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From "Erick Erickson (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 Fri, 20 Oct 2017 00:12:00 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16211959#comment-16211959
] 

Erick Erickson commented on LUCENE-7976:
----------------------------------------

What Yonik said.

+1 to working up a patch. I actually think this is pretty important.

bq: Also keep a lower bound check so users can't set a delete threshold below 20%.

Don't know. This is another arbitrary decision that may or may not apply. Perhaps a _strongly_
worded suggestion that this be the lower bound and a WARN message on startup if they specify
< 20%? 20% of a 10TB (aggregate across shards) index is still a lot. I don't have strong
feelings here though.

Hmmm. If you have a setter like setMaxDeletePctBeforeSingletonMerge(double pct) then through
reflection you can just specify
<double name="maxDeletePctBeforeSingletonMerge>5</double>
in the merge policy and it'll automagically get picked up. Then we don't advertise it, making
it truly expert.... 

bq:  ...pick a segment which is 5G in size and more than the threshold deletes...

Minor refinement. Pick a segment > 2.5G "live" documents and > X% deleted docs and merge
it. That way we merge a 4G segment with 20% deleted into a 3.2G segment. Rinse and repeat
until it had < 2.5G live docs at which point it's eligible for regular merging.

The sweet thing about this is that it would allow users to _recover_ from an optimize. Currently
if they do hit that big red button and optimize they can't recover deleted documents until
that single huge segment has < 2.5G live docs. Something like this will keep rewriting
that segment into smaller and smaller (though still large) segments and it'll eventually disappear.
Mind you it'll be painful, but at least it'll eventually get there.

I'm not sure whether to make this behavior the default for  TieredMergePolicy or not. Other
than rewriting very large segments, the current policy is essentially this with X being 50%.
Despite my comments about keeping reflection above, WDYT about just making this explicit?
That is, default a parameter like "largeSegmentMaxDeletePct" to 50?

And for a final thought, WDYT about Mike's idea of making optimize/forcemerge/expungeDeletes
respect maxSegemntSize? I think we still need to rewrite segments as this JIRA proposes since
the current policy can hover around 50%. I'm lukewarm to making optimize respect max segment
size since it would change that behavior, but I don't have strong feelings on it.

> 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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