Kudu relies on running background tasks for many important automatic maintenance activities. These tasks include flushing data from memory to disk, compacting data to improve performance, freeing up disk space, and more.
The maintenance manager schedules and runs background tasks. At any given point
in time, the maintenance manager is prioritizing the next task based on the
improvement needed at that moment, such as relieving memory pressure, improving
read performance, or freeing up disk space. The number of worker threads
dedicated to running background tasks can be controlled by setting
--maintenance_manager_num_threads.
Flushing data from memory to disk relieves memory pressure and can improve read
performance by switching from a write-optimized, row-oriented in-memory format
in the MemRowSet to a read-optimized, column-oriented format on disk.
Background tasks that flush data include FlushMRSOp and
FlushDeltaMemStoresOp.
The metrics associated with these ops have the prefix flush_mrs and
flush_dms, respectively.
Kudu constantly performs several types of 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. There are two types of delta store compaction operations
that may be run as well: MinorDeltaCompactionOp and MajorDeltaCompactionOp.
For more information on what these different types of compaction operations do, please 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.
Kudu maintains a write-ahead log (WAL) per tablet that is split into discrete
fixed-size segments. A tablet periodically rolls the WAL to a new log segment
when the active segment reaches a configured size (controlled by
--log_segment_size_mb). In order to save disk space and decrease startup
time, a background task called LogGCOp attempts to garbage-collect (GC) old
WAL segments by deleting them from disk once it is determined that they are no
longer needed by the local node for durability.
The metrics associated with this background task have the prefix log_gc.
Because Kudu uses a multiversion concurrency control (MVCC) mechanism to ensure that snapshot scans can proceeed isolated from new changes to a table, periodically old historical data should be garbage-collected (removed) to free up disk space. While Kudu never removes rows or data that are visible in the latest version of the data, Kudu does remove records of old changes that are no longer visible.
The point in time in the past beyond which historical MVCC data becomes
inaccessible and is free to be deleted is called the ancient history mark
(AHM). The AHM can be configured by setting --tablet_history_max_age_sec.
There are two background tasks that GC historical MVCC data older than the AHM:
the one that runs the merging compaction, called CompactRowSetsOp (see
above), and a separate background task that deletes old undo delta blocks,
called UndoDeltaBlockGCOp. Running UndoDeltaBlockGCOp reduces disk space
usage in all workloads, but particularly in those with a higher volume of
updates or upserts.
The metrics associated with this background task have the prefix
undo_delta_block.