We recently ran into a situation where both of our Cassandra Servers were unavailable to OpenNMS for a couple of minutes. Even though those Servers were able to respond a short while after, OpenNMS didn't try to reconnect to them and didn't send any collected performance data to Casssandra. The logs revealed an exception which occured because of the connection loss. Only after restarting OpenNMS, the connection was re-established.
Please change the behaviour of OpenNMS in case of connection loss to all Cassandra Servers, so it tries to reconnect "some time" after the incident.