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| <div class="section" id="storage-engine"> |
| <h1>Storage Engine<a class="headerlink" href="#storage-engine" title="Permalink to this headline">¶</a></h1> |
| <div class="section" id="commitlog"> |
| <span id="commit-log"></span><h2>CommitLog<a class="headerlink" href="#commitlog" title="Permalink to this headline">¶</a></h2> |
| <div class="admonition-todo admonition" id="index-0"> |
| <p class="first admonition-title">Todo</p> |
| <p class="last">todo</p> |
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| </div> |
| <div class="section" id="memtables"> |
| <span id="id1"></span><h2>Memtables<a class="headerlink" href="#memtables" title="Permalink to this headline">¶</a></h2> |
| <p>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 <a class="reference internal" href="#sstables">SSTables</a>. This can be triggered in several |
| ways:</p> |
| <ul class="simple"> |
| <li>The memory usage of the memtables exceeds the configured threshold (see <code class="docutils literal notranslate"><span class="pre">memtable_cleanup_threshold</span></code>)</li> |
| <li>The <a class="reference internal" href="#commit-log"><span class="std std-ref">CommitLog</span></a> approaches its maximum size, and forces memtable flushes in order to allow commitlog segments to |
| be freed</li> |
| </ul> |
| <p>Memtables may be stored entirely on-heap or partially off-heap, depending on <code class="docutils literal notranslate"><span class="pre">memtable_allocation_type</span></code>.</p> |
| </div> |
| <div class="section" id="sstables"> |
| <h2>SSTables<a class="headerlink" href="#sstables" title="Permalink to this headline">¶</a></h2> |
| <p>SSTables are the immutable data files that Cassandra uses for persisting data on disk.</p> |
| <p>As SSTables are flushed to disk from <a class="reference internal" href="#memtables"><span class="std std-ref">Memtables</span></a> 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.</p> |
| <p>Each SSTable is comprised of multiple components stored in separate files:</p> |
| <dl class="docutils"> |
| <dt><code class="docutils literal notranslate"><span class="pre">Data.db</span></code></dt> |
| <dd>The actual data, i.e. the contents of rows.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">Index.db</span></code></dt> |
| <dd>An index from partition keys to positions in the <code class="docutils literal notranslate"><span class="pre">Data.db</span></code> file. For wide partitions, this may also include an |
| index to rows within a partition.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">Summary.db</span></code></dt> |
| <dd>A sampling of (by default) every 128th entry in the <code class="docutils literal notranslate"><span class="pre">Index.db</span></code> file.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">Filter.db</span></code></dt> |
| <dd>A Bloom Filter of the partition keys in the SSTable.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">CompressionInfo.db</span></code></dt> |
| <dd>Metadata about the offsets and lengths of compression chunks in the <code class="docutils literal notranslate"><span class="pre">Data.db</span></code> file.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">Statistics.db</span></code></dt> |
| <dd>Stores metadata about the SSTable, including information about timestamps, tombstones, clustering keys, compaction, |
| repair, compression, TTLs, and more.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">Digest.crc32</span></code></dt> |
| <dd>A CRC-32 digest of the <code class="docutils literal notranslate"><span class="pre">Data.db</span></code> file.</dd> |
| <dt><code class="docutils literal notranslate"><span class="pre">TOC.txt</span></code></dt> |
| <dd>A plain text list of the component files for the SSTable.</dd> |
| </dl> |
| <p>Within the <code class="docutils literal notranslate"><span class="pre">Data.db</span></code> 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, <code class="docutils literal notranslate"><span class="pre">Murmur3Partition</span></code>, is used). Within a partition, rows are |
| stored in the order of their clustering keys.</p> |
| <p>SSTables can be optionally compressed using block-based compression.</p> |
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