org.apache.lucene.index

Class TieredMergePolicy

java.lang.Object

org.apache.lucene.index.MergePolicy

org.apache.lucene.index.TieredMergePolicy

public class TieredMergePolicy
extends MergePolicy

Merges segments of approximately equal size, subject to an allowed number of segments per tier. This is similar to LogByteSizeMergePolicy, except this merge policy is able to merge non-adjacent segment, and separates how many segments are merged at once (setMaxMergeAtOnce(int)) from how many segments are allowed per tier (setSegmentsPerTier(double)). This merge policy also does not over-merge (i.e. cascade merges).

For normal merging, this 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 (pro-rating by 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 segment divided by smallest segment), total merge size and percent 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 setMaxMergedSegmentMB(double), 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 policy freely merges non-adjacent segments; if this is a problem, use LogMergePolicy.

NOTE: This policy always merges by byte size of the segments, always pro-rates by percent deletes

NOTE Starting with Lucene 7.5, there are several changes: - findForcedMerges and findForcedDeletesMerges) respect the max segment size by default. - When findforcedmerges is called with maxSegmentCount other than 1, the resulting index is not guaranteed to have <= maxSegmentCount segments. Rather it is on a "best effort" basis. Specifically the theoretical ideal segment size is calculated and a "fudge factor" of 25% is added as the new maxSegmentSize, which is respected. - findForcedDeletesMerges will not produce segments greater than maxSegmentSize.

WARNING: This API is experimental and might change in incompatible ways in the next release.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	TieredMergePolicy.MergeScore Holds score and explanation for a single candidate merge.

Nested classes/interfaces inherited from class org.apache.lucene.index.MergePolicy

MergePolicy.MergeAbortedException, MergePolicy.MergeContext, MergePolicy.MergeException, MergePolicy.MergeSpecification, MergePolicy.OneMerge, MergePolicy.OneMergeProgress

Field Summary

Fields

Modifier and Type	Field and Description
static double	DEFAULT_NO_CFS_RATIO Default noCFSRatio.

Fields inherited from class org.apache.lucene.index.MergePolicy

DEFAULT_MAX_CFS_SEGMENT_SIZE, maxCFSSegmentSize, noCFSRatio

Constructor Summary

Constructors

Constructor and Description
TieredMergePolicy() Sole constructor, setting all settings to their defaults.

Method Summary

Modifier and Type	Method and Description
MergePolicy.MergeSpecification	findForcedDeletesMerges(SegmentInfos infos, MergePolicy.MergeContext mergeContext) Determine what set of merge operations is necessary in order to expunge all deletes from the index.
MergePolicy.MergeSpecification	findForcedMerges(SegmentInfos infos, int maxSegmentCount, Map<SegmentCommitInfo,Boolean> segmentsToMerge, MergePolicy.MergeContext mergeContext) Determine what set of merge operations is necessary in order to merge to <= the specified segment count.
MergePolicy.MergeSpecification	findMerges(MergeTrigger mergeTrigger, SegmentInfos infos, MergePolicy.MergeContext mergeContext) Determine what set of merge operations are now necessary on the index.
double	getDeletesPctAllowed() Returns the current deletesPctAllowed setting.
double	getFloorSegmentMB() Returns the current floorSegmentMB.
double	getForceMergeDeletesPctAllowed() Returns the current forceMergeDeletesPctAllowed setting.
int	getMaxMergeAtOnce() Returns the current maxMergeAtOnce setting.
int	getMaxMergeAtOnceExplicit() Returns the current maxMergeAtOnceExplicit setting.
double	getMaxMergedSegmentMB() Returns the current maxMergedSegmentMB setting.
double	getSegmentsPerTier() Returns the current segmentsPerTier setting.
protected TieredMergePolicy.MergeScore	score(List<SegmentCommitInfo> candidate, boolean hitTooLarge, Map<SegmentCommitInfo,org.apache.lucene.index.TieredMergePolicy.SegmentSizeAndDocs> segmentsSizes) Expert: scores one merge; subclasses can override.
TieredMergePolicy	setDeletesPctAllowed(double v) Controls the maximum percentage of deleted documents that is tolerated in the index.
TieredMergePolicy	setFloorSegmentMB(double v) Segments smaller than this are "rounded up" to this size, ie treated as equal (floor) size for merge selection.
TieredMergePolicy	setForceMergeDeletesPctAllowed(double v) When forceMergeDeletes is called, we only merge away a segment if its delete percentage is over this threshold.
TieredMergePolicy	setMaxMergeAtOnce(int v) Maximum number of segments to be merged at a time during "normal" merging.
TieredMergePolicy	setMaxMergeAtOnceExplicit(int v) Maximum number of segments to be merged at a time, during forceMerge or forceMergeDeletes.
TieredMergePolicy	setMaxMergedSegmentMB(double v) Maximum sized segment to produce during normal merging.
TieredMergePolicy	setSegmentsPerTier(double v) Sets the allowed number of segments per tier.
String	toString()

Methods inherited from class org.apache.lucene.index.MergePolicy

assertDelCount, findFullFlushMerges, getMaxCFSSegmentSizeMB, getNoCFSRatio, isMerged, keepFullyDeletedSegment, message, numDeletesToMerge, segString, setMaxCFSSegmentSizeMB, setNoCFSRatio, size, useCompoundFile, verbose

Methods inherited from class java.lang.Object

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

Field Detail

DEFAULT_NO_CFS_RATIO

public static final double DEFAULT_NO_CFS_RATIO

Default noCFSRatio. If a merge's size is >= 10% of the index, then we disable compound file for it.

See Also:

MergePolicy.setNoCFSRatio(double), Constant Field Values

Constructor Detail

TieredMergePolicy

public TieredMergePolicy()

Sole constructor, setting all settings to their defaults.

