Because virtualization offers so much cost savings and agility, many organizations are going the whole hog and virtualizing mission-critical systems they wouldn't have dreamed of virtualizing a little while ago. Who would have thought, for example, that we'd see enterprises deploy Oracle virtually rather than on physical hardware?
It's no surprise there's increasing interest in wringing every last drop of disaster recovery and disaster avoidance out of virtualization infrastructures. Most commonly, this is manifest in a replicated, dual-site architecture where virtualization resources can be quickly failed over to a secondary data center in the event that catastrophic failure strikes the primary site.
For some organizations, however, this isn't enough. They want the capability to seamlessly migrate workloads from one site to another in order to utilize computing resources at both sites. Moreover, they want the infrastructure to heal itself automatically in the event of a site or hardware failure -- just as single-site virtualization clusters are able to do. Hence the new buzz phrase "stretched cluster."