How hadoop1.x NameNode and SecondNameNode work
The Datanode backup mechanism is controlled by each dn node and is not initiated by the client. Because the remote transmission consumption of the client is higher than that of each dn node (usually the backup node is in a computer room and the transmission speed is fast)
The safe mode is similar to system initialization. This period is usually the process that namenode loads metadata,datanode to report the location information of each node block to namednode. Safe mode can be forced to leave by command, but
Not recommended
