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| <div class="section" id="bloom-filters"> |
| <h1>Bloom Filters<a class="headerlink" href="#bloom-filters" title="Permalink to this headline">¶</a></h1> |
| <p>In the read path, Cassandra merges data on disk (in SSTables) with data in RAM (in memtables). To avoid checking every |
| SSTable data file for the partition being requested, Cassandra employs a data structure known as a bloom filter.</p> |
| <p>Bloom filters are a probabilistic data structure that allows Cassandra to determine one of two possible states: - The |
| data definitely does not exist in the given file, or - The data probably exists in the given file.</p> |
| <p>While bloom filters can not guarantee that the data exists in a given SSTable, bloom filters can be made more accurate |
| by allowing them to consume more RAM. Operators have the opportunity to tune this behavior per table by adjusting the |
| the <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> to a float between 0 and 1.</p> |
| <p>The default value for <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> is 0.1 for tables using LeveledCompactionStrategy and 0.01 for all |
| other cases.</p> |
| <p>Bloom filters are stored in RAM, but are stored offheap, so operators should not consider bloom filters when selecting |
| the maximum heap size. As accuracy improves (as the <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> gets closer to 0), memory usage |
| increases non-linearly - the bloom filter for <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span> <span class="pre">=</span> <span class="pre">0.01</span></code> will require about three times as much |
| memory as the same table with <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span> <span class="pre">=</span> <span class="pre">0.1</span></code>.</p> |
| <p>Typical values for <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> are usually between 0.01 (1%) to 0.1 (10%) false-positive chance, where |
| Cassandra may scan an SSTable for a row, only to find that it does not exist on the disk. The parameter should be tuned |
| by use case:</p> |
| <ul class="simple"> |
| <li><p>Users with more RAM and slower disks may benefit from setting the <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> to a numerically lower |
| number (such as 0.01) to avoid excess IO operations</p></li> |
| <li><p>Users with less RAM, more dense nodes, or very fast disks may tolerate a higher <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> in order to |
| save RAM at the expense of excess IO operations</p></li> |
| <li><p>In workloads that rarely read, or that only perform reads by scanning the entire data set (such as analytics |
| workloads), setting the <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> to a much higher number is acceptable.</p></li> |
| </ul> |
| <div class="section" id="changing"> |
| <h2>Changing<a class="headerlink" href="#changing" title="Permalink to this headline">¶</a></h2> |
| <p>The bloom filter false positive chance is visible in the <code class="docutils literal notranslate"><span class="pre">DESCRIBE</span> <span class="pre">TABLE</span></code> output as the field |
| <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code>. Operators can change the value with an <code class="docutils literal notranslate"><span class="pre">ALTER</span> <span class="pre">TABLE</span></code> statement:</p> |
| <div class="highlight-none notranslate"><div class="highlight"><pre><span></span>ALTER TABLE keyspace.table WITH bloom_filter_fp_chance=0.01 |
| </pre></div> |
| </div> |
| <p>Operators should be aware, however, that this change is not immediate: the bloom filter is calculated when the file is |
| written, and persisted on disk as the Filter component of the SSTable. Upon issuing an <code class="docutils literal notranslate"><span class="pre">ALTER</span> <span class="pre">TABLE</span></code> statement, new |
| files on disk will be written with the new <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code>, but existing sstables will not be modified until |
| they are compacted - if an operator needs a change to <code class="docutils literal notranslate"><span class="pre">bloom_filter_fp_chance</span></code> to take effect, they 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, regenerating the bloom filters in the progress.</p> |
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