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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-2773) Don't create compound file for large segments by default
Date Wed, 24 Nov 2010 13:34:13 GMT


Michael McCandless commented on LUCENE-2773:

bq. But, lets say you have a 100gb size index, you will end up with 50 segments, no?

So, in the "worst" case, yes... but in the "best" case you could end up with 5 segments. 
This threshold applies to segments-to-be-merged, so if you have a bunch of segments just under
2 GB, they will get merged and make a nearly 20 GB segment, which would then be merged.

So basically this setting is terribly coarse.  I think this can be improved (eg something
the lines of BalancedSegmentMergePolicy), perhaps by merging (much) fewer than mergeFactor
segments at a time to keep the immense merges "smallish".  But until we cutover to a better
merge policy, we're stuck with this coarse setting...

So... maybe 5 GB?

But, on the deletes... in 2.9.x and 3.0.x we do NOT in fact take deletions into account by
default; I think, along with this change, we should also fix 2.9.x and 3.0.x to take deletions
into account.

> Don't create compound file for large segments by default
> --------------------------------------------------------
>                 Key: LUCENE-2773
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 2.9.4, 3.0.3, 3.1, 4.0
>         Attachments: LUCENE-2773.patch
> Spinoff from LUCENE-2762.
> CFS is useful for keeping the open file count down.  But, it costs
> some added time during indexing to build, and also ties up temporary
> disk space, causing eg a large spike on the final merge of an
> optimize.
> Since MergePolicy dictates which segments should be CFS, we can
> change it to only build CFS for "smallish" merges.
> I think we should also set a maxMergeMB by default so that very large
> merges aren't done.

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