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Merge
editMerge
editA 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.
Policy
editThe 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
.
tiered
editMerges 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 themax_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 to2.0
. -
index.compound_format
-
Should the index be stored in compound format or not. Defaults to
false
. Seeindex.compound_format
in Index Settings.
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_size
editA 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 |
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 |
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_doc
editA 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 |
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 |
index.merge.policy.max_merge_docs |
Determines the largest segment (measured by document count) that may be merged with other segments. Defaults to unbounded. |
Scheduling
editThe 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
.
ConcurrentMergeScheduler
editA 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
|
SerialMergeScheduler
editA merge scheduler that simply does each merge sequentially using the calling thread (blocking the operations that triggered the merge, the index operation).