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| <div class="section" id="compression"> |
| <h1>Compression<a class="headerlink" href="#compression" title="Permalink to this headline">¶</a></h1> |
| <p>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 <code class="docutils literal notranslate"><span class="pre">chunk_length_in_kb</span></code>. As 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).</p> |
| <p>Compression algorithms typically trade off between the following three areas:</p> |
| <ul class="simple"> |
| <li><strong>Compression speed</strong>: How fast does the compression algorithm compress data. This is critical in the flush and |
| compaction paths because data must be compressed before it is written to disk.</li> |
| <li><strong>Decompression speed</strong>: How fast does the compression algorithm de-compress data. This is critical in the read |
| and compaction paths as data must be read off disk in a full chunk and decompressed before it can be returned.</li> |
| <li><strong>Ratio</strong>: By what ratio is the uncompressed data reduced by. Cassandra typically measures this as the size of data |
| on disk relative to the uncompressed size. For example a ratio of <code class="docutils literal notranslate"><span class="pre">0.5</span></code> means that the data on disk is 50% the size |
| of the uncompressed data. Cassandra exposes this ratio per table as the <code class="docutils literal notranslate"><span class="pre">SSTable</span> <span class="pre">Compression</span> <span class="pre">Ratio</span></code> field of |
| <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">tablestats</span></code>.</li> |
| </ul> |
| <p>Cassandra offers five compression algorithms by default that make different tradeoffs in these areas. While |
| benchmarking compression algorithms depends on many factors (algorithm parameters such as compression level, |
| the compressibility of the input data, underlying processor class, etc …), the following table should help you pick |
| a starting point based on your application’s requirements with an extremely rough grading of the different choices |
| by their performance in these areas (A is relatively good, F is relatively bad):</p> |
| <table border="1" class="docutils"> |
| <colgroup> |
| <col width="39%" /> |
| <col width="20%" /> |
| <col width="11%" /> |
| <col width="13%" /> |
| <col width="6%" /> |
| <col width="11%" /> |
| </colgroup> |
| <thead valign="bottom"> |
| <tr class="row-odd"><th class="head">Compression Algorithm</th> |
| <th class="head">Cassandra Class</th> |
| <th class="head">Compression</th> |
| <th class="head">Decompression</th> |
| <th class="head">Ratio</th> |
| <th class="head">C* Version</th> |
| </tr> |
| </thead> |
| <tbody valign="top"> |
| <tr class="row-even"><td><a class="reference external" href="https://lz4.github.io/lz4/">LZ4</a></td> |
| <td><code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code></td> |
| <td>A+</td> |
| <td>A+</td> |
| <td>C+</td> |
| <td><code class="docutils literal notranslate"><span class="pre">>=1.2.2</span></code></td> |
| </tr> |
| <tr class="row-odd"><td><a class="reference external" href="https://lz4.github.io/lz4/">LZ4HC</a></td> |
| <td><code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code></td> |
| <td>C+</td> |
| <td>A+</td> |
| <td>B+</td> |
| <td><code class="docutils literal notranslate"><span class="pre">>=</span> <span class="pre">3.6</span></code></td> |
| </tr> |
| <tr class="row-even"><td><a class="reference external" href="https://facebook.github.io/zstd/">Zstd</a></td> |
| <td><code class="docutils literal notranslate"><span class="pre">ZstdCompressor</span></code></td> |
| <td>A-</td> |
| <td>A-</td> |
| <td>A+</td> |
| <td><code class="docutils literal notranslate"><span class="pre">>=</span> <span class="pre">4.0</span></code></td> |
| </tr> |
| <tr class="row-odd"><td><a class="reference external" href="http://google.github.io/snappy/">Snappy</a></td> |
| <td><code class="docutils literal notranslate"><span class="pre">SnappyCompressor</span></code></td> |
| <td>A-</td> |
| <td>A</td> |
| <td>C</td> |
| <td><code class="docutils literal notranslate"><span class="pre">>=</span> <span class="pre">1.0</span></code></td> |
| </tr> |
| <tr class="row-even"><td><a class="reference external" href="https://zlib.net">Deflate (zlib)</a></td> |
| <td><code class="docutils literal notranslate"><span class="pre">DeflateCompressor</span></code></td> |
| <td>C</td> |
| <td>C</td> |
| <td>A</td> |
| <td><code class="docutils literal notranslate"><span class="pre">>=</span> <span class="pre">1.0</span></code></td> |
| </tr> |
| </tbody> |
| </table> |
| <p>Generally speaking for a performance critical (latency or throughput) application <code class="docutils literal notranslate"><span class="pre">LZ4</span></code> is the right choice as it |
| gets excellent ratio per CPU cycle spent. This is why it is the default choice in Cassandra.</p> |
