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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-845) If you "flush by RAM usage" then IndexWriter may over-merge
Date Fri, 17 Aug 2007 20:46:31 GMT


Michael McCandless commented on LUCENE-845:

Agreed.  OK, I think this is a dead end: it adds complexity and won't
help in "typical" uses of Lucene.

So ... my plan of action is to assess the "actual" O(N^2) cost for
IndexWriter to keep the tail short, add a parameter to LogMergePolicy
so that it "floors" the level and always merges segments less than
this floor together, despite the O(N^2) cost.  And then pick a
reasonable default for this floor.

> If you "flush by RAM usage" then IndexWriter may over-merge
> -----------------------------------------------------------
>                 Key: LUCENE-845
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Index
>    Affects Versions: 2.1
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>            Priority: Minor
>         Attachments: LUCENE-845.patch
> I think a good way to maximize performance of Lucene's indexing for a
> given amount of RAM is to flush (writer.flush()) the added documents
> whenever the RAM usage (writer.ramSizeInBytes()) has crossed the max
> RAM you can afford.
> But, this can confuse the merge policy and cause over-merging, unless
> you set maxBufferedDocs properly.
> This is because the merge policy looks at the current maxBufferedDocs
> to figure out which segments are level 0 (first flushed) or level 1
> (merged from <mergeFactor> level 0 segments).
> I'm not sure how to fix this.  Maybe we can look at net size (bytes)
> of a segment and "infer" level from this?  Still we would have to be
> resilient to the application suddenly increasing the RAM allowed.
> The good news is to workaround this bug I think you just need to
> ensure that your maxBufferedDocs is less than mergeFactor *
> typical-number-of-docs-flushed.

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