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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>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</li>
<li>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</li>
<li>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.</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>
</div>
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