src/doc/3.11.10/_sources/architecture/storage_engine.rst.txt

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Storage Engine
--------------

.. _commit-log:

CommitLog
^^^^^^^^^

Commitlogs are an append only log of all mutations local to a Cassandra node. Any data written to Cassandra will first be written to a commit log before being written to a memtable. This provides durability in the case of unexpected shutdown. On startup, any mutations in the commit log will be applied to memtables.

All mutations write optimized by storing in commitlog segments, reducing the number of seeks needed to write to disk. Commitlog Segments are limited by the "commitlog_segment_size_in_mb" option, once the size is reached, a new commitlog segment is created. Commitlog segments can be archived, deleted, or recycled once all its data has been flushed to SSTables.  Commitlog segments are truncated when Cassandra has written data older than a certain point to the SSTables. Running "nodetool drain" before stopping Cassandra will write everything in the memtables to SSTables and remove the need to sync with the commitlogs on startup.

- ``commitlog_segment_size_in_mb``: The default size is 32, which is almost always fine, but if you are archiving commitlog segments (see commitlog_archiving.properties), then you probably want a finer granularity of archiving; 8 or 16 MB is reasonable. Max mutation size is also configurable via max_mutation_size_in_kb setting in cassandra.yaml. The default is half the size commitlog_segment_size_in_mb * 1024.

***NOTE: If max_mutation_size_in_kb is set explicitly then commitlog_segment_size_in_mb must be set to at least twice the size of max_mutation_size_in_kb / 1024***

*Default Value:* 32

Commitlogs are an append only log of all mutations local to a Cassandra node. Any data written to Cassandra will first be written to a commit log before being written to a memtable. This provides durability in the case of unexpected shutdown. On startup, any mutations in the commit log will be applied.

- ``commitlog_sync``: may be either “periodic” or “batch.”

  - ``batch``: In batch mode, Cassandra won’t ack writes until the commit log has been fsynced to disk. It will wait "commitlog_sync_batch_window_in_ms" milliseconds between fsyncs. This window should be kept short because the writer threads will be unable to do extra work while waiting. You may need to increase concurrent_writes for the same reason.

    - ``commitlog_sync_batch_window_in_ms``: Time to wait between "batch" fsyncs
    *Default Value:* 2

  - ``periodic``: In periodic mode, writes are immediately ack'ed, and the CommitLog is simply synced every "commitlog_sync_period_in_ms" milliseconds.

    - ``commitlog_sync_period_in_ms``: Time to wait between "periodic" fsyncs
    *Default Value:* 10000

*Default Value:* periodic

*** NOTE: In the event of an unexpected shutdown, Cassandra can lose up to the sync period or more if the sync is delayed. If using "batch" mode, it is recommended to store commitlogs in a separate, dedicated device.**


- ``commitlog_directory``: This option is commented out by default When running on magnetic HDD, this should be a separate spindle than the data directories. If not set, the default directory is $CASSANDRA_HOME/data/commitlog.

*Default Value:* /var/lib/cassandra/commitlog

- ``commitlog_compression``: Compression to apply to the commitlog. If omitted, the commit log will be written uncompressed. LZ4, Snappy, Deflate and Zstd compressors are supported.

(Default Value: (complex option)::

    #   - class_name: LZ4Compressor
    #     parameters:
    #         -

- ``commitlog_total_space_in_mb``: Total space to use for commit logs on disk.

If space gets above this value, Cassandra will flush every dirty CF in the oldest segment and remove it. So a small total commitlog space will tend to cause more flush activity on less-active columnfamilies.

The default value is the smaller of 8192, and 1/4 of the total space of the commitlog volume.

*Default Value:* 8192

.. _memtables:

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 :ref:`commit-log` 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 :ref:`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.