I am always intrigued by the notion that Cloud Computing should provide the “illusion of infinite capacity.” Illusion seems to belong to the domain of magicians. How does it fit into the realm of so scientific a discipline as Information Technology?

Most people are familiar with the quote from SF author Arthur C. Clarke (Clarkes’s 3rd Law.) “Any sufficiently advanced technology is indistinguishable from magic. “ All magicians use “gimmicks” and in this case, the gimmick is called Virtualization. Who would have imagined a decade ago that we would be capable of running 80 VMs on a physical server or bouncing them back and forth among hosts without user intervention? Those of us who struggled shoehorning multiple applications into a 640K memory space never dreamed of the capability of sharing 640 GIGABYTES among server Operating systems, “stealing from Peter to pay Paul” and not seeing these machines blue screen on a regular basis.

Capacity Planning in the old days meant considering the worst case scenario for our computing needs, and then purchasing MORE resources than were required (just in case.) The science/art even became codified under ITIL where an entire discipline is devoted to this practice. ITIL 3 (which is more oriented toward the business view of IT) views capacity management as comprising three sub-processes: business capacity management, service capacity management, and component capacity management (known as resource capacity management in ITIL version 2). All are inter-related.
So, let’s get back to the “illusion.” In virtualization, and especially in Cloud Computing, the key to providing this illusion is called “over-subscription.”

The practice of over-subscription predates virtualization. The Telecomm Industry and Disaster Recovery companies have relied on this model forever. Whenever resources are shared by multiple parties, there is usually the assumption that 100% of the users will never request the resources concurrently. Anyone who has ever shown up at an airport holding a boarding pass yet has still been denied a flight understands the potential fallacy in this thinking. HOWEVER, because the SLA to which the airlines are required to adhere typically states “in small print” that they will do their best to get you on the flight, you may still need to foot your own hotel bill, and pay for an extra day of parking until they have a resource available to accommodate you. If you are lucky, you might get a voucher for an atrocious airport meal.

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Read the rest of the article here at Cloud Lab - by jklincewicz