32 lines
1.2 KiB
Plaintext
32 lines
1.2 KiB
Plaintext
[[index-modules-merge]]
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== Merge
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A shard in Elasticsearch is a Lucene index, and a Lucene index is broken down
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into segments. Segments are internal storage elements in the index where the
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index data is stored, and are immutable. Smaller segments are periodically
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merged into larger segments to keep the index size at bay and to expunge
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deletes.
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The merge process uses auto-throttling to balance the use of hardware
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resources between merging and other activities like search.
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[discrete]
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[[merge-scheduling]]
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=== Merge scheduling
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The merge scheduler (ConcurrentMergeScheduler) controls the execution of merge
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operations when they are needed. Merges run in separate threads, and when the
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maximum number of threads is reached, further merges will wait until a merge
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thread becomes available.
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The merge scheduler supports the following _dynamic_ setting:
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`index.merge.scheduler.max_thread_count`::
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The maximum number of threads on a single shard that may be merging at once.
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Defaults to
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`Math.max(1, Math.min(4, <<node.processors, node.processors>> / 2))` which
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works well for a good solid-state-disk (SSD). If your index is on spinning
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platter drives instead, decrease this to 1.
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