Christine Poerschke created LUCENE-7523:
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Summary: UpgradeIndexMergePolicy: beyond one-off use, monster
segment avoidance
Key: LUCENE-7523
URL: https://issues.apache.org/jira/browse/LUCENE-7523
Project: Lucene - Core
Issue Type: Task
Reporter: Christine Poerschke
Priority: Minor
(Was looking at UpgradeIndexMergePolicy as part of SOLR-9648 and came up with
these possibilities here, what do people think?)
Currently one probably would not configure use of the
{{UpgradeIndexMergePolicy}} (UIMP) permanently since
[findForcedMerges|https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/index/UpgradeIndexMergePolicy.java#L74]
becomes a no-op after all segments have been upgraded.
* How about adding an optional {{fallbackToInnerAfterUpgrade}} flag? That way
UIMP.findForcedMerges could fallback to its inner/delegate merge policy's
findForcedMerges call after all segments have been upgraded.
Currently UIMP.findForcedMerges identifies all the segments to be upgraded and
then asks its inner/delegate merge policy to come up with a MergeSpecification
for those segments. If the inner/delegate merge policy does not supply a merge
for all the segments to be upgraded then UIMP merges the remaining segments
into _one_ new segment. That extra new segment could be quite a large 'monster'
segment.
* How about adding an optional {{upgradeUnmergedSegmentsIndividually}} flag?
That way UIMP.findForcedMerges could upgrade (but not merge) the remaining
segments.
* Or indeed should 'upgradeUnmergedSegmentsIndividually' be the default
behaviour?
Noticed whilst looking at the code:
*
[UIMP.findMerges|https://github.com/apache/lucene-solr/blob/master/lucene/core/src/java/org/apache/lucene/index/UpgradeIndexMergePolicy.java#L69]
does not pass the mergeTrigger to the inner/delegate merge policy.
** If we can figure out why that is, let's add a comment to say why that is.
** Understanding why that is would also be needed before proceeding with beyond
one-off use of UIMP.
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