| <p>For storage critical applications (disk footprint), however, <code class="docutils literal notranslate"><span class="pre">Zstd</span></code> may be a better choice as it can get significant |
| additional ratio to <code class="docutils literal notranslate"><span class="pre">LZ4</span></code>.</p> |
| <p><code class="docutils literal notranslate"><span class="pre">Snappy</span></code> is kept for backwards compatibility and <code class="docutils literal notranslate"><span class="pre">LZ4</span></code> will typically be preferable.</p> |
| <p><code class="docutils literal notranslate"><span class="pre">Deflate</span></code> is kept for backwards compatibility and <code class="docutils literal notranslate"><span class="pre">Zstd</span></code> will typically be preferable.</p> |
| <div class="section" id="configuring-compression"> |
| <h2>Configuring Compression<a class="headerlink" href="#configuring-compression" title="Permalink to this headline">¶</a></h2> |
| <p>Compression is configured on a per-table basis as an optional argument to <code class="docutils literal notranslate"><span class="pre">CREATE</span> <span class="pre">TABLE</span></code> or <code class="docutils literal notranslate"><span class="pre">ALTER</span> <span class="pre">TABLE</span></code>. Three |
| options are available for all compressors:</p> |
| <ul class="simple"> |
| <li><code class="docutils literal notranslate"><span class="pre">class</span></code> (default: <code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code>): specifies the compression class to use. The two “fast” |
| compressors are <code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code> and <code class="docutils literal notranslate"><span class="pre">SnappyCompressor</span></code> and the two “good” ratio compressors are <code class="docutils literal notranslate"><span class="pre">ZstdCompressor</span></code> |
| and <code class="docutils literal notranslate"><span class="pre">DeflateCompressor</span></code>.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">chunk_length_in_kb</span></code> (default: <code class="docutils literal notranslate"><span class="pre">16KiB</span></code>): specifies the number of kilobytes of data per compression chunk. The main |
| tradeoff here is that larger chunk sizes give compression algorithms more context and improve their ratio, but |
| require reads to deserialize and read more off disk.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">crc_check_chance</span></code> (default: <code class="docutils literal notranslate"><span class="pre">1.0</span></code>): determines how likely Cassandra is to verify the checksum on each compression |
| chunk during reads to protect against data corruption. Unless you have profiles indicating this is a performance |
| problem it is highly encouraged not to turn this off as it is Cassandra’s only protection against bitrot.</li> |
| </ul> |
| <p>The <code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code> supports the following additional options:</p> |
| <ul class="simple"> |
| <li><code class="docutils literal notranslate"><span class="pre">lz4_compressor_type</span></code> (default <code class="docutils literal notranslate"><span class="pre">fast</span></code>): specifies if we should use the <code class="docutils literal notranslate"><span class="pre">high</span></code> (a.k.a <code class="docutils literal notranslate"><span class="pre">LZ4HC</span></code>) ratio version |
| or the <code class="docutils literal notranslate"><span class="pre">fast</span></code> (a.k.a <code class="docutils literal notranslate"><span class="pre">LZ4</span></code>) version of <code class="docutils literal notranslate"><span class="pre">LZ4</span></code>. The <code class="docutils literal notranslate"><span class="pre">high</span></code> mode supports a configurable level, which can allow |
| operators to tune the performance <-> ratio tradeoff via the <code class="docutils literal notranslate"><span class="pre">lz4_high_compressor_level</span></code> option. Note that in |
| <code class="docutils literal notranslate"><span class="pre">4.0</span></code> and above it may be preferable to use the <code class="docutils literal notranslate"><span class="pre">Zstd</span></code> compressor.</li> |
| <li><code class="docutils literal notranslate"><span class="pre">lz4_high_compressor_level</span></code> (default <code class="docutils literal notranslate"><span class="pre">9</span></code>): A number between <code class="docutils literal notranslate"><span class="pre">1</span></code> and <code class="docutils literal notranslate"><span class="pre">17</span></code> inclusive that represents how much |
| CPU time to spend trying to get more compression ratio. Generally lower levels are “faster” but they get less ratio |
| and higher levels are slower but get more compression ratio.</li> |
| </ul> |
| <p>The <code class="docutils literal notranslate"><span class="pre">ZstdCompressor</span></code> supports the following options in addition:</p> |
