Hadoop Namenode Not Starting
My namenode was not starting because I had the wrong host configured in yarn-site.xml, mapred-site.xml, and core-site.xml.When you're running start-dfs.sh on your namenode, ensure the line that says "starting namenode" shows "/usr/local/hadoop/logs/hadoop-ubuntu-namenode-<your namenode's hostname>.out" in the output. This is how you know your configuration is correct.
You can check your server's hostname on Ubuntu by running "echo $(hostname)".