I have heard and worked on Clariion and got to know that with SPS communicate with each other by CMI (Clariion Messaging Interface). Whenever the SP1 goes down then Automatically SP2 will take over and serve data.
But I have a query that what make to communicate both the controllers in NetApp. AS we all know and also we provide solution that NetApp has a good Clustering solution Between both the controllers.
I'd say it is very similar on NetApp - both nodes from a HA pair communicate over cluster interconnect, plus some extras: NVRAM is being mirrored for consistency & there is also a mailbox on the disks.