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1. A index batch write (IndexWriter.close()) will result in creation of a new set of segment files. This could trigger a segment merge operation which could be resource intensive (think compaction in LSM).
2. A large number of segments would increase search latency.
3. Lucene buffers documents in memory (writer.setMaxBufferedDocs and writer.setRAMBufferSizeMB).  More RAM size means larger segments means less merging later.
4. Searchers will not set see any changes till IndexWriter is closed.
5. Optimizations
   1. If a large amount of data is to be indexed, then it is better to build N smaller indexes and combine using writer.addIndexesNoOptimize
      

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