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| <article class="doc"> |
| <h1 class="page">Bloom Filters</h1> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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>bloom_filter_fp_chance</code> to a float |
| between 0 and 1.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The default value for <code>bloom_filter_fp_chance</code> is 0.1 for tables using |
| LeveledCompactionStrategy and 0.01 for all other cases.</p> |
| </div> |
| <div class="paragraph"> |
| <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>bloom_filter_fp_chance</code> gets closer to 0), |
| memory usage increases non-linearly - the bloom filter for |
| <code>bloom_filter_fp_chance = 0.01</code> will require about three times as much |
| memory as the same table with <code>bloom_filter_fp_chance = 0.1</code>.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Typical values for <code>bloom_filter_fp_chance</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> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p>Users with more RAM and slower disks may benefit from setting the |
| <code>bloom_filter_fp_chance</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>bloom_filter_fp_chance</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>bloom_filter_fp_chance</code> to a much higher number is acceptable.</p> |
| </li> |
| </ul> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="changing"><a class="anchor" href="#changing"></a>Changing</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>The bloom filter false positive chance is visible in the |
| <code>DESCRIBE TABLE</code> output as the field <code>bloom_filter_fp_chance</code>. Operators |
| can change the value with an <code>ALTER TABLE</code> statement: :</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlightjs highlight"><code class="language-none hljs" data-lang="none">ALTER TABLE keyspace.table WITH bloom_filter_fp_chance=0.01</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <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>ALTER TABLE</code> statement, new files on disk will be written with the new |
| <code>bloom_filter_fp_chance</code>, but existing sstables will not be modified |
| until they are compacted - if an operator needs a change to |
| <code>bloom_filter_fp_chance</code> to take effect, they can trigger an SSTable |
| rewrite using <code>nodetool scrub</code> or <code>nodetool upgradesstables -a</code>, both of |
| which will rebuild the sstables on disk, regenerating the bloom filters |
| in the progress.</p> |
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