I want it rotating by time and not file size to make deploying additional slaves easier for me.

I thought "6 Hour" means 6H00 12H00 18H00 and 24H00. At the same time I altered the event I also made a tweak to /etc/mysql/my.cnf:

expire_logs_days=7

And then I did

systemctl restart mariadb.service

The master-bin rotated when MariaDB re-stared. It created master-bin.000006. When I restarted MariaDB it was ~5:21. Does this mean the master-bin will rotate at 11:21, 17:21, etc.?

For full disclosure I am also including:

MariaDB [mysql]> show events \G
*************************** 1. row ***************************
                  Db: mysql
                Name: flush_logs_event
             Definer: root@localhost
           Time zone: SYSTEM
                Type: RECURRING
          Execute at: NULL
      Interval value: 6
      Interval field: HOUR
              Starts: 2021-12-09 05:21:23
                Ends: NULL
              Status: ENABLED
          Originator: 1
character_set_client: utf8mb4
collation_connection: utf8mb4_general_ci
  Database Collation: utf8mb4_general_ci

Thanks.

Answer 1

I thought "6 Hour" means 6H00 12H00 18H00 and 24H00.

No. This means "6 hours from previous start". You have no specidfied any timepoint as a base - and server uses previous event starting time for this purposes.

If you want the event to be fired at listed hours you must specify STARTS schedule option - in this case the specified value will be used as a base. Like

ALTER EVENT flush_logs_event 
ON SCHEDULE EVERY 6 HOUR 
            STARTS '2021-12-09 00:00:00'
COMMENT 'Rotate /var/log/mysql/master-bin*' 
DO FLUSH LOGS;