We have been having some problems with a Cassandra cluster, so I wanted to look at the java heap space. Unfortunately, jstat cannot find the pid. And, yes, it is the right PID!

Looking in /tmp/hsperfdata_cassandra/, there’s no file! Reading through the whole line where Cassandra is running, I noticed +PerfDisableSharedMem … that’d do it!

It looks like they intentionally set +PerfDisableSharedMem in the Cassandra startup script. I assume their rational is still reasonable … so wouldn’t remove the parameter for day-to-day operation. But, when there’s a problem … restarting Cassandra without this parameter allows us to check how garbage collection is going.