RAM
Cassandra runs on a JVM, and therefore needs to have a sufficient amount of RAM available. The JVM heap size will be the primary consideration, as most of the memory that Cassandra will use will be on-heap. The off-heap activity will be largely driven by how much is configured for off-heap memtables (see the following section on configuring Cassandra). And of course, the operating system needs to have its share of RAM as well.
Most Cassandra production deployments should have between 32 GB and 64 GB per instance. Cloud instances with as little as 16 GB of RAM can be effective as well. While Cassandra can be made to work on less, great care should be taken if a production cluster is built on instances with a smaller amount of RAM. Compare ...