title: Are MongoDB indexes persistent across restarts?
body: <p>Referencing the guide here, <a href="http://docs.mongodb.org/manual/core/indexes/" rel="nofollow">http://docs.mongodb.org/manual/core/indexes/</a> I cannot tell if Mongo indexes for fields are stored persistently.  </p>
  
  <p>If <code>ensureIndex()</code> is called (and completes) within an application using <code>MongoDB</code>, what happens if: </p>
  
  <ol>
  <li>The application using <code>MongoDB</code> is restarted.  Will a subsequent call to <code>ensureIndex()</code> cause a complete reindex?</li>
  <li>The <code>MongoDB</code> server is restarted.  Would a later call of <code>ensureIndex()</code> from a client application rebuild?</li>
  <li>Is any of this affected by having multiple client sessions?  I assume indexing is global across the entire collection per the documentation: <code>"MongoDB defines indexes on a per-collection level."</code></li>
  </ol>