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https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16427642#comment-16427642
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Erick Erickson commented on LUCENE-7976:
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This is coming together, here's a preliminary patch. It has nocommits and
several rough spots/hard-coded numbers, code commented out etc.
I'm putting it up in case anyone interested in this wants to take a look at the
_approach_ and poke holes in it. Please raise any concerns but also please
don't spend a lot of time on the details before I wrap up things I _know_ will
need addressing.
Current state:
0> refactors a bit of findMerges to gather the stats into a separate class as
that method was getting quite hard follow. I haven't made use of that new class
in forceMerge or expungeDeletes yet.
1> forceMerge and expungeDeletes respect maxMergedSegmentSizeMB
2> regular merging will do "singleton merges" on overly-large segments when
they're more than 20% deleted docs. 20% is completely arbitrary, don't quite
know the correct numbers yet. That handles the case of a single-segment
optimize not getting merged away for a long time.
3> forceMerge will purge all deleted docs. It tries to assemble max-sized
segments. Any segments where the live docs are larger than
maxMergedSegmentSizeMB get a singleton merge.
4> fixes the annoying bit where segments reported on the admin UI are
improperly proportioned
5> expungeDeletes now tries to assemble max sized segments from all segments
with > 10% deleted docs. If a segment has > 10% deleted docs _and_ it's
liveDocs > maxMergedSegmentSizeMB it gets a singleton merge.
What's left to do:
1> more rigorous testing. So far I've just been looking at the admin UI
segments screen and saying "that looks about right".
2> Normal merging rewrites the largest segment too often until it gets to max
segment size. I think it also merges dissimilar-sized segments too often.
3> compare the total number of bytes written for one of my test runs between
the old and new versions. I'm sure this does more writing, just not sure how
much.
4> allow forceMerge to merge down to one segment without having to change
solrconfig.xml.
5> perhaps refactor, findMerges, forceMerge and findForcedDeletesMerges to make
use of common code.
6> ????
> Make TieredMergePolicy respect maxSegmentSizeMB and allow singleton merges of
> very large segments
> -------------------------------------------------------------------------------------------------
>
> Key: LUCENE-7976
> URL: https://issues.apache.org/jira/browse/LUCENE-7976
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
> Attachments: LUCENE-7976.patch, LUCENE-7976.patch
>
>
> 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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