Question: Where is the my.cnf file?
Answer: In Clustrix there is no my.cnf file as all configuration is done through the sql interface.
You can find more information on configuring global variables on the Clustrix documentation Global Variables Page.
No external configuration file is needed as Global Variables are persisted in the database (just like the rest of the data in the cluster).
Changes to global variables are runtime mutable. All nodes are immediately aware of global variable changes. Global variables that are also session variables will only affect new sessions, example: session_log_slow_threshold_ms as described in: Query Logging.
For information regarding Replication configuration, please see the Replication section of the guide.