1.Minimum required Java version increased from Java 7 to Java 8
java最低支持版本变成java8
2.Support for erasure encoding in HDFS
关于EC技术，参考http://blog.csdn.net/hellojoy/article/details/54744353
3.YARN Timeline Service v.2
参考http://han-zw.iteye.com/blog/2332924
4. Shell script rewrite
5. Support for more than 2 NameNodes
主备nn变为支持多个个nn，从最多只允许一个nn坏掉，到支持多个个nn坏掉仍保持集群健康