Datanode process not running in Hadoop

You need to do something like this:

• bin/stop-all.sh (or stop-dfs.sh and stop-yarn.sh in the 2.x serie)
• rm -Rf /app/tmp/hadoop-your-username/*
• bin/hadoop namenode -format (or hdfs in the 2.x serie)

the solution was taken from:
http://pages.cs.brandeis.edu/~cs147a/lab/hadoop-troubleshooting/. Basically it consists in restarting from scratch, so make sure you won’t loose data by formating the hdfs.