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Compression
-----------

Cassandra offers operators the ability to configure compression on a per-table basis. Compression reduces the size of
data on disk by compressing the SSTable in user-configurable compression ``chunk_length_in_kb``. Because Cassandra
SSTables are immutable, the CPU cost of compressing is only necessary when the SSTable is written - subsequent updates
to data will land in different SSTables, so Cassandra will not need to decompress, overwrite, and recompress data when
UPDATE commands are issued. On reads, Cassandra will locate the relevant compressed chunks on disk, decompress the full
chunk, and then proceed with the remainder of the read path (merging data from disks and memtables, read repair, and so
on).

Configuring Compression
^^^^^^^^^^^^^^^^^^^^^^^

Compression is configured on a per-table basis as an optional argument to ``CREATE TABLE`` or ``ALTER TABLE``. By
default, three options are relevant:

- ``class`` specifies the compression class - Cassandra provides four classes (``LZ4Compressor``,
  ``SnappyCompressor``, ``DeflateCompressor`` and ``ZstdCompressor``). The default is ``LZ4Compressor``.
- ``chunk_length_in_kb`` specifies the number of kilobytes of data per compression chunk. The default is 64KB.
- ``crc_check_chance`` determines how likely Cassandra is to verify the checksum on each compression chunk during
  reads. The default is 1.0.
- ``compression_level`` is only applicable for ``ZstdCompressor`` and accepts values between ``-131072`` and ``22``.
    The lower the level, the faster the speed (at the cost of compression). Values from 20 to 22 are called
    "ultra levels" and should be used with caution, as they require more memory. The default is 3.

Users can set compression using the following syntax:

::

    CREATE TABLE keyspace.table (id int PRIMARY KEY) WITH compression = {'class': 'LZ4Compressor'};

Or

::

    ALTER TABLE keyspace.table WITH compression = {'class': 'SnappyCompressor', 'chunk_length_in_kb': 128, 'crc_check_chance': 0.5};

Once enabled, compression can be disabled with ``ALTER TABLE`` setting ``enabled`` to ``false``:

::

    ALTER TABLE keyspace.table WITH compression = {'enabled':'false'};

Operators should be aware, however, that changing compression is not immediate. The data is compressed when the SSTable
is written, and as SSTables are immutable, the compression will not be modified until the table is compacted. Upon
issuing a change to the compression options via ``ALTER TABLE``, the existing SSTables will not be modified until they
are compacted - if an operator needs compression changes to take effect immediately, the operator can trigger an SSTable
rewrite using ``nodetool scrub`` or ``nodetool upgradesstables -a``, both of which will rebuild the SSTables on disk,
re-compressing the data in the process.

Benefits and Uses
^^^^^^^^^^^^^^^^^

Compression's primary benefit is that it reduces the amount of data written to disk. Not only does the reduced size save
in storage requirements, it often increases read and write throughput, as the CPU overhead of compressing data is faster
than the time it would take to read or write the larger volume of uncompressed data from disk.

Compression is most useful in tables comprised of many rows, where the rows are similar in nature. Tables containing
similar text columns (such as repeated JSON blobs) often compress very well.

Operational Impact
^^^^^^^^^^^^^^^^^^

- Compression metadata is stored off-heap and scales with data on disk.  This often requires 1-3GB of off-heap RAM per
  terabyte of data on disk, though the exact usage varies with ``chunk_length_in_kb`` and compression ratios.

- Streaming operations involve compressing and decompressing data on compressed tables - in some code paths (such as
  non-vnode bootstrap), the CPU overhead of compression can be a limiting factor.

- The compression path checksums data to ensure correctness - while the traditional Cassandra read path does not have a
  way to ensure correctness of data on disk, compressed tables allow the user to set ``crc_check_chance`` (a float from
  0.0 to 1.0) to allow Cassandra to probabilistically validate chunks on read to verify bits on disk are not corrupt.

Advanced Use
^^^^^^^^^^^^

Advanced users can provide their own compression class by implementing the interface at
``org.apache.cassandra.io.compress.ICompressor``.