Configuring memtable throughput can improve write performance. Cassandra flushes memtables to disk, creating SSTables when the commit log space threshold has been exceeded. Configure the commit log space threshold per node in the cassandra.yaml. How you tune memtable thresholds depends on your data and write load. Increase memtable throughput under either of these conditions:
Allocating memory for memtables reduces the memory available for caching and other internal Cassandra structures, so tune carefully and in small increments.