While the ONTAP VASA Provider is an appliance, SolidFire's implementation is a part of the cluster. Both models have their advantages and disadvantages, unfortunately I don't know the engineering or product management decision process that led to each implementation.
If you haven't already, I highly recommend you check out SolidFire's VVOLs features. There is lots of really great functionality there, and it further removes storage management overhead from the virtualization administrator. It's a great example of just how simple storage for virtualization can be, while at the same time not sacrificing features or functionality.
Hope that helps.
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