title: Block assignation using network topology
body: <p>If I well understood principles when applying network topology, blocks are written:</p>
  
  <ul>
  <li>On the client server if hosting a datanode</li>
  <li>On a second server defined on a different rack</li>
  <li>On a third server defined on the same rack as #2</li>
  </ul>
  
  <p>Is this policy configurable or it is “hard-written” in class? Of course, I do not want to modify any class by myself…</p>
  
  <p>Basically, I would like to:</p>
  
  <ol>
  <li><p>Take into account datacenter (according to what I read, HDFS do not care datacenters even if using network topology)</p></li>
  <li><p>Force the write in 3 distinct racks</p></li>
  </ol>
  
  <p>How do I do that?</p>