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From "Jason Rutherglen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2680) Improve how IndexWriter flushes deletes against existing segments
Date Wed, 10 Nov 2010 23:54:14 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2680?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12930845#action_12930845
] 

Jason Rutherglen commented on LUCENE-2680:
------------------------------------------

I think I've isolated this test failure to recording the applied deletes.
Because we're using last segment index/info, I was adding deletes that may
or may not have been applied to a particular segment to the last segment
info. I'm not sure what to do in this case as if we record the applied
terms per segment, but keep the pending terms in last segment info, we're
effectively not gaining anything from using last segment info because
we're then recording all of the terms per-segment anyways. In fact, this
is how I've isolated that this is the issue, I simply removed the usage of
last segment info, and instead went to maintaining pending deletes
per-segment. I'll give it some thought.

In conclusion, when deletes are recorded per-segment with no last segment
info, the test passes after 200 times. 

> 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
>
>
> 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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