Sounds like it would make tuning your RF for durability a pain in the ass if you run multiple cassandra nodes on the same box. Cassandra uses non-JVM managed me

Sounds like it would make tuning your RF for durability a pain in the ass if you run multiple cassandra nodes on the same box. Cassandra uses non-JVM managed memory for a fair amount of work and relies heavily on the OS filesystem cache (much like Pg), unless you are consolidating multiple Cassandra clusters to one box I would keep one node per physical machine for durability reasons alone.