org.elasticsearch.index.shard

Class MergePolicyConfig

java.lang.Object

org.elasticsearch.index.shard.MergePolicyConfig

All Implemented Interfaces:

IndexSettingsService.Listener

public final class MergePolicyConfig
extends Object
implements IndexSettingsService.Listener

A shard in elasticsearch is a Lucene index, and a Lucene index is broken down into segments. Segments are internal storage elements in the index where the index data is stored, and are immutable up to delete markers. Segments are, periodically, merged into larger segments to keep the index size at bay and expunge deletes.

Merges select segments of approximately equal size, subject to an allowed number of segments per tier. The merge policy is able to merge non-adjacent segments, and separates how many segments are merged at once from how many segments are allowed per tier. It also does not over-merge (i.e., cascade merges).

All merge policy settings are dynamic and can be updated on a live index. The merge policy has the following settings:

• index.merge.policy.expunge_deletes_allowed: When expungeDeletes is called, we only merge away a segment if its delete percentage is over this threshold. Default is 10.
• index.merge.policy.floor_segment: Segments smaller than this are "rounded up" to this size, i.e. treated as equal (floor) size for merge selection. This is to prevent frequent flushing of tiny segments, thus preventing a long tail in the index. Default is 2mb.
• index.merge.policy.max_merge_at_once: Maximum number of segments to be merged at a time during "normal" merging. Default is 10.
• index.merge.policy.max_merge_at_once_explicit: Maximum number of segments to be merged at a time, during optimize or expungeDeletes. Default is 30.
• index.merge.policy.max_merged_segment: Maximum sized segment to produce during normal merging (not explicit optimize). This setting is approximate: the estimate of the merged segment size is made by summing sizes of to-be-merged segments (compensating for percent deleted docs). Default is 5gb.
• index.merge.policy.segments_per_tier: Sets the allowed number of segments per tier. Smaller values mean more merging but fewer segments. Default is 10. Note, this value needs to be >= than the max_merge_at_once otherwise you'll force too many merges to occur.
• index.merge.policy.reclaim_deletes_weight: Controls how aggressively merges that reclaim more deletions are favored. Higher values favor selecting merges that reclaim deletions. A value of 0.0 means deletions don't impact merge selection. Defaults to 2.0.

For normal merging, the policy first computes a "budget" of how many segments are allowed to be in the index. If the index is over-budget, then the policy sorts segments by decreasing size (proportionally considering percent deletes), and then finds the least-cost merge. Merge cost is measured by a combination of the "skew" of the merge (size of largest seg divided by smallest seg), total merge size and pct deletes reclaimed, so that merges with lower skew, smaller size and those reclaiming more deletes, are favored.

If a merge will produce a segment that's larger than max_merged_segment then the policy will merge fewer segments (down to 1 at once, if that one has deletions) to keep the segment size under budget.

Note, this can mean that for large shards that holds many gigabytes of data, the default of max_merged_segment (5gb) can cause for many segments to be in an index, and causing searches to be slower. Use the indices segments API to see the segments that an index has, and possibly either increase the max_merged_segment or issue an optimize call for the index (try and aim to issue it on a low traffic time).

Field Summary

Fields

Modifier and Type	Field and Description
static double	DEFAULT_EXPUNGE_DELETES_ALLOWED
static ByteSizeValue	DEFAULT_FLOOR_SEGMENT
static int	DEFAULT_MAX_MERGE_AT_ONCE
static int	DEFAULT_MAX_MERGE_AT_ONCE_EXPLICIT
static ByteSizeValue	DEFAULT_MAX_MERGED_SEGMENT
static double	DEFAULT_RECLAIM_DELETES_WEIGHT
static double	DEFAULT_SEGMENTS_PER_TIER
static String	INDEX_COMPOUND_FORMAT
static String	INDEX_MERGE_ENABLED
static String	INDEX_MERGE_POLICY_EXPUNGE_DELETES_ALLOWED
static String	INDEX_MERGE_POLICY_FLOOR_SEGMENT
static String	INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE
static String	INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE_EXPLICIT
static String	INDEX_MERGE_POLICY_MAX_MERGED_SEGMENT
static String	INDEX_MERGE_POLICY_RECLAIM_DELETES_WEIGHT
static String	INDEX_MERGE_POLICY_SEGMENTS_PER_TIER

