title: Couldn't start hadoop datanode normally
body: <p>I'd started datanode successfully before, but when I tried today it shows the following info.  It sounds like I have not mkdir the /home/hadoop/appdata/hadoopdata directory,but I confirmed that the directory already exists in my computer. So what's the problem? Why I couldn't start the datanode normally?</p>
  
  <p>Ex:I've tried to delete /home/hadoop/appdata/ and mkdir a new one, but it still doesn't work.</p>
  
  <p>I've also deleted /home/hadoop/tmp/hadoop_tmp and mkdir a new one, it still doesn't work...</p>
  
  <pre><code>2014-03-04 09:30:30,106 WARN org.apache.hadoop.metrics2.impl.MetricsSystemImpl: Source name ugi already exists!
  
  2014-03-04 09:30:30,349 INFO org.apache.hadoop.hdfs.server.common.Storage: Cannot access storage directory /home/hadoop/appdata/hadoopdata
  
  2014-03-04 09:30:30,350 INFO org.apache.hadoop.hdfs.server.common.Storage: Storage directory /home/hadoop/appdata/hadoopdata does not exist
  
  2014-03-04 09:30:30,453 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: 
  
  java.io.IOException: All specified directories are not accessible or do not exist.
  
      at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:139)
      at org.apache.hadoop.hdfs.server.datanode.DataNode.startDataNode(DataNode.java:414)
      at org.apache.hadoop.hdfs.server.datanode.DataNode.&lt;init&gt;(DataNode.java:321)
      at org.apache.hadoop.hdfs.server.datanode.DataNode.makeInstance(DataNode.java:1712)
      at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1651)
      at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1669)
      at org.apache.hadoop.hdfs.server.datanode.DataNode.secureMain(DataNode.java:1795)
  </code></pre>