Why do I have a deletable file (and old segment files remain) after running optimize?
This is normal behavior on Windows whenever you also have readers (IndexReaders or IndexSearchers) open against the index you are optimizing. Lucene tries to remove old segments files once they have been merged (optimized). However, because Windows does not allow removing files that are open for reading, Lucene catches an IOException deleting these files and and then records these pending deletable files into the “deletable” file. On the next segments merge, which happens with explicit optimize() or close() calls and also whenever the IndexWriter flushes its internal RAMDirectory to disk (every IndexWriter.DEFAULT_MAX_BUFFERED_DOCS (default 10) addDocuments), Lucene will try again to delete these files (and additional ones) and any that still fail will be rewritten to the deletable file. Note that as of 2.1 the deletable file is no longer used. Instead, Lucene computes which files are no longer referenced by the index and removes them whenever a writer is created.
