Compacting on-disk data

Kudu constantly performs several compaction tasks in order to maintain consistent read and write performance over time.

• A merging compaction, which combines multiple DiskRowSets together into a single DiskRowSet, is run by CompactRowSetsOp.
• Kudu also runs two types of delta store compaction operations: MinorDeltaCompactionOp and MajorDeltaCompactionOp.

  For more information on what these compaction operations do, see the Kudu Tablet design document.

The metrics associated with these tasks have the prefix compact_rs, delta_minor_compact_rs, and delta_major_compact_rs, respectively.