What are Memtables in Cassandra?

Memtables in Cassandra are a type of in-memory data structure that temporarily holds data that has been recently written to the database before it is flushed to disk as an SSTable (Sorted String Table). They play a crucial role in the write path of Cassandra, allowing for fast data access and ensuring that writes are quickly acknowledged without the need for immediate disk I/O operations.

By keeping this data in memory, Cassandra can efficiently batch the writes, which enhances performance. Once the Memtable reaches a certain threshold, it is flushed to disk, creating a new SSTable. This mechanism ensures that writes are handled quickly and helps maintain the overall performance of the database, especially in scenarios with high write throughput.

This understanding highlights why the other options do not accurately describe Memtables. They are not immutable disk files or backup files, nor are they simply a temporary cache for query results, as their primary function is to facilitate fast handling of recent writes before they are durably stored on disk.