首先，在搭建Hadoop 的3节点集群时，安装基本的步骤，配置好以下几个文件
1.core-site.xml
2.hadoop-env.sh
3.hdfs-site.xml
4.yarn-env.sh
5.yarn-site.xml
6.slaves
之后就是格式化NameNode节点，
[root@spark1 hadoop]# hdfs namenode -format
启动hdfs集群
[root@spark1 hadoop]# start-dfs.sh
查询各个节点是否运行成功。
spark1 :
[root@spark1 hadoop]# jps
5575 SecondaryNameNode
5722 Jps
5443 DataNode
5336 NameNode
spark2:
[root@spark2 hadoop]# jps
1859 Jps
1795 DataNode
spark3:
[root@spark3 ~]# jps
1748 DataNode
1812 Jps
集群搭建过程核心文件配置没问题，可是，就是在使用50070端口检测的时候，显示livenode为1，而且只是spark1！
于是，经过对问题的排查，发现，最终因为前期配置/etc/ hosts 的时候，配置分别为：
spark1:
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
192.168.30.111 spark1
spark2:
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
192.168.30.112 spark2
spark3:
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
192.168.30.113 spark3
现在，统一改为：
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
192.168.30.113 spark3
192.168.30.111 spark1
192.168.30.112 spaqk2
ok ,问题得到解决。
利用代码：
[root@spark1 hadoop]# hadoop dfsadmin -report
DEPRECATED: Use of this script to execute hdfs command is deprecated.
Instead use the hdfs command for it.
Configured Capacity: 55609774080 (51.79 GB)
Present Capacity: 47725793280 (44.45 GB)
DFS Remaining: 47725719552 (44.45 GB)
DFS Used: 73728 (72 KB)
DFS Used%: 0.00%
Under replicated blocks: 0
Blocks with corrupt replicas: 0
Missing blocks: 0
-------------------------------------------------
Datanodes available: 3 (3 total, 0 dead)
Live datanodes:
Name: 192.168.30.111:50010 (spark1)
Hostname: spark1
Decommission Status : Normal
Configured Capacity: 18536591360 (17.26 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 2628579328 (2.45 GB)
DFS Remaining: 15907987456 (14.82 GB)
DFS Used%: 0.00%
DFS Remaining%: 85.82%
Configured Cache Capacity: 0 (0 B)
Cache Used: 0 (0 B)
Cache Remaining: 0 (0 B)
Cache Used%: 100.00%
Cache Remaining%: 0.00%
Last contact: Wed Aug 09 05:03:06 CST 2017
Name: 192.168.30.113:50010 (spark3)
Hostname: spark3
Decommission Status : Normal
Configured Capacity: 18536591360 (17.26 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 2627059712 (2.45 GB)
DFS Remaining: 15909507072 (14.82 GB)
DFS Used%: 0.00%
DFS Remaining%: 85.83%
Configured Cache Capacity: 0 (0 B)
Cache Used: 0 (0 B)
Cache Remaining: 0 (0 B)
Cache Used%: 100.00%
Cache Remaining%: 0.00%
Last contact: Wed Aug 09 05:03:05 CST 2017
Name: 192.168.30.112:50010 (spark2)
Hostname: spark2
Decommission Status : Normal
Configured Capacity: 18536591360 (17.26 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 2628341760 (2.45 GB)
DFS Remaining: 15908225024 (14.82 GB)
DFS Used%: 0.00%
DFS Remaining%: 85.82%
Configured Cache Capacity: 0 (0 B)
Cache Used: 0 (0 B)
Cache Remaining: 0 (0 B)
Cache Used%: 100.00%
Cache Remaining%: 0.00%
Last contact: Wed Aug 09 05:03:05 CST 2017
代码部分，可以看出，连接的datanode 为3个。
分别为 192.168.30.111
192.168.30.112
192.168.30.113