title: Namenode doesn't detect datanodes failure
body: <p>I have set up a Hadoop high available cluster including 3 nodes as masters (3 journal nodes, active namenode, and standby namenode, with no secondary namenode) and 3 datanodes.
  using commands</p>
  
  <p><code>hadoop-daemon.sh start journalnode
   hadoop-daemon.sh start namenode
   hadoop-daemon.sh start zkfc</code></p>
  
  <p>I start namenode services and using command <code>hadoop-daemon.sh start datanode</code> I start datanode services.</p>
  
  <p>The problem is when I stop a datanode intentionally using command <code>hadoop-daemon.sh stop datanode</code>, In the namenodes WebUI,(both active and standby) even after some minutes, it is still considered as an alive node and I think namenodes don't detect datanode's failure!</p>