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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>. 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).</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>. By |
| default, three options are relevant:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">class</span></code> specifies the compression class - Cassandra provides three classes (<code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code>, |
| <code class="docutils literal notranslate"><span class="pre">SnappyCompressor</span></code>, and <code class="docutils literal notranslate"><span class="pre">DeflateCompressor</span></code> ). The default is <code class="docutils literal notranslate"><span class="pre">LZ4Compressor</span></code>.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">chunk_length_in_kb</span></code> specifies the number of kilobytes of data per compression chunk. The default is 64KB.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">crc_check_chance</span></code> determines how likely Cassandra is to verify the checksum on each compression chunk during |
| reads. The default is 1.0.</p></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': 'SnappyCompressor', 'chunk_length_in_kb': 128, '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.</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><p>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.</p></li> |
| <li><p>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.</p></li> |
| <li><p>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.</p></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> |
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