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https://issues.apache.org/jira/browse/LUCENE-8263?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16543616#comment-16543616
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Marc Morissette commented on LUCENE-8263:
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I would like to argue against a 20% floor.
Some indexes contain documents of wildly different sizes with the larger
documents experiencing much higher turnover. I have seen indexes with around
20% deletions that were more than 2x their optimized size because of this
phenomenon.
I such situations, deletesPctAllowed around 10-15% would make a lot of sense. I
say keep the floor at 10%.
> Add indexPctDeletedTarget as a parameter to TieredMergePolicy to control more
> aggressive merging
> ------------------------------------------------------------------------------------------------
>
> Key: LUCENE-8263
> URL: https://issues.apache.org/jira/browse/LUCENE-8263
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
> Attachments: LUCENE-8263.patch
>
>
> Spinoff of LUCENE-7976 to keep the two issues separate.
> The current TMP allows up to 50% deleted docs, which can be wasteful on large
> indexes. This parameter will do more aggressive merging of segments with
> deleted documents when the _total_ percentage of deleted docs in the entire
> index exceeds it.
> Setting this to 50% should approximate current behavior. Setting it to 20%
> caused the first cut at this to increase I/O roughly 10%. Setting it to 10%
> caused about a 50% increase in I/O.
> I was conflating the two issues, so I'll change 7976 and comment out the bits
> that reference this new parameter. After it's checked in we can bring this
> back. That should be less work than reconstructing this later.
> Among the questions to be answered:
> 1> what should the default be? I propose 20% as it results in significantly
> less space wasted and helps control heap usage for a modest increase in I/O.
> 2> what should the floor be? I propose 10% with _strong_ documentation
> warnings about not setting it below 20%.
> 3> should there be two parameters? I think this was discussed somewhat in
> 7976. The first cut at this used this number for two purposes:
> 3a> the total percentage of deleted docs index-wide to trip this trigger
> 3b> the percentage of an _individual_ segment that had to be deleted if the
> segment was over maxSegmentSize/2 bytes in order to be eligible for merging.
> Empirically, using the same percentage for both caused the merging to hover
> around the value specified for this parameter.
> My proposal for <3> would be to have the parameter do double-duty. Assuming
> my preliminary results hold, you specify this parameter at, say, 20% and once
> the index hits that % deleted docs it hovers right around there, even if
> you've forceMerged earlier down to 1 segment. This seems in line with what
> I'd expect and adding another parameter seems excessively complicated to no
> good purpose. We could always add something like that later if we wanted.
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