| <ul class="simple"> |
| <li><code class="docutils literal notranslate"><span class="pre">compression_level</span></code> (default <code class="docutils literal notranslate"><span class="pre">3</span></code>): A number between <code class="docutils literal notranslate"><span class="pre">-131072</span></code> and <code class="docutils literal notranslate"><span class="pre">22</span></code> inclusive that represents how much CPU |
| time to spend trying to get more compression ratio. The lower the level, the faster the speed (at the cost of ratio). |
| Values from 20 to 22 are called “ultra levels” and should be used with caution, as they require more memory. |
| The default of <code class="docutils literal notranslate"><span class="pre">3</span></code> is a good choice for competing with <code class="docutils literal notranslate"><span class="pre">Deflate</span></code> ratios and <code class="docutils literal notranslate"><span class="pre">1</span></code> is a good choice for competing |
| with <code class="docutils literal notranslate"><span class="pre">LZ4</span></code>.</li> |
| </ul> |
| <p>Users can set compression using the following syntax:</p> |
| <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>CREATE TABLE keyspace.table (id int PRIMARY KEY) WITH compression = {'class': 'LZ4Compressor'}; |
| </pre></div> |
| </div> |
| <p>Or</p> |
| <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>ALTER TABLE keyspace.table WITH compression = {'class': 'LZ4Compressor', 'chunk_length_in_kb': 64, 'crc_check_chance': 0.5}; |
| </pre></div> |
| </div> |
| <p>Once enabled, compression can be disabled with <code class="docutils literal notranslate"><span class="pre">ALTER</span> <span class="pre">TABLE</span></code> setting <code class="docutils literal notranslate"><span class="pre">enabled</span></code> to <code class="docutils literal notranslate"><span class="pre">false</span></code>:</p> |
| <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>ALTER TABLE keyspace.table WITH compression = {'enabled':'false'}; |
| </pre></div> |
| </div> |
| <p>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 <code class="docutils literal notranslate"><span class="pre">ALTER</span> <span class="pre">TABLE</span></code>, 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 <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">scrub</span></code> or <code class="docutils literal notranslate"><span class="pre">nodetool</span> <span class="pre">upgradesstables</span> <span class="pre">-a</span></code>, both of which will rebuild the SSTables on disk, |
| re-compressing the data in the process.</p> |
| </div> |
| <div class="section" id="benefits-and-uses"> |
| <h2>Benefits and Uses<a class="headerlink" href="#benefits-and-uses" title="Permalink to this headline">¶</a></h2> |
| <p>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.</p> |
| <p>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. Tables containing data that has already |
| been compressed or random data (e.g. benchmark datasets) do not typically compress well.</p> |
| </div> |
| <div class="section" id="operational-impact"> |
| <h2>Operational Impact<a class="headerlink" href="#operational-impact" title="Permalink to this headline">¶</a></h2> |
| <ul class="simple"> |
| <li>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 <code class="docutils literal notranslate"><span class="pre">chunk_length_in_kb</span></code> and compression ratios.</li> |
| <li>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.</li> |
| <li>To prevent slow compressors (<code class="docutils literal notranslate"><span class="pre">Zstd</span></code>, <code class="docutils literal notranslate"><span class="pre">Deflate</span></code>, <code class="docutils literal notranslate"><span class="pre">LZ4HC</span></code>) from blocking flushes for too long, all three |
| flush with the default fast <code class="docutils literal notranslate"><span class="pre">LZ4</span></code> compressor and then rely on normal compaction to re-compress the data into the |
| desired compression strategy. See <cite>CASSANDRA-15379 <https://issues.apache.org/jira/browse/CASSANDRA-15379></cite> for more |
| details.</li> |
| <li>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 <code class="docutils literal notranslate"><span class="pre">crc_check_chance</span></code> (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.</li> |
| </ul> |
| </div> |
| <div class="section" id="advanced-use"> |
| <h2>Advanced Use<a class="headerlink" href="#advanced-use" title="Permalink to this headline">¶</a></h2> |
| <p>Advanced users can provide their own compression class by implementing the interface at |
| <code class="docutils literal notranslate"><span class="pre">org.apache.cassandra.io.compress.ICompressor</span></code>.</p> |
| </div> |
| </div> |
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