The Flocker processes running on the nodes will write their logs to /var/log/flocker/. The log files are named <processname>-<pid>.log, e.g. flocker-volume-1234.log.
Logs from the Docker containers are written to systemd’s journal with a unit name constructed with a ctr- prefix. For example if you’ve started an application called mymongodb you can view its logs by running the following command on the node where the application was started:
$ journalctl -u ctr-mymongodb