背景

hadoop集群启动后，无有效的datanode节点，表现形式为：
使用hdfs dfsadmin -report查看节点的情况时，都是0

xf@master01:~/hadoop-2.6.5$ hdfs dfsadmin -report
Configured Capacity: 0 (0 B)
Present Capacity: 0 (0 B)
DFS Remaining: 0 (0 B)
DFS Used: 0 (0 B)
DFS Used%: NaN%
Under replicated blocks: 0
Blocks with corrupt replicas: 0
Missing blocks: 0

排查解决

• 问题排查：检查datanode的安全模式是否关闭
  网上有一些地方说可能是因为datanode开启了安全模式导致的。所以使用命令hdfs dfsadmin -safemode get，查看datanode的安全模式是否关闭，但是却获得如下报错：

safemode: Call From slave01/127.0.1.1 to master01:9000 failed on connection exception: java.net.ConnectException: Connection refused; For more details see:  http://wiki.apache.org/hadoop/ConnectionRefused

• 问题跟踪：通过报错信息中的链接 http://wiki.apache.org/hadoop/ConnectionRefused ，看到这么一行
  

Check that there isn’t an entry for your hostname mapped to 127.0.0.1 or 127.0.1.1 in /etc/hosts (Ubuntu is notorious for this).

大概意思是要查看hosts配置文件里面是否把主机名映射到了127.0.0.1 或者127.0.1.1 ，特别是Ubuntu容易出现这种异常情况！

• 尝试解决：我的系统就是Ubuntu，打开主机master01的/etc/hosts 一看，确实是这个情况！！！

127.0.0.1       localhost
127.0.1.1       master01      --- 把这一行删掉就行了！！！

192.168.0.111   master01
192.168.0.112   master02
192.168.0.121   slave01
192.168.0.122   slave02
192.168.0.123   slave03

• 结果验证
  把集群各个节点下的hosts文件中的 127.0.1.1 主机名 这一行删除后，在slave节点重新执行hdfs dfsadmin -safemode get，可以看到

Safe mode is OFF

说明datanode的安全模式已经关闭

最后，在master节点执行hdfs dfsadmin -report验证，确实正常了，Live datanodes (3) ，而不是原来的0了

Configured Capacity: 25990606848 (24.21 GB)
Present Capacity: 16147132416 (15.04 GB)
DFS Remaining: 16147058688 (15.04 GB)
DFS Used: 73728 (72 KB)
DFS Used%: 0.00%
Under replicated blocks: 0
Blocks with corrupt replicas: 0
Missing blocks: 0

-------------------------------------------------
Live datanodes (3):

Name: 192.168.0.122:50010 (slave02)
Hostname: slave02
Decommission Status : Normal
Configured Capacity: 8663535616 (8.07 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 3268976640 (3.04 GB)
DFS Remaining: 5394534400 (5.02 GB)
DFS Used%: 0.00%
DFS Remaining%: 62.27%
Configured Cache Capacity: 0 (0 B)
Cache Used: 0 (0 B)
Cache Remaining: 0 (0 B)
Cache Used%: 100.00%
Cache Remaining%: 0.00%
Xceivers: 1
Last contact: Wed Sep 09 23:04:24 CST 2020


Name: 192.168.0.121:50010 (slave01)
Hostname: slave01
Decommission Status : Normal
Configured Capacity: 8663535616 (8.07 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 3268980736 (3.04 GB)
DFS Remaining: 5394530304 (5.02 GB)
DFS Used%: 0.00%
DFS Remaining%: 62.27%
Configured Cache Capacity: 0 (0 B)
Cache Used: 0 (0 B)
Cache Remaining: 0 (0 B)
Cache Used%: 100.00%
Cache Remaining%: 0.00%
Xceivers: 1
Last contact: Wed Sep 09 23:04:22 CST 2020


Name: 192.168.0.123:50010 (slave03)
Hostname: slave03
Decommission Status : Normal
Configured Capacity: 8663535616 (8.07 GB)
DFS Used: 24576 (24 KB)
Non DFS Used: 3305517056 (3.08 GB)
DFS Remaining: 5357993984 (4.99 GB)
DFS Used%: 0.00%
DFS Remaining%: 61.85%
Configured Cache Capacity: 0 (0 B)
Cache Used: 0 (0 B)
Cache Remaining: 0 (0 B)
Cache Used%: 100.00%
Cache Remaining%: 0.00%
Xceivers: 1
Last contact: Wed Sep 09 23:04:23 CST 2020