Mergeedit

A shard in elasticsearch is a Lucene index, and a Lucene index is broken down into segments. Segments are internal storage elements in the index where the index data is stored, and are immutable up to delete markers. Segments are, periodically, merged into larger segments to keep the index size at bay and expunge deletes.

The more segments one has in the Lucene index means slower searches and more memory used. Segment merging is used to reduce the number of segments, however merges can be expensive to perform, especially on low IO environments. Merges can be throttled using store level throttling.

Policyedit

The index merge policy module allows one to control which segments of a shard index are to be merged. There are several types of policies with the default set to tiered.

tierededit

Merges segments of approximately equal size, subject to an allowed number of segments per tier. This is similar to log_bytes_size merge policy, except this merge policy is able to merge non-adjacent segment, and separates how many segments are merged at once from how many segments are allowed per tier. This merge policy also does not over-merge (i.e., cascade merges).

This policy has the following settings:

index.merge.policy.expunge_deletes_allowed
When expungeDeletes is called, we only merge away a segment if its delete percentage is over this threshold. Default is 10.
index.merge.policy.floor_segment
Segments smaller than this are "rounded up" to this size, i.e. 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 2mb.
index.merge.policy.max_merge_at_once
Maximum number of segments to be merged at a time during "normal" merging. Default is 10.
index.merge.policy.max_merge_at_once_explicit
Maximum number of segments to be merged at a time, during optimize or expungeDeletes. Default is 30.
index.merge.policy.max_merged_segment
Maximum sized segment to produce during normal merging (not explicit optimize). 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 5gb.
index.merge.policy.segments_per_tier
Sets the allowed number of segments per tier. Smaller values mean more merging but fewer segments. Default is 10. Note, this value needs to be >= then the max_merge_at_once otherwise you’ll force too many merges to occur.
index.reclaim_deletes_weight
Controls how aggressively merges that reclaim more deletions are favored. Higher values favor selecting merges that reclaim deletions. A value of 0.0 means deletions don’t impact merge selection. Defaults to 2.0.
index.compound_format
Should the index be stored in compound format or not. Defaults to false. See index.compound_format in Index Settingsedit.

For normal merging, this policy first computes a "budget" of how many segments are allowed by 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 seg divided by smallest seg), total merge size and pct 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 max_merged_segment 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 can mean that for large shards that holds many gigabytes of data, the default of max_merged_segment (5gb) can cause for many segments to be in an index, and causing searches to be slower. Use the indices segments API to see the segments that an index have, and possibly either increase the max_merged_segment or issue an optimize call for the index (try and aim to issue it on a low traffic time).

log_byte_sizeedit

A merge policy that merges segments into levels of exponentially increasing byte size, where each level has fewer segments than the value of the merge factor. Whenever extra segments (beyond the merge factor upper bound) are encountered, all segments within the level are merged.

This policy has the following settings:

Setting Description

index.merge.policy.merge_factor

Determines how often segment indices are merged by index operation. With smaller values, less RAM is used while indexing, and searches on unoptimized indices are faster, but indexing speed is slower. With larger values, more RAM is used during indexing, and while searches on unoptimized indices are slower, indexing is faster. Thus larger values (greater than 10) are best for batch index creation, and smaller values (lower than 10) for indices that are interactively maintained. Defaults to 10.

index.merge.policy.min_merge_size

A size setting type which sets the minimum size for the lowest level segments. Any segments below this size are considered to be on the same level (even if they vary drastically in size) and will be merged whenever there are mergeFactor of them. This effectively truncates the "long tail" of small segments that would otherwise be created into a single level. If you set this too large, it could greatly increase the merging cost during indexing (if you flush many small segments). Defaults to 1.6mb

index.merge.policy.max_merge_size

A size setting type which sets the largest segment (measured by total byte size of the segment’s files) that may be merged with other segments. Defaults to unbounded.

index.merge.policy.max_merge_docs

Determines the largest segment (measured by document count) that may be merged with other segments. Defaults to unbounded.

log_docedit

A merge policy that tries to merge segments into levels of exponentially increasing document count, where each level has fewer segments than the value of the merge factor. Whenever extra segments (beyond the merge factor upper bound) are encountered, all segments within the level are merged.

Setting Description

index.merge.policy.merge_factor

Determines how often segment indices are merged by index operation. With smaller values, less RAM is used while indexing, and searches on unoptimized indices are faster, but indexing speed is slower. With larger values, more RAM is used during indexing, and while searches on unoptimized indices are slower, indexing is faster. Thus larger values (greater than 10) are best for batch index creation, and smaller values (lower than 10) for indices that are interactively maintained. Defaults to 10.

index.merge.policy.min_merge_docs

Sets the minimum size for the lowest level segments. Any segments below this size are considered to be on the same level (even if they vary drastically in size) and will be merged whenever there are mergeFactor of them. This effectively truncates the "long tail" of small segments that would otherwise be created into a single level. If you set this too large, it could greatly increase the merging cost during indexing (if you flush many small segments). Defaults to 1000.

index.merge.policy.max_merge_docs

Determines the largest segment (measured by document count) that may be merged with other segments. Defaults to unbounded.

Schedulingedit

The merge schedule controls the execution of merge operations once they are needed (according to the merge policy). The following types are supported, with the default being the ConcurrentMergeScheduler.

ConcurrentMergeScheduleredit

A merge scheduler that runs merges using a separated thread, until the maximum number of threads at which when a merge is needed, the thread(s) that are updating the index will pause until one or more merges completes.

The scheduler supports the following settings:

Setting Description

index.merge.scheduler.max_thread_count

The maximum number of threads to perform the merge operation. Defaults to Math.max(1, Math.min(3, Runtime.getRuntime().availableProcessors() / 2)).

SerialMergeScheduleredit

A merge scheduler that simply does each merge sequentially using the calling thread (blocking the operations that triggered the merge, the index operation).