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From "Erick Erickson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-7976) Make TieredMergePolicy respect maxSegmentSizeMB and allow singleton merges of very large segments
Date Tue, 01 May 2018 05:25:00 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-7976?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16459460#comment-16459460
] 

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

OK, I think this is getting quite close. The place I'm most uncomfortable is in findForcedMerges.
See the TODO around line 778 plus the fact that there's a bunch of special handling depending
on whether we're forceMerging to 1 segment, a max count or respecting max segments size. I
want to make one more pass though it, there _ought_ to be a more organized way of doing this.

Also, when giving a maximum number of segments, we calculate the ideal segment size and then
I increase it by 25% on the theory that segments won't fit perfectly, so allow some extra
space in hopes that all the segments will be fit into the max segment count the first time
through.

However, there are still edge cases I think where that won't necessarily work on the first
pass, especially if there are very many segments. In that case, at the very end there's a
loop essentially saying "go through as many iterations as necessary increasing the max segment
size by 25% each time until you can fit them all in the required number of segments". This
really means that in this case you could rewrite the entire index twice. Is that OK? I don't
want to spend a lot of time on this case though, it seems to me that if you specify this you'll
have to live with this edge case.

[~mikemccand] There's another departure from the old process here. If there are multiple passes
for forceMerge, I keep returning null in until there aren't any current merges running involving
the original segments. Is there any real point in trying to create another merge specification
if there are merges from previous passes going on? This is around line 684.

I beasted all of Mikes failures 120 times along with TestTieredMergePolicy and no failures.
All tests pass and precommit worked.

Then, of course I made one tiny change so I'll have to go 'round that testing again. I also
have to make another couple of runs at counting the total bytes written to see if something
crept in.

That said, I think this is the last major rearrangement I want to do. If my additional testing
succeeds and there are no objections, I'll probably commit sometime this weekend.

Thanks to all who've looked at this!

> 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, LUCENE-7976.patch, LUCENE-7976.patch,
LUCENE-7976.patch, 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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