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| <article> |
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| <h1 class="entry-title">New Range Partitioning Features in Kudu 0.10</h1> |
| <p class="meta">Posted 23 Aug 2016 by Dan Burkert</p> |
| </header> |
| <div class="entry-content"> |
| <p>Kudu 0.10 is shipping with a few important new features for range partitioning. |
| These features are designed to make Kudu easier to scale for certain workloads, |
| like time series. This post will introduce these features, and discuss how to use |
| them to effectively design tables for scalability and performance.</p> |
| |
| <!--more--> |
| |
| <p>Since Kudu’s initial release, tables have had the constraint that once created, |
| the set of partitions is static. This forces users to plan ahead and create |
| enough partitions for the expected size of the table, because once the table is |
| created no further partitions can be added. When using hash partitioning, |
| creating more partitions is as straightforward as specifying more buckets. For |
| range partitioning, however, knowing where to put the extra partitions ahead of |
| time can be difficult or impossible.</p> |
| |
| <p>The common solution to this problem in other distributed databases is to allow |
| range partitions to split into smaller child range partitions. Unfortunately, |
| range splitting typically has a large performance impact on running tables, |
| since child partitions need to eventually be recompacted and rebalanced to a |
| remote server. Range splitting is particularly thorny with Kudu, because rows |
| are stored in tablets in primary key sorted order, which does not necessarily |
| match the range partitioning order. If the range partition key is different than |
| the primary key, then splitting requires inspecting and shuffling each |
| individual row, instead of splitting the tablet in half.</p> |
| |
| <h2 id="adding-and-dropping-range-partitions">Adding and Dropping Range Partitions</h2> |
| |
| <p>As an alternative to range partition splitting, Kudu now allows range partitions |
| to be added and dropped on the fly, without locking the table or otherwise |
| affecting concurrent operations on other partitions. This solution is not |
| strictly as powerful as full range partition splitting, but it strikes a good |
| balance between flexibility, performance, and operational overhead. |
| Additionally, this feature does not preclude range splitting in the future if |
| there is a push to implement it. To support adding and dropping range |
| partitions, Kudu had to remove an even more fundamental restriction when using |
| range partitions.</p> |
| |
| <p>Previously, range partitions could only be created by specifying split points. |
| Split points divide an implicit partition covering the entire range into |
| contiguous and disjoint partitions. When using split points, the first and last |
| partitions are always unbounded below and above, respectively. A consequence of |
| the final partition being unbounded is that datasets which are range-partitioned |
| on a column that increases in value over time will eventually have far more rows |
| in the last partition than in any other. Unbalanced partitions are commonly |
| referred to as hotspots, and until Kudu 0.10 they have been difficult to avoid |
| when storing time series data in Kudu.</p> |
| |
| <p><img src="/img/2016-08-23-new-range-partitioning-features/range-partitioning-on-time.png" alt="png" class="img-responsive" /></p> |
| |
| <p>The figure above shows the tablets created by two different attempts to |
| partition a table by range on a timestamp column. The first, above in blue, uses |
| split points. The second, below in green, uses bounded range partitions |
| specified during table creation. With bounded range partitions, there is no |
| longer a guarantee that every possible row has a corresponding range partition. |
| As a result, Kudu will now reject writes which fall in a ‘non-covered’ range.</p> |
| |
| <p>Now that tables are no longer required to have range partitions covering all |
| possible rows, Kudu can support adding range partitions to cover the otherwise |
| unoccupied space. Dropping a range partition will result in unoccupied space |
| where the range partition was previously. In the example above, we may want to |
| add a range partition covering 2017 at the end of the year, so that we can |
| continue collecting data in the future. By lazily adding range partitions we |
| avoid hotspotting, avoid the need to specify range partitions up front for time |
| periods far in the future, and avoid the downsides of splitting. Additionally, |
| historical data which is no longer useful can be efficiently deleted by dropping |
| the entire range partition.</p> |
| |
| <h2 id="what-about-hash-partitioning">What About Hash Partitioning?</h2> |
| |
| <p>Since Kudu’s hash partitioning feature originally shipped in version 0.6, it has |
| been possible to create tables which combine hash partitioning with range |
| partitioning. The new range partitioning features continue to work seamlessly |
| when combined with hash partitioning. Just as before, the number of tablets |
| which comprise a table will be the product of the number of range partitions and |
| the number of hash partition buckets. Adding or dropping a range partition will |
| result in the creation or deletion of one tablet per hash bucket.</p> |
| |
| <p><img src="/img/2016-08-23-new-range-partitioning-features/range-and-hash-partitioning.png" alt="png" class="img-responsive" /></p> |
| |
| <p>The diagram above shows a time series table range-partitioned on the timestamp |
| and hash-partitioned with two buckets. The hash partitioning could be on the |
| timestamp column, or it could be on any other column or columns in the primary |
| key. In this example only two years of historical data is needed, so at the end |
| of 2016 a new range partition is added for 2017 and the historical 2014 range |
| partition is dropped. This causes two new tablets to be created for 2017, and |
| the two existing tablets for 2014 to be deleted.</p> |
| |
| <h2 id="getting-started">Getting Started</h2> |
| |
| <p>Beginning with the Kudu 0.10 release, users can add and drop range partitions |
| through the Java and C++ client APIs. Range partitions on existing tables can be |
| dropped and replacements added, but it requires the servers and all clients to |
| be updated to 0.10.</p> |
| |
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