1，防火墙没关闭、或者没有启动 yarn 集群中错误首先看防火墙。
2，datanode 不被 namenode 识别问题
Namenode 在 format 初始化的时候会形成两个标识，blockPoolId 和
clusterId。新的datanode 加入时，会获取这两个标识作为自己工作目录中的
标识。一旦 namenode 重新 format 后，namenode 的身份标识已变，而
datanode 如果依然持有原来的 id，就不会被 namenode 识别。
解决办法，删除 datanode 节点中的数据后，再次重新格式化 namenode。

3 ，jps 不生效。
原因：全局变量 hadoop java 没有生效，需要 source /etc/profile 文件
4，8088 端口连接不上
$ cat /etc/hosts
注释掉如下代码

127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4

::1 hadoop102

5，Hadoop集群从节点出现错误： Retrying connect to server:
0.0.0.0/0.0.0.0:8031. Already tried 0 time(s);
6，在使用启动集群的命令start-dfs.sh时，出现下面的警告：
WARN util.NativeCodeLoader: Unable to load native-hadoop library for your
platform... using builtin-java classes where applicable
或者另一个错误：
-c: Unknown cipher type 'cd'
admin1: stopping namenode
try: ssh: Could not resolve hostname try: Name or service not known
disabled: ssh: Could not resolve hostname disabled: Name or service not known
VM: ssh: Could not resolve hostname VM: Name or service not known
will: ssh: Could not resolve hostname will: Name or service not known
recommended: ssh: Could not resolve hostname recommended: Name or service not known
It's: ssh: Could not resolve hostname It's: Name or service not known
HotSpot(TM): ssh: Could not resolve hostname HotSpot(TM): Name or service not known
have: ssh: Could not resolve hostname have: Name or service not known
The: ssh: Could not resolve hostname The: Name or service not known
VM: ssh: Could not resolve hostname VM: Name or service not known
which: ssh: Could not resolve hostname which: Name or service not known
have: ssh: Could not resolve hostname have: Name or service not known
might: ssh: Could not resolve hostname might: Name or service not known
Client: ssh: Could not resolve hostname Client: Name or service not known
the: ssh: Could not resolve hostname the: Name or service not known
highly: ssh: Could not resolve hostname highly: Name or service not known
that: ssh: Could not resolve hostname that: Name or service not known
'execstack: ssh: Could not resolve hostname 'execstack: Name or service not known
stack: ssh: Could not resolve hostname stack: Name or service not known
guard: ssh: Could not resolve hostname guard: Name or service not known
library: ssh: Could not resolve hostname library: Name or service not known
guard.: ssh: Could not resolve hostname guard.: Name or service not known
You: ssh: Could not resolve hostname You: Name or service not known
warning:: ssh: Could not resolve hostname warning:: Name or service not known
loaded: ssh: Could not resolve hostname loaded: Name or service not known
fix: ssh: Could not resolve hostname fix: Name or service not known
it: ssh: Could not resolve hostname it: Name or service not known
to: ssh: Could not resolve hostname to: Name or service not known
stack: ssh: Could not resolve hostname stack: Name or service not known
Java: ssh: Could not resolve hostname Java: Name or service not known
now.: ssh: Could not resolve hostname now.: Name or service not known
library: ssh: Could not resolve hostname library: Name or service not known
the: ssh: Could not resolve hostname the: Name or service not known
you: ssh: Could not resolve hostname you: Name or service not known
fix: ssh: Could not resolve hostname fix: Name or service not known
'-z: ssh: Could not resolve hostname '-z: Name or service not known
or: ssh: Could not resolve hostname or: Name or service not known
with: ssh: Could not resolve hostname with: Name or service not known
<libfile>',: ssh: Could not resolve hostname <libfile>',: Name or service not known
link: ssh: Could not resolve hostname link: Name or service not known
noexecstack'.: ssh: Could not resolve hostname noexecstack'.: Name or service not known
with: ssh: Could not resolve hostname with: Name or service not known

7，在hadoop集群的master中用命令运行例子易出现的故障：

1. 错误现象：java.lang.OutOfMemoryError: Java heap space.

** 原因：**JVM内存不够。

** 解决方法**：修改mapred-site.xml中mapred.child.java.opts属性的值，其默认值是-Xmx200m 可根据需要适当增大 该值。