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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2680) Improve how IndexWriter flushes deletes against existing segments
Date Wed, 24 Nov 2010 18:55:13 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2680?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12935441#action_12935441
] 

Michael McCandless commented on LUCENE-2680:
--------------------------------------------

What a nice small patch :)

I think the getDeletesSegmentsForward shouldn't be folding in the
deletesInRAM?  Ie, that newly flushed info will have carried over the
previous deletes in RAM?

I think pushDeletes/pushSegmentDeletes should be merged, and we should
nuke DocumentsWriter.deletesFlushed?  Ie, we should push directly from
deletesInRAM to the new SegmentInfo?  EG you are now pushing all
deletesFlushed into the new SegmentInfo when actually you should only
push the deletes for that one segment.

We shouldn't do the remap deletes anymore.  We can remove
DocumentsWriter.get/set/updateFlushedDocCount too.

Hmm... so what are we supposed to do if someone opens IW, does a bunch
of deletes, then commits?  Ie flushDocs is false, so there's no new
SegmentInfo.  I think in this case we can stick the deletes against
the last segment in the index, with the docidUpto set to the maxDoc()
of that segment?


> Improve how IndexWriter flushes deletes against existing segments
> -----------------------------------------------------------------
>
>                 Key: LUCENE-2680
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2680
>             Project: Lucene - Java
>          Issue Type: Improvement
>            Reporter: Michael McCandless
>             Fix For: 4.0
>
>         Attachments: LUCENE-2680.patch, LUCENE-2680.patch, LUCENE-2680.patch, LUCENE-2680.patch,
LUCENE-2680.patch, LUCENE-2680.patch, LUCENE-2680.patch, LUCENE-2680.patch, LUCENE-2680.patch,
LUCENE-2680.patch
>
>
> IndexWriter buffers up all deletes (by Term and Query) and only
> applies them if 1) commit or NRT getReader() is called, or 2) a merge
> is about to kickoff.
> We do this because, for a large index, it's very costly to open a
> SegmentReader for every segment in the index.  So we defer as long as
> we can.  We do it just before merge so that the merge can eliminate
> the deleted docs.
> But, most merges are small, yet in a big index we apply deletes to all
> of the segments, which is really very wasteful.
> Instead, we should only apply the buffered deletes to the segments
> that are about to be merged, and keep the buffer around for the
> remaining segments.
> I think it's not so hard to do; we'd have to have generations of
> pending deletions, because the newly merged segment doesn't need the
> same buffered deletions applied again.  So every time a merge kicks
> off, we pinch off the current set of buffered deletions, open a new
> set (the next generation), and record which segment was created as of
> which generation.
> This should be a very sizable gain for large indices that mix
> deletes, though, less so in flex since opening the terms index is much
> faster.

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