Final answer:
In Cassandra write operations, the biggest performance gain is usually achieved by storing the commit log on a separate disk from the data files, thus allowing for parallel writes and reducing I/O contention.
Step-by-step explanation:
The biggest performance gain in Cassandra write operations is often realized when the commit log is stored on a separate disk from the data files. This configuration allows for parallel writes to occur: one drive can focus on writing the commit log, while another drive handles writing the SSTables (data files). This reduces the I/O contention between the commit log and the data file writes, which can be a significant bottleneck when both are contending for the same disk's I/O. By separating these onto different drives, Cassandra can leverage the full potential of concurrent disk I/O, leading to a substantial improvement in write throughput and latency.