In the Migrating databases to MariaDB instances procedure - an operator should know how to get SOURCE_MARIADB_IP from 17.1 control plane.

For example:

In director host, get the internal_api VIP from this file:

[stack@director ~]$ cat /home/stack/templates/overcloud-vip-deployed.yaml

[...]

  VipPortMap:

    external:

      ip_address: 172.0.0.206

      ip_address_uri: 172.0.0.206

      ip_subnet: 172.0.0.206/24

    internal_api:

      ip_address: 172.16.2.220

      ip_address_uri: 172.16.2.220

      ip_subnet: 172.16.2.220/24

    storage:

      ip_address: 172.16.1.129

      ip_address_uri: 172.16.1.129

      ip_subnet: 172.16.1.129/24

    storage_mgmt:

      ip_address: 172.16.3.124

      ip_address_uri: 172.16.3.124

      ip_subnet: 172.16.3.124/24

[...]

You can also connect to the controller0 and list the pacemaker resources:

(undercloud) [stack@director ~]$ ssh tripleo-admin@172.25.250.201

[tripleo-admin@overcloud-controller-0 ~]$ sudo -i

[tripleo-admin@overcloud-controller-0 ~]$ pcs status

[...]

Full List of Resources:

  * ip-172.25.250.49    (ocf:heartbeat:IPaddr2):         Started overcloud-controller-0

  * ip-172.16.2.220     (ocf:heartbeat:IPaddr2):         Started overcloud-controller-1

  * ip-172.16.1.129     (ocf:heartbeat:IPaddr2):         Started overcloud-controller-2

  * ip-172.16.3.124     (ocf:heartbeat:IPaddr2):         Started overcloud-controller-0

[...]

SOURCE_MARIADB_IP is then 172.16.2.220 member of the internal_api network