In addition to keeping an eye on JMX statistics, there are several levels of log files that should be monitored so that they can be analyzed in case of failure. Ideally, you should be using some sort of log aggregation (such as Flume, FluentD, Splunk, or other similar tools) to make it easier to make sense of logs. Also, aggregation ensures that catastrophic node failures don't prevent you from recovering logs from the problematic hosts, which may be the most important bit of diagnostic data available.
Cassandra itself provides two logs, and both are located in the configured logging directory, which is /var/log/cassandra by default. The first, system.log, is a rolling log of Cassandra's logback output. The second, output.log, shows standard out and standard error and is overwritten on startup.
If you are experiencing an issue that warrants lower-level logging than the default INFO level, you can adjust the logging level by editing the logback.xml file (in the conf directory...
