1) Shutdown/Stop hosts that are accessing the XIO array to stop IO to the array. 
2) From the CLI run the 
show-clusters-performance command to verify that there is no IO running to the array. Once you no longer see IO you can proceed with the shutdown procedure. 
3) Run the 
show-clusters command to verify that the array status is ACTIVE CONNECTED
 
4) Issue the 
stop-cluster-unorderly command to stop the XIO array.
5) Verify that the XIO array is in a shutdown state by running the 
show-clusters command. 
6) Once you have verified that the array state is stopped (unorderly) you can safely remove power from the rack PDU that the XIO array is installed in. 
BONUS CONTENT
1) In order to shutdown the XMS server run the 
shutdown-xms shutdown-type=machine