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https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16213213#comment-16213213
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Yonik Seeley commented on LUCENE-7976:
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It's not a bug, it's a feature. It's an explicit request that *may* or *may
not* be a mistake on the part of the user, and it can certainly be a judgement
call. Given that it's explicit and we don't know if is advisable or not, we
should do what is requested.
The root cause of the problem here seems to be that we have only one variable
(maxSegmentSize) and multiple use-cases we're forcing on it:
1) the max segment size that can be create automatically just by adding
documents (this is maxSegmentSize currently)
2) the max segment size that can *ever* be created, even through explicit
forceMerge (this is more for Tim's usecase... certain filesystems or transports
may break if you go over certain limits)
There is no variable/setting for #2 currently, but we should not re-use the
current maxSegmentSize for this as it conflates the two use-cases.
Perhaps something like hardMaxSegmentSize or something?
> 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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