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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-2324) Per thread DocumentsWriters that write their own private segments
Date Fri, 26 Mar 2010 09:48:27 GMT


Michael McCandless commented on LUCENE-2324:

Good question...

When we buffer delete Term/Query we record the current docID as of when that delete had arrived
(so that interleaved delete/adds are resolved properly).  The docID we record is "absolute"
(ie, adds in the base flushedDocCount), so that we can decouple when deletes are materialized
(moved into the deletedDocs BitVectors) from when new segments are flushed.

I think we have a couple options.

Option 1 is to use a relative (within the current segment) docID when the deleted Term/Query/docID
is first buffered, but then make it absolute only when the segment is finally flushed.

Option 2 is to use a relative docID, but do away with the decoupling, ie force deletions to
always flush at the same time the segment is flushed.

I think I like option 1 the best -- I suspect the decoupling gains us performance as it allows
us to batch up more deletions (doing deletions in batch gets better locality, and also means
opening/closing readers left often, in the non-pooling case).

> Per thread DocumentsWriters that write their own private segments
> -----------------------------------------------------------------
>                 Key: LUCENE-2324
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: 3.1
> See LUCENE-2293 for motivation and more details.
> I'm copying here Mike's summary he posted on 2293:
> Change the approach for how we buffer in RAM to a more isolated
> approach, whereby IW has N fully independent RAM segments
> in-process and when a doc needs to be indexed it's added to one of
> them. Each segment would also write its own doc stores and
> "normal" segment merging (not the inefficient merge we now do on
> flush) would merge them. This should be a good simplification in
> the chain (eg maybe we can remove the *PerThread classes). The
> segments can flush independently, letting us make much better
> concurrent use of IO & CPU.

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