If you get this error

Jul 14 13:00:20 mariadb systemd[1]: Starting MariaDB 10.3.15 database server...
Jul 14 13:00:21 mariadb systemd[1296]: mariadb.service: Failed to set up mount namespacing: Permission denied
Jul 14 13:00:21 mariadb systemd[1296]: mariadb.service: Failed at step NAMESPACE spawning /usr/sbin/mysqld: Permission denied
Jul 14 13:00:21 mariadb systemd[1]: mariadb.service: Main process exited, code=exited, status=226/NAMESPACE
Jul 14 13:00:21 mariadb systemd[1]: mariadb.service: Failed with result 'exit-code'.
Jul 14 13:00:21 mariadb systemd[1]: Failed to start MariaDB 10.3.15 database server.

The workaround is to override the mariadb.service file

cat <<EOF >/etc/systemd/system/mariadb.service.d/override.conf
[Service]
ProtectSystem=off
PrivateDevices=false
ProtectHome=false
EOF

If running a container in Proxmox you can also enable Nesting from web gui container > Options > Edit > Features > Nesting

If starting with systemctl start mariadb times out, try checking if the instance is part of Galera cluster. It might be that other instances need to be started as well.

• https://mariadb.com/docs/server/server-management/starting-and-stopping-mariadb/what-to-do-if-mariadb-doesnt-start