Grummel Yes, but w/o setting the timeout to infinity I would have had no success. For whatever reason, the default timeout limit of 90 sec. (and even 600 sec. I tried as well) was too short for my VM. In general I have no performance problems, but in this case... I'm glad it went so smoothly at the end. Only one or two grey hairs more... :-)
BusinessTux Grummel Perhaps your installation needed more time to migrate. I found the following message on the upgrade from 15.3 to 15.4 Message from package mariadb: WARNING: You are upgrading from different stable version of MySQL! Your database will be migrated automatically during next restart of MySQL. Before you do that make sure you have up to date backup of your data. It should be mainly in /var/lib/mysql directory.
crpb BusinessTux It wouldn't have worked as he said because somehow his VM didn't perform well for whatever reason. Here you can see the .service-File which hasn't got any "TimeOutStartSec"-Setting and therefore defaults to the Value of '90' from /etc/systemd/system.conf (DefaultTimeoutStartSec). So if the Service won't signal it's successfull Start within those 90 seconds it will always "fail" and stop.
WalterH BusinessTux simply do a systemctl restart mysql, this will upgrade the database, if it fails, you have an mysql issue.