Virtual Storage Equals Real ConfusionWhile doing research for a current book project on storage virtualization, I found it useful to keep a large bottle of aspirin handy for the headaches that were bound to ensue. Storage virtualization is the elephant that vendors and customers blinded by today's marketing hype lay hands on in total darkness and walk away with completely different impressions about what they encountered.
Attempts to make the technology understandable are sometimes helpful, but usually do not lift the veil of confusion from the subject. For example, in one explanation I saw, the mystery is reduced to the mundane by the declaration that virtualization is all around us. Even driving a car is a virtual experience since we don't have to get out and move the wheels every time we want to turn. Storage virtualization is, after all, just one variation of a very familiar phenomenon.
In another explanations, boxes representing storage, servers and a SAN are connected by lines of varying thickness, or by overlapping circles or explosive triangles. Arrows point to where the virtualizing business occurs, and sidebars explain the benefits. Despite these well-intended explanations, however, storage virtualization remains a complex technology whose intricate inner workings, ironically, are meant to make the life of the storage administrator simpler.
A Logical Look at Virtualization
The current market confusion over storage virtualization stems in part from the diversity of strategies for hiding the complexities of physical storage and presenting a streamlined logical view of storage assets. Storage virtualization creates an abstraction layer between physical and logical storage. That abstraction may occur in hosts, in storage arrays or within the SAN. It may also be performed in-band (a.k.a. symmetrical) with control and data on the same path, or out-of-band (a.k.a. asymmetrical) with control and data taking separate paths. In each case, however, there are no standard requirements for how storage virtualization must be implemented.
Unlike SCSI, Fibre Channel or iSCSI technologies, there are no objective standards that define how storage virtualization will perform. The closest the technology has come to standardization has been through the work of the Storage Networking Industry Association (SNIA). The SNIA's Technical Council and virtualization workgroup have created a taxonomy to explain basic concepts and associations. SNIA tutorials on virtualization have been presented at Storage Networking World conferences and are available online. Despite these noble efforts, the diversity of offerings and lack of standardization conspire to make storage virtualization a fertile ground for migraines for anyone who ventures there.
Market confusion over storage virtualization is also compounded by the intermixing of storage virtualization methods with the services that virtualization facilitate. Most virtualization vendor literature, for example, focuses on auxiliary services such as point-in-time copy (snapshots), data replication, utilization of storage capacity, support for heterogeneous storage arrays and so on. These storage services are enhanced by, but not dependent on, storage virtualization and so can be delivered by other means. Separating what storage virtualization is from what it does is a useful exercise for understanding the common denominators of the fractional approaches vendors have taken.