1、Hadoop 安装 pdsh localhost： Connection refused
Hadoop安装过程中使用
$ sbin/start-dfs.sh
启动节点时，发生错误提示：
pdsh@xxx: localhost: connect: Connection refused
解决办法：
其原因是 pdsh 默认采用的是rsh登录，修改成ssh登录即可，可以通过在环境变量 /etc/profile 里加入：
export PDSH_RCMD_TYPE=ssh
然后再执行 source /etc/profile 使修改生效，再启动查看即可看到hdfs服务可以启动。
2、执行 sbin/start-dfs.sh 时出现 “ ERROR: Attempting to operate on hdfs namenode as root ” 等错误
如下：
[root@server1 hadoop-3.1.0]# sbin/start-dfs.sh Starting namenodes on [server1] ERROR: Attempting to operate on hdfs namenode as root ERROR: but there is no HDFS_NAMENODE_USER defined. Aborting operation. Starting datanodes ERROR: Attempting to operate on hdfs datanode as root ERROR: but there is no HDFS_DATANODE_USER defined. Aborting operation. Starting secondary namenodes [server1] ERROR: Attempting to operate on hdfs secondarynamenode as root ERROR: but there is no HDFS_SECONDARYNAMENODE_USER defined. Aborting operation. 2018-05-21 14:49:57,012 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
解决办法：
在 start-dfs.sh 和 stop-dfs.sh 两个文件中添加下面的配置参数：
HDFS_DATANODE_USER=root HADOOP_SECURE_DN_USER=hdfs HDFS_NAMENODE_USER=root HDFS_SECONDARYNAMENODE_USER=root