It sounds like the latency jumped to 150ms because the newly written file was not in the OS cache.
By default, Cassandra will use a 1GB heap, as set in bin/cassandra.in.sh
. You can adjust the jvm heap there via the -Xmx option, but generally you want to balance the jvm vs the OS cache. With 6GB, I would probably give 2GB to the jvm, but if you aren't having issues now increasing the jvm's memory probably won't provide any performance gains, but it's worth noting that with row cache in 0.6 this may change.