With the argument –log-level it’s possible to start with a certain log level. The default log level is info. If you want the log level to change while being online, this command can be used. It will not be saved when the server is restarted.
Valid log levels are “debug”, “info”, “warning”, “error” and “critical”
# Change the log-level to "debug" alter server f851c6a4-820e-11e5-9661-080027f37001 set log_level debug