Storage Engine¶

CommitLog¶

Todo

todo

Memtables¶

Memtables are in-memory structures where Cassandra buffers writes. In general, there is one active memtable per table. Eventually, memtables are flushed onto disk and become immutable SSTables. This can be triggered in several ways:

The memory usage of the memtables exceeds the configured threshold (see memtable_cleanup_threshold)
The CommitLog approaches its maximum size, and forces memtable flushes in order to allow commitlog segments to be freed

Memtables may be stored entirely on-heap or partially off-heap, depending on memtable_allocation_type.

SSTables¶

SSTables are the immutable data files that Cassandra uses for persisting data on disk.

As SSTables are flushed to disk from Memtables or are streamed from other nodes, Cassandra triggers compactions which combine multiple SSTables into one. Once the new SSTable has been written, the old SSTables can be removed.

Each SSTable is comprised of multiple components stored in separate files:

Data.db: The actual data, i.e. the contents of rows.
Index.db: An index from partition keys to positions in the Data.db file. For wide partitions, this may also include an index to rows within a partition.
Summary.db: A sampling of (by default) every 128th entry in the Index.db file.
Filter.db: A Bloom Filter of the partition keys in the SSTable.
CompressionInfo.db: Metadata about the offsets and lengths of compression chunks in the Data.db file.
Statistics.db: Stores metadata about the SSTable, including information about timestamps, tombstones, clustering keys, compaction, repair, compression, TTLs, and more.
Digest.crc32: A CRC-32 digest of the Data.db file.
TOC.txt: A plain text list of the component files for the SSTable.

Within the Data.db file, rows are organized by partition. These partitions are sorted in token order (i.e. by a hash of the partition key when the default partitioner, Murmur3Partition, is used). Within a partition, rows are stored in the order of their clustering keys.

SSTables can be optionally compressed using block-based compression.