Configuring compaction

As discussed in the Compaction topic, the compaction process merges keys, combines columns, evicts tombstones, consolidates SSTables, and creates a new index in the merged SSTable.

In the cassandra.yaml file, you configure these global compaction parameters:

• snapshot_before_compaction
• concurrent_compactors
• compaction_throughput_mb_per_sec

The compaction_throughput_mb_per_sec parameter is designed for use with large partitions. Cassandra throttles compaction to this rate across the entire system.

Cassandra provides a start-up option for testing compaction strategies without affecting the production workload.

Cassandra supports the following compaction strategies, which you can configure using CQL:

• LeveledCompactionStrategy (LCS): The leveled compaction strategy creates SSTables of a fixed, relatively small size (160 MB by default) that are grouped into levels. Within each level, SSTables are guaranteed to be non-overlapping. Each level (L0, L1, L2 and so on) is 10 times as large as the previous. Disk I/O is more uniform and predictable on higher than on lower levels as SSTables are continuously being compacted into progressively larger levels. At each level, row keys are merged into non-overlapping SSTables in the next level. This process can improve performance for reads, because Cassandra can determine which SSTables in each level to check for the existence of row key data. This compaction strategy is modeled after Google's LevelDB implementation. Also see LCS compaction subproperties.
• SizeTieredCompactionStrategy (STCS): The default compaction strategy. This strategy triggers a minor compaction when there are a number of similar sized SSTables on disk as configured by the table subproperty, min_threshold. A minor compaction does not involve all the tables in a keyspace. Also see STCS compaction subproperties.
• TimeWindowCompactionStrategy (TWCS) This strategy is an alternative for time series data. TWCS compacts SSTables using a series of time windows. While with a time window, TWCS compacts all SSTables flushed from memory into larger SSTables using STCS. At the end of the time window, all of these SSTables are compacted into a single SSTable. Then the next time window starts and the process repeats. The duration of the time window is the only setting required. See TWCS compaction subproperties. For more information about TWCS, see How is data maintained?.
• DateTieredCompactionStrategy (DTCS): This strategy is an alternative for time series data. It is deprecated in Cassandra 3.0.8/3.8 and later. DTCS stores data written within a certain period of time in the same SSTable. For example, Cassandra stores the last hour of data in one SSTable time window, and the next 4 hours of data in another time window, and so on. Compactions are triggered when the min_threshold (4 by default) for SSTables in those windows is reached. The most common queries for time series workloads retrieve the last hour/day/month of data. Cassandra can limit SSTables returned to those having the relevant data. Also, Cassandra can store data that has been set to expire using TTL in an SSTable with other data scheduled to expire at approximately the same time. Cassandra can then drop the SSTable without doing any compaction. Also see DTCS compaction subproperties.

To configure the compaction strategy property and CQL compaction subproperties, such as the maximum number of SSTables to compact and minimum SSTable size, use CREATE TABLE or ALTER TABLE.