Decrease TieredMergePolicy's default number of segments per tier to 8. #14823
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`TieredMergePolicy` currently allows 10 segments per tier. With Lucene being increasingly deployed with separate indexing and search tiers that get updated via segment-based replication, I believe that it would make sense for Lucene to have more aggressive merging defaults, a price that is only paid once on the indexing tier, but that benefits all search nodes that serve queries for this index. Note that this is still a somewhat conservative default, applications with low latency requirements and low update rates will likely want to go even further, with 4 segments per tier, or even 2. `BaseMergePolicyTestCase#testSimulateAppendOnly` reports a write amplification increase from 3.4 to 3.8, while `BaseMergePolicyTestCase#testSimulateUpdates` reports a write amplification increase from 4.5 to 4.9. In exchange, the number of segments between the floor and max segment sizes decreases by about 20%. This should especially help queries that have a high per-segment overhead: PK lookups, point queries, multi-term queries and vector searches.
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+1, this is a great idea -- more aggessive merging by default makes sense. |
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Thanks @mikemccand ! I'll wait a few days before merging to give others a chance to take a look. |
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TieredMergePolicycurrently allows 10 segments per tier. With Lucene being increasingly deployed with separate indexing and search tiers that get updated via segment-based replication, I believe that it would make sense for Lucene to have more aggressive merging defaults, a price that is only paid once on the indexing tier, but that benefits all search nodes that serve queries for this index.Note that this is still a somewhat conservative default, applications with low latency requirements and low update rates will likely want to go even further, with 4 segments per tier, or even 2.
BaseMergePolicyTestCase#testSimulateAppendOnlyreports a write amplification increase from 3.4 to 3.8, whileBaseMergePolicyTestCase#testSimulateUpdatesreports a write amplification increase from 4.5 to 4.9. In exchange, the number of segments between the floor and max segment sizes decreases by about 20%.This should especially help queries that have a high per-segment overhead: PK lookups, point queries, multi-term queries and vector searches.