SANs and Virtualization

So, if virtualization is a pedigreed concept in computing architecture and one that has enjoyed such widespread use in the IT environment, why is it that virtualization has become a "dirty word" within the context of storage area networking today? Many vendors are reluctant to use the term to describe software technologies that they are promoting for use in managing SAN infrastructure. Survey after survey reflects consumer disdain for the term "SAN virtualization."

There are many possible explanations for the phenomenon , which has stymied the marketing efforts of even the leading vendors in the storage space. Many vendors and analysts suggest that virtualization is too confusing to resonate with consumers. Like the term SAN, virtualization has been bent and twisted by marketeers so that it seems to mean different things depending on the vendor to whom one speaks. The only two points of universal agreement when it comes to virtualization are 1) that it is essential to have virtualization in order for SANs to deliver on their value proposition and 2) that there is absolute disagreement about the best strategy for realizing it.

SANs require virtualization if they are to deliver on their value proposition of reduced storage- related downtime and improved capacity allocation efficiency. SANs promise to reduce downtime by dynamically scaling storage behind an application so that applications never encounter a disk-full error message.

This value proposition is an extension of a claim long made by vendors of high-end arrays. As a function of controller intelligence and microcode , arrays enabled many physical disk drives to be joined together and presented as a single disk volume (typically identified by a Logical Unit Number or LUN) to the operating systems of connected servers. Increasingly, array vendors provided configuration tools to IT technicians that would enable them to add more physical disks to a LUN or to combine two or more LUNs to make larger volumes .

While these capabilities were useful, they often ran afoul of constraints within server operating systems themselves . Some operating systems identified volume capacity at the time of boot-strapping (start-up) and needed to be quiesced, powered down, then rebooted, to identify and begin using newly expanded volumes. This process disrupted normal operations and became more painful as IT shops began operating on "Internet time" ”365 days per year, 24 hours per day, 7 days per week.

In the parlance of storage area networks (SANs), which may themselves be conceived of as very large disk arrays (or, more confusingly, as "arrays of arrays"), virtualization is used both as a verb, referring to the process of creating scalable volumes from storage nodes connected in the SAN fabric, and as a noun, a device or software component that provides a role comparable to that of a controller in a single disk array. In general, SAN virtualization refers to techniques for combining or aggregating LUNs.

Vendors differ in their preference for locating this aggrandized array controller functionality in the SAN. Figure 7-2 surveys the popular options: on the host, in the wire (in-band) on a switch or intelligent appliance, out-of- band on a server or appliance, or at the array controller.

Figure 7-2. Options for locating virtualization intelligence in a SAN.