配置NFS

      $sudo systemctl enable nfs-server.service    #开机启动NFS
      $sudo systemctl start nfs-server.service     #立即启动NFS
      $sudo systemctl restart nfs-server.service   #立即重启NFS
   ＃关闭防火墙， 否则会出现（mount: RPC: Unable to receive; errno = No route to host）
      $sudo systemctl status  firewalld.service
      $sudo systemctl stop firewalld.service
      $sudo systemctl disable firewalld.service