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
. The difference this time is that we have implemented the change and
tested its performance, as described below.
We propose adding a "deleteDocuments(Term term)" method to IndexWriter.
Using this method, inserts and deletes can be interleaved using the same
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 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.
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.
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