[Solr Wiki] Update of "FAQ" by HossMan

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The "FAQ" page has been changed by HossMan:
http://wiki.apache.org/solr/FAQ?action=diff&rev1=80&rev2=81

Comment:
MS Windows, deleted files, and old segments taking up disk

  
  If you only encounter this error infrequently because of fluke situations, you'll probably be ok just ignoring it.
  
+ == Why doesn't my index directory get smaller (immediately) when i delete documents? force a merge? optimize? ==
+ 
+ Because of the "inverted index" data structure, deleting documents only annotates them as deleted for the purpose of searching.  The space used by those documents will be reclaimed when the segments they are in are merged.
+ 
+ When segments are merged (either because of the Merge Policy as documents are added, or explicitly because of a forced merge or optimize command) then Solr attempts to delete old segment files, but on some filesystems Notably in Microsoft Windows) it is not possible to delete a file while the file is open for reading (Which is usually true since Solr is still serving requests against the old segments until the new Searcher is ready and has it's caches warmed).  When this happens, the older segment files are left on disk, and Solr will re-attempt to delete them later the next time a merge happens. 
+ 
  = Searching =
  
  == Why Aren't Scores returned as a percentage? How Do I normalize Scores? ==