title: Does the MongoDB "extent was empty" indicate a missing index?
body: <p>I have seen a lot of questions about the MongoDB log entry such as</p>
  
  <pre><code>Tue Mar 11 09:42:46.995 [conn1044] info DFM::findAll(): extent 0:2f000 was empty, skipping ahead. ns:mydb.mycoll
  </code></pre>
  
  <p>I know this is because there is allocated empty space in the MongoDB datafiles caused by deleting previous data. Commenters usually say you can ignore this message or compact the DB if you want to recover the diskspace.</p>
  
  <p>But why is MongoDB looking in an empty spot anyway? Does this message also indicate that I am missing an index and that the DB is scanning the datafiles?</p>