It seems like you can't talk to anyone in IT without hearing about "software-defined" something these days. Ever since Software-Defined Networking (SDN) burst on the scene, it's the hot trend. My world of storage is just as bad: It seems as if every vendor is claiming to sell "Software-Defined Storage" without much clarity about what exactly it is. Is SDS just the latest cloudy buzzword or does it have a real meaning?


Wikipedia, that inerrant font of all human knowledge, defines Software-Defined Storage (SDS) to be "policy-based provisioning and management of data storage independent of the underlying hardware." It goes on to talk about abstraction, automation, and commodity hardware. I can get behind that definition. Wikipedia also contrasts SDS with mere "Software-Based Storage", which pretty much encompasses "all storage" these days!


I've fought quite a few battles about what is (and isn't) "Software-Defined Storage", and I've listened to more than enough marketers twisting the term, so I think I can make some informed statements.


#1: Software-Defined Storage Isn't Just Software-Based Storage


Lots of marketers are slapping the "software-defined" name on everything they sell. I recently talked to one who, quite earnestly, insisted that five totally different products were all "SDS", including a volume manager, a scale-out NAS, and an API-driven cloud storage solution! Just about the only thing all these products have in common is that they all contain software. Clearly, "software" isn't sufficient for "SDS".


It would be pointless to try to imagine a storage system, software-denied or otherwise, that doesn't rely on software for the majority of its functionality.


#2: Commodity Hardware Isn't Sufficient For SDS Either


Truthfully, all storage these days is primarily software. Even the big fancy arrays from big-name vendors are based on the same x86 PC hardware as my home lab. That's the rise of commodity hardware for you. And it's not just x86: SAS and SATA, PCI Express, Ethernet, and just about every other technology used in storage arrays is common to PC's and servers too.


Commodity hardware is a great way to improve the economics of storage, but software running on x86 isn't sufficiently differentiated to be called "SDS" either.


#3: Data Plane + Control Plane = Integration and Automation


In the world of software-defined networking, you'll hear a lot about "separating the data plane from the control plane." We can't make the exact same analogy for storage since it's a fundamentally different technology, but there's an important conteptual seed here: SDN is about programmability and centralized control, and this architecture allows such a change. Software-defined storage should similarly allow centralization of control. That's what "policy-based provisioning and management" and "independent of the underlying hardware" are all about.


SDS, like SDN, is about integration and automation, even if the "control plane/data plane" concept isn't exactly the same.


#4: SDS Is Bigger Than Hardware


SDN was invented to transcend micro-management of independent switches, and SDS similarly must escape from the confines of a single array. The primary challenge in storage today is scalability and flexibility, not performance or reliability. Abstraction of storage software from underlying hardware doesn't just mean being able to use different hardware; abstraction also means being able to span devices, swap components, and escape from the confines of a "box."


SDS ought to allow storage continuity even as hardware changes.


My Ideal Software-Defined Storage Solution


Here's what my ideal SDS solution looks like:

  1. A software platform for storage that virtualizes and abstracts underlying components
  2. A scalable solution that can grow, shrink, and change according to the needs of the application and users
  3. API-driven control, management, provisioning, and reporting that allows the array to "disappear" in an integrated application platform


Any storage solution that meets these requirements is truly software-defined and will deliver transformative benefits to IT. We've already seen solutions like this (VSAN, Amazon S3, Nutanix) and they are all notable more for what they deliver to applications than the similarity or differences between their underlying components. Software-defined storage really is a different animal.


I am Stephen Foskett and I love storage. You can find more writing like this at, connect with me as @SFoskett on Twitter, and check out my Tech Field Day events.