docs/background_tasks.adoc - kudu - Git at Google

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 // regarding copyright ownership.  The ASF licenses this file
 // to you under the Apache License, Version 2.0 (the
 // "License"); you may not use this file except in compliance
 // with the License.  You may obtain a copy of the License at
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 // Unless required by applicable law or agreed to in writing,
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 // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 // KIND, either express or implied.  See the License for the
 // specific language governing permissions and limitations
 // under the License.

 [[background_tasks]]
 = Apache Kudu Background Maintenance Tasks
 :author: Kudu Team
 :imagesdir: ./images
 :icons: font
 :toc: left
 :toclevels: 3
 :doctype: book
 :backend: html5
 :sectlinks:
 :experimental:

 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.

 == Maintenance manager

 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 to disk

 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.

 == Compacting on-disk data

 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
 link:https://github.com/apache/kudu/blob/master/docs/design-docs/tablet.md[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.

 == Write-ahead log GC

 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`.

 == Tablet history GC and the ancient history mark

 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`.
	// Licensed to the Apache Software Foundation (ASF) under one
	// or more contributor license agreements. See the NOTICE file
	// distributed with this work for additional information
	// regarding copyright ownership. The ASF licenses this file
	// to you under the Apache License, Version 2.0 (the
	// "License"); you may not use this file except in compliance
	// with the License. You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing,
	// software distributed under the License is distributed on an
	// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
	// KIND, either express or implied. See the License for the
	// specific language governing permissions and limitations
	// under the License.

	[[background_tasks]]
	= Apache Kudu Background Maintenance Tasks
	:author: Kudu Team
	:imagesdir: ./images
	:icons: font
	:toc: left
	:toclevels: 3
	:doctype: book
	:backend: html5
	:sectlinks:
	:experimental:

	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.

	== Maintenance manager

	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 to disk

	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.

	== Compacting on-disk data

	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
	link:https://github.com/apache/kudu/blob/master/docs/design-docs/tablet.md[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.

	== Write-ahead log GC

	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`.

	== Tablet history GC and the ancient history mark

	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`.