Configuring the buffered read size can reduce problems with wasted disk I/) and unnecessary garbage collection.
On read workloads, Cassandra 2.1 reads drastically more data than it emits over the network. This causes problems throughput the system by wasting disk I/O and causing unnecessary garbage collection (GC).
Open the cassandra-env.sh file for editing and scroll down
Set a value for the buffered read size. By default, the size is 65536.