Interested in your opinion...
I really enjoyed >80% this article, beginning with the premise and ending just before you got to the point of defining SDS.
IMO the glaring omissions here are the market leaders, HP's StoreVirtual VSA (which evolved from LeftHand networks circa 2009) and DataCore's SANsymphony, (which started in 1998 and began blowing 'hardware defined' storage out of the water in price-performance with their first SPC-1 all the way back in 2003, and continues slaying the 'hardware defined storage' beasts even today). There are many more examples.
I have a slightly different take on what it means to be "Software Defined Storage". Call me simplistic, but in my mind the first criteria is that it must be software-only (i.e. not necessarily a 'hardware appliance' sold by a 'storage software' vendor) and secondly that it must NOT be "hardware-defined" (i.e it runs on any servers, including ones I already own).
Bottom line -- If I can completely separate my storage software purchase decision from my storage hardware purchase decision, including separating the hardware refresh cycle from the software upgrade cycle (like I do when I chose my database software vendor), then I have "Software Defined Storage".
Beyond that, what's the point of limiting SDS to "scale-out", 'Shared-nothing" and "API-based management"? Those are architectural considerations that have nothing to do with whether any given solution is "software defined" or "hardware defined".
Software-Defined Storage is any storage solution that completely disaggregates the software purchase decision from the hardware purchase decision. In this context, since I can't buy Nutanix without buying Nutanix-defined hardware appliances, then Nutanix is no different from Isilon, or any other hardware-defined solution. Nutanix then would not be 'software defined'. As a Nutanix customer, when you realize that your Nutanix software license expires when your appliance server hardware goes EOL, then you realize that Nutanix is not "software defined"...no?
If these seem like rhetorical questions, then they probably are.
Can a Software-Defined storage solution exist without some vendors proprietary file-system implementation? Here, DataCore, Falconstor and Starwind (among many others) come to mind.
What about IBM's decades-old GPFS (General Parallel File System) that runs on any servers/storage how is that not SDS?
Likewise, if IBM really does begin selling XIV as software-only "Spectrum Accelerate SDS" (rather than just marketing it that way), and I can then run it on whatever x86 servers/disks I want -- would that be SDS?
I can't help but notice that all of the companies and technologies you list to constrain the definition of SDS are either venture-capital fueled startup companies or newly minted open-source initiatives.
I'm puzzled by this but IMO, it's all just the Hype-Cycle talking.
Thanks for a thought-provoking article and for inviting the question(s).