Method Detail

setMaxMergeAtOnce

public TieredMergePolicy setMaxMergeAtOnce(int v)

Maximum number of segments to be merged at a time during "normal" merging. For explicit merging (eg, forceMerge or forceMergeDeletes was called), see setMaxMergeAtOnceExplicit(int). Default is 10.

getMaxMergeAtOnce

public int getMaxMergeAtOnce()

Returns the current maxMergeAtOnce setting.

See Also:

setMaxMergeAtOnce(int)

setMaxMergeAtOnceExplicit

public TieredMergePolicy setMaxMergeAtOnceExplicit(int v)

Maximum number of segments to be merged at a time, during forceMerge or forceMergeDeletes. Default is 30.

getMaxMergeAtOnceExplicit

public int getMaxMergeAtOnceExplicit()

Returns the current maxMergeAtOnceExplicit setting.

See Also:

setMaxMergeAtOnceExplicit(int)

setMaxMergedSegmentMB

public TieredMergePolicy setMaxMergedSegmentMB(double v)

Maximum sized segment to produce during normal merging. 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 5 GB.

getMaxMergedSegmentMB

public double getMaxMergedSegmentMB()

Returns the current maxMergedSegmentMB setting.

See Also:

setMaxMergedSegmentMB(double)

setDeletesPctAllowed

public TieredMergePolicy setDeletesPctAllowed(double v)

Controls the maximum percentage of deleted documents that is tolerated in the index. Lower values make the index more space efficient at the expense of increased CPU and I/O activity. Values must be between 20 and 50. Default value is 33.

getDeletesPctAllowed

public double getDeletesPctAllowed()

Returns the current deletesPctAllowed setting.

See Also:

setDeletesPctAllowed(double)

setFloorSegmentMB

public TieredMergePolicy setFloorSegmentMB(double v)

Segments smaller than this are "rounded up" to this size, ie treated as equal (floor) size for merge selection. This is to prevent frequent flushing of tiny segments from allowing a long tail in the index. Default is 2 MB.

getFloorSegmentMB

public double getFloorSegmentMB()

Returns the current floorSegmentMB.

See Also:

setFloorSegmentMB(double)

setForceMergeDeletesPctAllowed

public TieredMergePolicy setForceMergeDeletesPctAllowed(double v)

When forceMergeDeletes is called, we only merge away a segment if its delete percentage is over this threshold. Default is 10%.

getForceMergeDeletesPctAllowed

public double getForceMergeDeletesPctAllowed()

Returns the current forceMergeDeletesPctAllowed setting.

See Also:

setForceMergeDeletesPctAllowed(double)

setSegmentsPerTier

public TieredMergePolicy setSegmentsPerTier(double v)

Sets the allowed number of segments per tier. Smaller values mean more merging but fewer segments.

Default is 10.0.

getSegmentsPerTier

public double getSegmentsPerTier()

Returns the current segmentsPerTier setting.

See Also:

setSegmentsPerTier(double)

findMerges

public MergePolicy.MergeSpecification findMerges(MergeTrigger mergeTrigger,
                                                 SegmentInfos infos,
                                                 MergePolicy.MergeContext mergeContext)
                                          throws IOException

Description copied from class: MergePolicy

Determine what set of merge operations are now necessary on the index. IndexWriter calls this whenever there is a change to the segments. This call is always synchronized on the IndexWriter instance so only one thread at a time will call this method.

Specified by:

findMerges in class MergePolicy

Parameters:

mergeTrigger - the event that triggered the merge

infos - the total set of segments in the index

mergeContext - the IndexWriter to find the merges on

Throws:

IOException

score

protected TieredMergePolicy.MergeScore score(List<SegmentCommitInfo> candidate,
                                             boolean hitTooLarge,
                                             Map<SegmentCommitInfo,org.apache.lucene.index.TieredMergePolicy.SegmentSizeAndDocs> segmentsSizes)
                                      throws IOException

Expert: scores one merge; subclasses can override.

Throws:

IOException

findForcedMerges

public MergePolicy.MergeSpecification findForcedMerges(SegmentInfos infos,
                                                       int maxSegmentCount,
                                                       Map<SegmentCommitInfo,Boolean> segmentsToMerge,
                                                       MergePolicy.MergeContext mergeContext)
                                                throws IOException

Description copied from class: MergePolicy

Determine what set of merge operations is necessary in order to merge to <= the specified segment count. IndexWriter calls this when its IndexWriter.forceMerge(int) method is called. This call is always synchronized on the IndexWriter instance so only one thread at a time will call this method.

Specified by:

findForcedMerges in class MergePolicy

Parameters:

infos - the total set of segments in the index

maxSegmentCount - requested maximum number of segments in the index (currently this is always 1)

segmentsToMerge - contains the specific SegmentInfo instances that must be merged away. This may be a subset of all SegmentInfos. If the value is True for a given SegmentInfo, that means this segment was an original segment present in the to-be-merged index; else, it was a segment produced by a cascaded merge.

mergeContext - the MergeContext to find the merges on

Throws:

IOException

findForcedDeletesMerges

public MergePolicy.MergeSpecification findForcedDeletesMerges(SegmentInfos infos,
                                                              MergePolicy.MergeContext mergeContext)
                                                       throws IOException

Description copied from class: MergePolicy

Determine what set of merge operations is necessary in order to expunge all deletes from the index.

Specified by:

findForcedDeletesMerges in class MergePolicy

Parameters:

infos - the total set of segments in the index

mergeContext - the MergeContext to find the merges on

Throws:

IOException

toString

public String toString()

Overrides:

toString in class Object