Class 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, if you call IndexWriter.forceMerge(int) with this (default) merge policy, if setMaxMergedSegmentMB(double) is in conflict with maxNumSegments passed to IndexWriter.forceMerge(int) then maxNumSegments wins. For example, if your index has 50 1 GB segments, and you have setMaxMergedSegmentMB(double) at 1024 (1 GB), and you call forceMerge(10), the two settings are clearly in conflict. TieredMergePolicy will choose to break the setMaxMergedSegmentMB(double) constraint and try to merge down to at most ten segments, each up to 5 * 1.25 GB in size (since an extra 25% buffer increase in the expected segment size is targetted).

    findForcedDeletesMerges should never produce segments greater than maxSegmentSize.

    NOTE: This policy returns natural merges whose size is below the floor segment size for full-flush merges.

    WARNING: This API is experimental and might change in incompatible ways in the next release.
    • 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. Default is 10.
      • getMaxMergeAtOnce

        public int getMaxMergeAtOnce()
        Returns the current maxMergeAtOnce 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:
      • 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 5 and 50. Default value is 20.

        When the maximum delete percentage is lowered, the indexing thread will call for merges more often, meaning that write amplification factor will be increased. Write amplification factor measures the number of times each document in the index is written. A higher write amplification factor will lead to higher CPU and I/O activity as indicated above.

      • getDeletesPctAllowed

        public double getDeletesPctAllowed()
        Returns the current deletesPctAllowed setting.
        See Also:
      • setFloorSegmentMB

        public TieredMergePolicy setFloorSegmentMB​(double v)
        Segments smaller than this size are merged more aggressively:
        • They are candidates for full-flush merges, in order to reduce the number of segments in the index prior to opening a new point-in-time view of the index.
        • For background merges, smaller segments are "rounded up" to this size.
        In both cases, this helps prevent frequent flushing of tiny segments to create a long tail of small segments in the index. Default is 2MB.
      • 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%.
      • 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:
      • 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
        infos - the total set of segments in the index
        maxSegmentCount - requested maximum number of segments in the index
        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