http://hadoop.apache.org/docs/r2.7.3/hadoop-project-dist/hadoop-common/ClusterSetup.html
bin/hdfs namenode -format 如果出现 Unsupported major.minor version 51.0
则需要在 etc/hadoop_env.sh 中 export JAVA_HOME=xxxxxjava7 目录 java1.7目录
http://hadoop.apache.org/docs/r2.7.3/hadoop-project-dist/hadoop-common/SingleCluster.html