Package org.apache.lucene.index

Class TieredMergePolicy

java.lang.Object
org.apache.lucene.index.MergePolicy
org.apache.lucene.index.TieredMergePolicy
All Implemented Interfaces:
Closeable, AutoCloseable, Cloneable
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, and does not apply any maximum segment size during forceMerge (unlike LogByteSizeMergePolicy).

  • Field Details

  • Constructor Details

    • TieredMergePolicy

      public TieredMergePolicy()
      Sole constructor, setting all settings to their defaults.

  • Method Details

    • 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:

    • 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:

    • 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:

    • setReclaimDeletesWeight

      public TieredMergePolicy setReclaimDeletesWeight(double v)
      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.

    • getReclaimDeletesWeight

      public double getReclaimDeletesWeight()
      See setReclaimDeletesWeight(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:

    • 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:

    • setSegmentsPerTier

      public TieredMergePolicy setSegmentsPerTier(double v)
      Sets the allowed number of segments per tier. Smaller values mean more merging but fewer segments.

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

      Default is 10.0.

    • getSegmentsPerTier

      public double getSegmentsPerTier()
      Returns the current segmentsPerTier setting.
      See Also:

    • findMerges

      public MergePolicy.MergeSpecification findMerges(MergePolicy.MergeTrigger mergeTrigger, SegmentInfos infos) 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
      Throws:
      IOException

    • score

      protected TieredMergePolicy.MergeScore score(List<SegmentCommitInfo> candidate, boolean hitTooLarge, long mergingBytes) throws IOException
      Expert: scores one merge; subclasses can override.
      Throws:
      IOException

    • findForcedMerges

      public MergePolicy.MergeSpecification findForcedMerges(SegmentInfos infos, int maxSegmentCount, Map<SegmentCommitInfo,Boolean> segmentsToMerge) 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.
      Throws:
      IOException

    • findForcedDeletesMerges

      public MergePolicy.MergeSpecification findForcedDeletesMerges(SegmentInfos infos) 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
      Throws:
      IOException

    • close

      public void close()
      Description copied from class: MergePolicy
      Release all resources for the policy.
      Specified by:
      close in interface AutoCloseable
      Specified by:
      close in interface Closeable
      Specified by:
      close in class MergePolicy

    • toString

      public String toString()
      Overrides:
      toString in class Object