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https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16213175#comment-16213175
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Shawn Heisey commented on LUCENE-7976:
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Very interesting discussion and problem.
If we ignore for a moment what TMP actually does, and back up to the design
intent when the policy was made ... what would the designer have wanted to
happen in the case of a segment that's considerably larger than the configured
max size? Took me a while to find the right issue, which is LUCENE-854, work
by [~mikemccand].
I suspect that the current behavior, where a segment that's 20 times larger
than the configured max segment size is ineligible for automatic merging until
97.5 percent deleted docs, was not actually what was desired. Indexes with a
segment like might not have even been considered when TMP was new. I don't see
anything in LUCENE-854 that mentions it. I haven't checked all the later
issues where changes to TMP were made.
So, how do we deal with this problem? I see three options. We can design an
entirely new policy, and if its behavior becomes preferred, consider changing
the default at a later date. We can change TMP so it behaves better with very
large segments with no change in user code or config. We can add Erick's
suggested option. For any of these options, improved documentation is a must.
The second option (and the latter half of the first option) carries one risk
factor I can think of -- users complaining about new behavior in a similar
manner to what I've heard about when the default directory was changed to MMAP.
> 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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