Constructor Summary

Constructors

Constructor and Description
MergePolicyConfig(ESLogger logger, Settings indexSettings)

Method Summary

Methods

Modifier and Type	Method and Description
static String	formatNoCFSRatio(double ratio)
org.apache.lucene.index.MergePolicy	getMergePolicy()
void	onRefreshSettings(Settings settings)
static double	parseNoCFSRatio(String noCFSRatio)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_EXPUNGE_DELETES_ALLOWED

public static final double DEFAULT_EXPUNGE_DELETES_ALLOWED

See Also:

Constant Field Values

DEFAULT_FLOOR_SEGMENT

public static final ByteSizeValue DEFAULT_FLOOR_SEGMENT

DEFAULT_MAX_MERGE_AT_ONCE

public static final int DEFAULT_MAX_MERGE_AT_ONCE

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Constant Field Values

DEFAULT_MAX_MERGE_AT_ONCE_EXPLICIT

public static final int DEFAULT_MAX_MERGE_AT_ONCE_EXPLICIT

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Constant Field Values

DEFAULT_MAX_MERGED_SEGMENT

public static final ByteSizeValue DEFAULT_MAX_MERGED_SEGMENT

DEFAULT_SEGMENTS_PER_TIER

public static final double DEFAULT_SEGMENTS_PER_TIER

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Constant Field Values

DEFAULT_RECLAIM_DELETES_WEIGHT

public static final double DEFAULT_RECLAIM_DELETES_WEIGHT

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Constant Field Values

INDEX_COMPOUND_FORMAT

public static final String INDEX_COMPOUND_FORMAT

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Constant Field Values

INDEX_MERGE_POLICY_EXPUNGE_DELETES_ALLOWED

public static final String INDEX_MERGE_POLICY_EXPUNGE_DELETES_ALLOWED

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Constant Field Values

INDEX_MERGE_POLICY_FLOOR_SEGMENT

public static final String INDEX_MERGE_POLICY_FLOOR_SEGMENT

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Constant Field Values

INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE

public static final String INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE

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Constant Field Values

INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE_EXPLICIT

public static final String INDEX_MERGE_POLICY_MAX_MERGE_AT_ONCE_EXPLICIT

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Constant Field Values

INDEX_MERGE_POLICY_MAX_MERGED_SEGMENT

public static final String INDEX_MERGE_POLICY_MAX_MERGED_SEGMENT

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Constant Field Values

INDEX_MERGE_POLICY_SEGMENTS_PER_TIER

public static final String INDEX_MERGE_POLICY_SEGMENTS_PER_TIER

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Constant Field Values

INDEX_MERGE_POLICY_RECLAIM_DELETES_WEIGHT

public static final String INDEX_MERGE_POLICY_RECLAIM_DELETES_WEIGHT

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Constant Field Values

INDEX_MERGE_ENABLED

public static final String INDEX_MERGE_ENABLED

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Constant Field Values

Constructor Detail

MergePolicyConfig

public MergePolicyConfig(ESLogger logger,
                 Settings indexSettings)

Method Detail

getMergePolicy

public org.apache.lucene.index.MergePolicy getMergePolicy()

onRefreshSettings

public void onRefreshSettings(Settings settings)

Specified by:

onRefreshSettings in interface IndexSettingsService.Listener

parseNoCFSRatio

public static double parseNoCFSRatio(String noCFSRatio)

formatNoCFSRatio

public static String formatNoCFSRatio(double ratio)