Class TieredMergePolicy

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 (MaxMergeAtOnce) from how many segments are allowed per tier (SegmentsPerTier). 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 MaxMergedSegmentMB, 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, and does not apply any maximum segment size during forceMerge (unlike LogByteSizeMergePolicy).

Note

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

Inheritance
object
MergePolicy
TieredMergePolicy
Implements
Inherited Members
Namespace: Lucene.Net.Index
Assembly: Lucene.Net.dll
Syntax
public class TieredMergePolicy : MergePolicy, IDisposable

Constructors

TieredMergePolicy()

Sole constructor, setting all settings to their defaults.

Declaration
public TieredMergePolicy()

Fields

DEFAULT_NO_CFS_RATIO

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

Declaration
public static readonly double DEFAULT_NO_CFS_RATIO
Field Value
Type Description
double
See Also

Properties

FloorSegmentMB

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.

Declaration
public virtual double FloorSegmentMB { get; set; }
Property Value
Type Description
double

ForceMergeDeletesPctAllowed

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

Declaration
public virtual double ForceMergeDeletesPctAllowed { get; set; }
Property Value
Type Description
double

MaxMergeAtOnce

Gets or sets maximum number of segments to be merged at a time during "normal" merging. For explicit merging (eg, ForceMerge(int) or ForceMergeDeletes() was called), see MaxMergeAtOnceExplicit. Default is 10.

Declaration
public virtual int MaxMergeAtOnce { get; set; }
Property Value
Type Description
int

MaxMergeAtOnceExplicit

Gets or sets maximum number of segments to be merged at a time, during ForceMerge(int) or ForceMergeDeletes(). Default is 30.

Declaration
public virtual int MaxMergeAtOnceExplicit { get; set; }
Property Value
Type Description
int

MaxMergedSegmentMB

Gets or sets 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.

Declaration
public virtual double MaxMergedSegmentMB { get; set; }
Property Value
Type Description
double

ReclaimDeletesWeight

Controls how aggressively merges that reclaim more deletions are favored. Higher values will more aggressively target merges that reclaim deletions, but be careful not to go so high that way too much merging takes place; a value of 3.0 is probably nearly too high. A value of 0.0 means deletions don't impact merge selection.

Declaration
public virtual double ReclaimDeletesWeight { get; set; }
Property Value
Type Description
double

SegmentsPerTier

Gets or sets the allowed number of segments per tier. Smaller values mean more merging but fewer segments.

NOTE: this value should be >= the MaxMergeAtOnce otherwise you'll force too much merging to occur.

Default is 10.0.
Declaration
public virtual double SegmentsPerTier { get; set; }
Property Value
Type Description
double

Methods

Dispose(bool)

Release all resources for the policy.

Declaration
protected override void Dispose(bool disposing)
Parameters
Type Name Description
bool disposing
Overrides

FindForcedDeletesMerges(SegmentInfos)

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

Declaration
public override MergePolicy.MergeSpecification FindForcedDeletesMerges(SegmentInfos infos)
Parameters
Type Name Description
SegmentInfos infos
Returns
Type Description
MergePolicy.MergeSpecification
Overrides

FindForcedMerges(SegmentInfos, int, IDictionary<SegmentCommitInfo, bool>)

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

Declaration
public override MergePolicy.MergeSpecification FindForcedMerges(SegmentInfos infos, int maxSegmentCount, IDictionary<SegmentCommitInfo, bool> segmentsToMerge)
Parameters
Type Name Description
SegmentInfos infos
int maxSegmentCount

Requested maximum number of segments in the index (currently this is always 1)
IDictionary<SegmentCommitInfo, bool> 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.
Returns
Type Description
MergePolicy.MergeSpecification
Overrides

FindMerges(MergeTrigger, SegmentInfos)

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.

Declaration
public override MergePolicy.MergeSpecification FindMerges(MergeTrigger mergeTrigger, SegmentInfos infos)
Parameters
Type Name Description
MergeTrigger mergeTrigger

the event that triggered the merge
SegmentInfos infos
Returns
Type Description
MergePolicy.MergeSpecification
Overrides

Score(IList<SegmentCommitInfo>, bool, long)

Expert: scores one merge; subclasses can override.

Declaration
protected virtual TieredMergePolicy.MergeScore Score(IList<SegmentCommitInfo> candidate, bool hitTooLarge, long mergingBytes)
Parameters
Type Name Description
IList<SegmentCommitInfo> candidate
bool hitTooLarge
long mergingBytes
Returns
Type Description
TieredMergePolicy.MergeScore

ToString()

Returns a string that represents the current object.

Declaration
public override string ToString()
Returns
Type Description
string

A string that represents the current object.
Overrides

Implements