在Workstation12 中创建了3个安装了CentOS7.0的虚拟机，其中一个作为主节点master，两个作为从节点slave，各种准备环境搞好后安装了Hadoop2.6.4，各种配置文件啥的都弄好了，初始化也successful了，但是在主结点上执行启动命令后，却在master:50070和master:8088上看不到datanode，在网上尝试了各种方法都没起作用，最终应该是修改了/etc/hosts文件才搞定。总结下，发现问题最好的方法就是查看日志，根据日志提示找问题。
下面是查看slave结点上的datanode日志：发现是无法连接master上的9000端口。然后telnet master 9000发现refused connection，终于找到了问题，网上有说要关闭防火墙，但是CentOS7.0的默认好像没开，不过还是在3个结点上执行了关闭防火墙的命令，都不行，最后把/etc/hosts文件里的前两行注释掉才好了。
slave结点上的datanode日志：
2016-08-02 08:17:17,570 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 0 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:18,572 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 1 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:19,576 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 2 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:20,578 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 3 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:21,580 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 4 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:22,584 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 5 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:23,587 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 6 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:24,589 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 7 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:25,594 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 8 time(s); retry policy is RetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:26,596 INFO org.apache.hadoop.ipc.Client: Retrying connect to server: master/192.168.130.135:9000. Already tried 9 time(s); retry policy isRetryUpToMaximumCountWithFixedSleep(maxRetries=10, sleepTime=1000 MILLISECONDS)
2016-08-02 08:17:26,598 WARN org.apache.hadoop.hdfs.server.datanode.DataNode: Problem connecting to server: master/192.168.130.135:9000
总结一下几种排查方法：
1、首先排查4个配置文件项（core-site.xml、hdfs-site.xml、mapred-site.xml、yarn-site.xml）有没错误；
2、关闭防火墙，
systemctl stop firewalld.service # 关闭firewall
systemctl disable firewalld.service # 禁止firewall开机启动
3、telnet 主节点的9000端口（core-site.xml配置文件中的配置的端口）；
4、tmp、data、name文件权限问题；
5、有的说是三个结点的uuid不一致；
·········
注：最重要的是要重新格式化，在重新格式化前要把 tmp 和 hdfs/name、hdfs/data 下的文件都删除掉，这样也是为避免uuid不一致。