isi_for_array will send a shutdown -p command to all the nodes, but it's possible that the node where you initiate the isi_for_array starts to shut down before all nodes have received the shutdown -p. Thus, you have nodes that are left online that don't shutdown. isi config is designed with that scenario in mind.  /sbin/shutdown on OneFS is a wrapper for the real shutdown command and it does handle things like flushing caches, writing a backup journal, and unmounting /ifs. But, I agree with others here - isi config > shutdown all is the safest way to shutdown the cluster plus manual verification that all nodes are shut down.
Hello,  Please tell me about "How to Safely Shut Down an EMC Isilon Cluster". KB 16529 has described as to shutdown the Isilon Cluster by "isi config > shutdown" command. Why must I not shut it down by "isi_for_array shutdown -p now" command?  Regards, lucky-7
How to Safely Shut Down an Isilon Cluster