A log-structured merge-tree (LSM tree) is a data structure and storage architecture used by RocksDB.

SSTable (Sorted String Table)

An SSTable is essentially a file format. It’s a simple, immutable file on disk that stores key-value pairs, sorted strictly by their keys.

Sorted Strings Table (SSTable) is a persistent file format used by ScyllaDB, Apache Cassandra, and other NoSQL databases to take the in-memory data stored in memtables, order it for fast access, and store it on disk in a persistent, ordered, immutable set of files.

• When data is committed, ScyllaDB or Cassandra stores the changes in a commitlog, which is a file that only allows appending, so writes are quick.
• Simultaneously the data is written to an in-memory cache of key/column data called a memtable.
• Periodically the memtable is flushed to persistent storage in the form of SSTables on disk.
• SSTables in Cassandra or ScyllaDB serve as the building blocks of the total data stored in the database.
• SSTables are immutable, so updates to data create a new SSTable file instead of changing the existing ones.