What is an SSTable in Cassandra?

An SSTable, or Sorted String Table, in Cassandra is fundamentally designed as an immutable disk file that holds data. This structure allows for efficient storage and retrieval of data. When data is written to Cassandra, it is first stored in memory (in a structure called memtable), and once this in-memory data reaches a certain threshold, it is flushed to disk as an SSTable.

The immutability of SSTables ensures that once data is written, it cannot be altered, which greatly simplifies the management of data and supports Cassandra's eventual consistency model. Immutability allows Cassandra to perform efficient reads, avoids lock contention during writes, and makes it easier to manage data at scale.

SSTables are organized in a way that supports fast read operations, as they are sorted on disk, meaning that data retrieval can be optimized for disk access patterns. They also enable efficient data compaction processes which help in managing storage space and improving read performance over time. This structure is crucial to Cassandra's design as a distributed, scalable database that can handle large amounts of data across many servers while providing fast read and write capabilities.