hadoop中ssh无密码访问什么时候会用到？

在namenode执行start-all.sh和stop-all.sh时候会通过ssh执行相关命令

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cloudera网站：When SSH is and is not Used

It is a good idea to use SSH for remote administration purposes (instead of rlogin, for example) but note that Hadoop itself and the related services do not use SSH for communication as a matter of course. Some scripts, and in particular the Hadoop start-all and stop-all scripts, do use SSH, but otherwise SSH is not used for communication among the following:

Datanode
Namenode
TaskTracker
JobTracker
/etc/init.d scripts (which start daemons locally)?

参考：https://ccp.cloudera.com/display/CDHDOC/CDH3+Deployment+on+a+Cluster