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在主节点上执行start-dfs.sh命令启动hdfs，无法启动datanode：
1）看一下slaves文件有没有配置正确
2）检查从namenode节点到datanode节点的免密登录是否配置正确；
3）测试一下从namenode上能否ping通datanode，datanode节点的防火墙有没有关（或者如果你的本意就是不关闭防火墙，而是只给hadoop开启特定的端口的话，那就使用telnet命令检查一下从namenode节点到datanode节点的特定端口能不能连接上）
4）还有一种比较特殊的情况，就是你的namenode和datanode节点上，用于安装hadoop环境的用户不一样，比如在namenode节点上用u1用户安装hadoop环境，而在datanode节点上则是用u2用户安装的hadoop环境。这种时候就会有问题，因为当你在namenode上启动hdfs时，肯定是用u1用户执行的命令（因为你是用u1用户安装的hadoop嘛，环境变量是配在这个用户下的），那么当namenode节点要远程登录到datanode节点上去启动datanode时，namenode默认是会使用当前用户也就是u1用户去登录datanode节点的，这个时候就会出错，因为datanode节点上没有u1用户，根本无法登录上datanode，更别说去启动datanode了。对于这个问题，我在网上偶然看到有人的解决方法是在配置masters和slaves时，不单单是写IP（比如192.168.137.22），而是要写上用户名，比如“[email protected]”。正如我所说的，这是个比较特殊的问题，最好还是将hadoop集群的所有节点的安装用户保持一致。另外我感觉如果集群中的不同节点是用不同用户来安装hadoop环境的话，根本无法安装成功，虽然我在网上看到有朋友说他这么安装也成功了……我觉得无法安装成功是因为，这么来安装hadoop集群环境的话，首先namenode节点到datanode节点的免密登录就没法配置，因为在namenode节点上是以u1用户生成的公钥，因此在namenode节点上无法以u2用户的身份免密登录datanode节点——额…至少以我目前了解的ssh免密登录设置的知识来说是不行的…………
