Let's get straight to the point: Software-defined storage is a meaningless term that underscores, but sidesteps, the failure in most IT shops to manage the physical storage infrastructure. It's little more than a re-branding of the terms vendors have been tossing around for the last few years, such as storage hypervisors, private storage clouds and even storage virtualization. The idea of a software-defined storage architecture is to let anyone slice, dice and provision storage capacity and services such as various types of data protection. It's also supposed to help solve the problem of having storage volumes move around the infrastructure with virtual workload.
But software-defined storage (SDS) doesn't correct or address the underlying issue. What exactly impairs the efficiency of storage allocation, hampers storage resiliency and persistence, and drives the cost of storage so high is the lack of infrastructure monitoring and management. We react to hardware faults; we don't manage them. So, let's look at how we landed in this SDS craze and what it means for storage professionals.
The concept of software-defined storage is simple. The act of storing data to a volume is an inherently software-based function. The hardware for storing data is secondary and leverages commodity kits (all disks come from one of two vendors, and hardware controllers are increasingly server motherboards running a commodity OS and so on). Therefore, SDS advocates say abstracting software functionality away from hardware is a natural or evolutionary advancement in storage architecture.
The core objective of SDS is to make it much easier to provision and use storage resources. Gone is the need to worry about physical LUNs, World Wide Names or port addresses. In a virtualized storage infrastructure, aka software-defined storage architecture, that complexity is masked from users who require a storage volume resource that provides the capacity and performance attributes that are suited to the application workload they're running.
It's important to note that implicit in the description of (and case for) software-defined storage is the idea that expert storage administrators aren't as available (or affordable) in the current do-more-with-less climate that pervades today's IT shops. Virtual server administrators, who tend to know little about storage hardware or connection technologies, are being called upon to ensure that the right storage resources are allocated to applications and their data. Just as the operation of a coin-operated coffee machine doesn't require the skills of a barista, SDS advocates argue that storage resource provisioning shouldn't require any special skills in storage.
This concept is extremely dangerous, engendering greater dependency on hardware vendors to configure gear, tune it when problems arise and repair it when components fail -- all without involving the customer IT folks (except when it comes to handling the bill). It could also be argued that outsourcing responsibility for the physical infrastructure to external agents (vendors) blunts the ability of consumers to innovate their storage architecture by limiting their ability to manage what they build. IT managers already complain of skills shortages in job applicants; SDS doesn't resolve the problem, it just layers on a better user interface.
Another argument for software-defined storage is that it enables the storage resource to be more agile. When a virtualized workload transitions from server host to server host (aka vMotion), its connections to back-end storage should update automatically. That way, the consequences of re-hosting workloads (for example, adjusting to different physical routes to storage) are made transparent to both applications and the workload.
Not surprisingly, the current fascination with SDS follows the 2012 acquisition (at an extremely high dollar cost) of software-defined networking company Nicira Inc. by VMware Inc. Now, virtually all infrastructure software is hyped as software-defined.
Many forms of storage virtualization exist in storage systems today, including RAID, file systems and various types of storage virtualization software. However, current generation storage virtualization software (DataCore Software SANsymphony-V) and/or hardware/software appliances (such as IBM's SAN Volume Controller) are more or less both hardware-agnostic (it doesn't matter whose branding is on the physical hardware) and workload-agnostic (it doesn't matter what hypervisor or application software is running on the server), so current SDS offerings tend to be part of a proprietary software stack, such as vSphere.
The goal of software-defined storage is to divorce the storage control plane from the hardware plane so that resources can be presented simply to end users and apps. To realize the purported value of SDS, consumers would be well advised to buy technology that's truly independent from both hardware and server hypervisors to avoid costly lock-ins.