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https://issues.apache.org/jira/browse/LUCENE-2814?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12971234#action_12971234
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Michael McCandless commented on LUCENE-2814:
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bq. Does this mean we will be able to merge non consecutive segments?
Well... it's rather decoupled from this issue. Meaning, we were always free to
allow non-contiguous merges, and segment would simply have forced the merge of
the doc stores in that case.
But, yes, because of the same reasons above, such a merge policy is much lower
perf hit than it used to be. And so now it is again compelling to explore
this.... we just have to go and do LUCENE-1076. I think it should not be hard;
we just have to relax the merge methods in IW that check that the segments are
contiguous.
This added freedom for the merge policy would be compelling, ie, it can then do
a much better job cherry picking similarly sized segments for merging. I think
it'd also simplify merge policies like BalancedSegMP, eg (I think?) no longer
requiring a viterbi search to pick the best merges.
> stop writing shared doc stores across segments
> ----------------------------------------------
>
> Key: LUCENE-2814
> URL: https://issues.apache.org/jira/browse/LUCENE-2814
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Affects Versions: 3.1, 4.0
> Reporter: Michael McCandless
> Assignee: Michael McCandless
>
> Shared doc stores enables the files for stored fields and term vectors to be
> shared across multiple segments. We've had this optimization since 2.1 I
> think.
> It works best against a new index, where you open an IW, add lots of docs,
> and then close it. In that case all of the written segments will reference
> slices a single shared doc store segment.
> This was a good optimization because it means we never need to merge these
> files. But, when you open another IW on that index, it writes a new set of
> doc stores, and then whenever merges take place across doc stores, they must
> now be merged.
> However, since we switched to shared doc stores, there have been two
> optimizations for merging the stores. First, we now bulk-copy the bytes in
> these files if the field name/number assignment is "congruent". Second, we
> now force congruent field name/number mapping in IndexWriter. This means
> this optimization is much less potent than it used to be.
> Furthermore, the optimization adds *a lot* of hair to
> IndexWriter/DocumentsWriter; this has been the source of sneaky bugs over
> time, and causes odd behavior like a merge possibly forcing a flush when it
> starts. Finally, with DWPT (LUCENE-2324), which gets us truly concurrent
> flushing, we can no longer share doc stores.
> So, I think we should turn off the write-side of shared doc stores to pave
> the path for DWPT to land on trunk and simplify IW/DW. We still must support
> reading them (until 5.0), but the read side is far less hairy.
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