[
http://issues.apache.org/jira/browse/LUCENE-565?page=comments#action_12459506 ]
Ning Li commented on LUCENE-565:
--------------------------------
Here is the design overview. Minor changes were made because of lock-less
commits.
In the current IndexWriter, newly added documents are buffered in ram in the
form of one-doc segments.
When a flush is triggered, all ram documents are merged into a single segment
and written to disk.
Further merges of disk segments may be triggered.
NewIndexModifier extends IndexWriter and supports document deletion in addition
to document addition.
NewIndexModifier not only buffers newly added documents in ram, but also
buffers deletes in ram.
The following describes what happens when a flush is triggered:
1 merge ram documents into one segment and written to disk
do not commit - segmentInfos is updated in memory, but not written to disk
2 for each disk segment to which a delete may apply
open reader
delete docs*, write new .delN file (* Care is taken to ensure that an
interleaved sequence of
inserts and deletes for the same document are properly serialized.)
close reader, but do not commit - segmentInfos is updated in memory, but
not written to disk
3 commit - write new segments_N to disk
Further merges for disk segments work the same as before.
As an option, we can cache readers to minimize the number of reader
opens/closes. In other words,
we can trade memory for better performance. The design would be modified as
follows:
1 same as above
2 for each disk segment to which a delete may apply
open reader and cache it if not already opened/cached
delete docs*, write new .delN file
3 commit - write new segments_N to disk
The logic for disk segment merge changes accordingly: open reader if not
already opened/cached;
after a merge is complete, close readers for the segments that have been merged.
> Supporting deleteDocuments in IndexWriter (Code and Performance Results
> Provided)
> ---------------------------------------------------------------------------------
>
> Key: LUCENE-565
> URL: http://issues.apache.org/jira/browse/LUCENE-565
> Project: Lucene - Java
> Issue Type: Bug
> Components: Index
> Reporter: Ning Li
> Attachments: NewIndexModifier.Sept21.patch, perf-test-res.JPG,
> perf-test-res2.JPG, perfres.log, TestBufferedDeletesPerf.java
>
>
> Today, applications have to open/close an IndexWriter and open/close an
> IndexReader directly or indirectly (via IndexModifier) in order to handle a
> mix of inserts and deletes. This performs well when inserts and deletes
> come in fairly large batches. However, the performance can degrade
> dramatically when inserts and deletes are interleaved in small batches.
> This is because the ramDirectory is flushed to disk whenever an IndexWriter
> is closed, causing a lot of small segments to be created on disk, which
> eventually need to be merged.
> We would like to propose a small API change to eliminate this problem. We
> are aware that this kind change has come up in discusions before. See
> http://www.gossamer-threads.com/lists/lucene/java-dev/23049?search_string=indexwriter%20delete;#23049
> . The difference this time is that we have implemented the change and
> tested its performance, as described below.
> API Changes
> -----------
> We propose adding a "deleteDocuments(Term term)" method to IndexWriter.
> Using this method, inserts and deletes can be interleaved using the same
> IndexWriter.
> Note that, with this change it would be very easy to add another method to
> IndexWriter for updating documents, allowing applications to avoid a
> separate delete and insert to update a document.
> Also note that this change can co-exist with the existing APIs for deleting
> documents using an IndexReader. But if our proposal is accepted, we think
> those APIs should probably be deprecated.
> Coding Changes
> --------------
> Coding changes are localized to IndexWriter. Internally, the new
> deleteDocuments() method works by buffering the terms to be deleted.
> Deletes are deferred until the ramDirectory is flushed to disk, either
> because it becomes full or because the IndexWriter is closed. Using Java
> synchronization, care is taken to ensure that an interleaved sequence of
> inserts and deletes for the same document are properly serialized.
> We have attached a modified version of IndexWriter in Release 1.9.1 with
> these changes. Only a few hundred lines of coding changes are needed. All
> changes are commented by "CHANGE". We have also attached a modified version
> of an example from Chapter 2.2 of Lucene in Action.
> Performance Results
> -------------------
> To test the performance our proposed changes, we ran some experiments using
> the TREC WT 10G dataset. The experiments were run on a dual 2.4 Ghz Intel
> Xeon server running Linux. The disk storage was configured as RAID0 array
> with 5 drives. Before indexes were built, the input documents were parsed
> to remove the HTML from them (i.e., only the text was indexed). This was
> done to minimize the impact of parsing on performance. A simple
> WhitespaceAnalyzer was used during index build.
> We experimented with three workloads:
> - Insert only. 1.6M documents were inserted and the final
> index size was 2.3GB.
> - Insert/delete (big batches). The same documents were
> inserted, but 25% were deleted. 1000 documents were
> deleted for every 4000 inserted.
> - Insert/delete (small batches). In this case, 5 documents
> were deleted for every 20 inserted.
> current current new
> Workload IndexWriter IndexModifier IndexWriter
> -----------------------------------------------------------------------
> Insert only 116 min 119 min 116 min
> Insert/delete (big batches) -- 135 min 125 min
> Insert/delete (small batches) -- 338 min 134 min
> As the experiments show, with the proposed changes, the performance
> improved by 60% when inserts and deletes were interleaved in small batches.
> Regards,
> Ning
> Ning Li
> Search Technologies
> IBM Almaden Research Center
> 650 Harry Road
> San Jose, CA 95120
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