title: checkpointing: Is fsimage always copied from namenode
body: <p>In checkpointing, Definitive Guides say</p>
  
  <pre><code>1. The secondary asks the primary to roll its edits file, so new edits goes to a new file
  2. The secondary retrieves fsimage and edits from primary (using HTTP GET)
  </code></pre>
  
  <p>and at the end of checkpointing secondary namenode sends updated fsimage to namenode.</p>
  
  <p>Now secondary namenode has latest fsimage, in next checkpointing will secondary namenode again copy fsimage from namenode?? If so why?? can't it simply compare two using checksum</p>