hadoop 启动时namenode和datanode可以启动，使用jps命令也可以看到进程，但是在浏览器中输入master:50070却没有显示datanode
查看datanode的log日志：
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
-- ::, INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.200.128:. Already tried time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=, sleepTime= SECONDS)
2016-05-06 13:22:50,240 INFO org.apache.hadoop.ipc.RPC: Server at master/192.168.200.128:9000 not available yet, Zzzzz...
发现连接不上namenode。
所以在master（也就是namenode）中配置etc/hosts文件，将120.0.0.1 master注释掉，然后添加192.168.200.131 master（如果不注释可能会优先匹配），重新hadoop namenode -format 然后启动就可以了。