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From "Mike Sokolov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-7976) Add a parameter to TieredMergePolicy to merge segments that have more than X percent deleted documents
Date Wed, 04 Oct 2017 16:41:00 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16191541#comment-16191541
] 

Mike Sokolov commented on LUCENE-7976:
--------------------------------------

Is it reasonable to modify the delete percentage in the policy while leaving the max in place?


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> Add a parameter to TieredMergePolicy to merge segments that have more than X percent
deleted documents
> ------------------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-7976
>                 URL: https://issues.apache.org/jira/browse/LUCENE-7976
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Erick Erickson
>
> We're seeing situations "in the wild" where there are very large indexes (on disk) handled
quite easily in a single Lucene index. This is particularly true as features like docValues
move data into MMapDirectory space. The current TMP algorithm allows on the order of 50% deleted
documents as per a dev list conversation with Mike McCandless (and his blog here:  https://www.elastic.co/blog/lucenes-handling-of-deleted-documents).
> Especially in the current era of very large indexes in aggregate, (think many TB) solutions
like "you need to distribute your collection over more shards" become very costly. Additionally,
the tempting "optimize" button exacerbates the issue since once you form, say, a 100G segment
(by optimizing/forceMerging) it is not eligible for merging until 97.5G of the docs in it
are deleted (current default 5G max segment size).
> The proposal here would be to add a new parameter to TMP, something like <maxAllowedPctDeletedInBigSegments>
(no, that's not serious name, suggestions welcome) which would default to 100 (or the same
behavior we have now).
> So if I set this parameter to, say, 20%, and the max segment size stays at 5G, the following
would happen when segments were selected for merging:
> > any segment with > 20% deleted documents would be merged or rewritten NO MATTER
HOW LARGE. There are two cases,
> >> the segment has < 5G "live" docs. In that case it would be merged with smaller
segments to bring the resulting segment up to 5G. If no smaller segments exist, it would just
be rewritten
> >> The segment has > 5G "live" docs (the result of a forceMerge or optimize).
It would be rewritten into a single segment removing all deleted docs no matter how big it
is to start. The 100G example above would be rewritten to an 80G segment for instance.
> Of course this would lead to potentially much more I/O which is why the default would
be the same behavior we see now. As it stands now, though, there's no way to recover from
an optimize/forceMerge except to re-index from scratch. We routinely see 200G-300G Lucene
indexes at this point "in the wild" with 10s of  shards replicated 3 or more times. And that
doesn't even include having these over HDFS.
> Alternatives welcome! Something like the above seems minimally invasive. A new merge
policy is certainly an alternative.



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