| Virtualization as High Availability or High-Availability as Virtualization? |
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Many pundits and other folks like VMware's CEO Diane Greene have touted virtualization as being the "cure" to disaster recovery, many for the past several years. Disaster recovery can be pretty reasonably viewed as being high-availability over distance, so it makes some sense to see how DR, HA and virtualization fit together. What's hype here, and what's real? Let's look and see what we find out.
What is virtualization? Wikipedia defines virtualization like this: In computing, virtualization is a broad term that refers to the abstraction of computer resources. One useful definition is "a technique for hiding the physical characteristics of computing resources from the way in which other systems, applications or end users interact with those resources". At the moment, this term is most commonly used to refer to machine or OS-level virtualization, storage virtualization and something I'll call container virtualization. For now, let's ignore storage virtualization. Machine-level virtualization means basically making it difficult to tell exactly which physical machine an OS is running on at any given point in time. There are many other kinds of virtualization as well - for example service virtualization. Service virtualization doesn't hide or abstract physical machines, but instead virtualizes or hides services. That is, there is no fixed binding between services and physical machines. This is, in fact, exactly what classic HA software does - software like Linux-HA . What an HA system does is use service virtualization to recover from server failures, and monitors services so that it can restart them - either locally or on another server. Customers care about services, not about OSes or physical machines, so from their point of view, machine and service virtualization are similar. Both provide the potential for recovering from failures of physical servers and OSes. However, since machine virtualization doesn't have any visibility of the individual services and the dependencies between them, it can't monitor them and restart them if they fail. But, the ideas of virtualization need management entities to oversee them and decide to move virtual entities, and coordinate the movement of these virtual entities from one place to another. VMware will soon have their Site Recovery Manager, and Linux has Linux-HA (among other solutions). The purpose of such management entities is to detect and respond to failures and administrative requests by starting, stopping, and migrating virtual entities in their purview to different sets of physical servers. The author of this excellent article, Alan Roberston, covers the following points on his blog
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