Cassandra uses a storage structure similar to a Log-Structured Merge Tree, unlike a typical relational database that uses a B-Tree. The storage engine writes sequentially to disk in append mode and stores data contiguously. Operations are parallel across nodes and within an individual machine. Because Cassandra does not use a B-tree, concurrency control is unnecessary. Nothing needs to be updated when writing.
Cassandra accommodates modern solid-state disks (SSDs) extremely well. Inexpensive, consumer SSDs are fine for use with Cassandra because Cassandra minimizes wear and tear on an SSD. The disk I/O performed by Cassandra is minimal.