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Due to the fact that usually IaaS provide VMs you can think that there is a direct correlation between virtualization (in this case system virtualization) and cloud. But this correlation could exists or also not!
Virtualization is stricly related to the technologies and the infrastructure aspect. Cloud more on high-level aspects, like type of services, services level, …
Cloud imply virtualization?
Virtualization it’s just an infrastructure layer that could be used in a cloud infrastructure, but is not strictly mandatory!
Of course, for IaaS probably a virtualization is implemented (although also IaaS, in some limited cases, could be implement without a virtualization layer), but for a PaaS and SaaS virtualization could not exist, and some of its services (like high availability, resources migration, …) could be implemented at higher level.
For example first release of Microsoft SaaS offer (called BPOS, and now Office 365) was based on physical systems (not sure if now is implemented on virtual environments). Google also has is how (physical) platform.
And from the user point of view, if virtualization exist or not is absolutely non relevant, because is not a layer that he must manage. Of course is not the same from the provider point of view, and if virtualization can bring benefit and advantages to the infrastructure, then is reasonable to use it.
Virtualization imply cloud?
By using a virtualization solution it does not mean that you have a cloud solution! And have a system virtualization does not imply have also a private cloud!
Cloud (by the dal NIST definition) imply:
- On-demand self-service
- Broad network access
- Resource pooling
- Rapid elasticity
- Measured Service
Virtualization (plus automation) could be a first step to move to a cloud approach (IaaS based) and could be a requiments to implement an hybrid cloud (still IaaS based).
- Cloud Computing Goes Far Beyond Virtualization (good post about Virtualization vs. Private Cloud)
- Is Cloud Computing Just Virtualization and Automation? No!!
- The Importance of Knowing the Difference Between Virtualization and Cloud Computing
- Microsoft: ‘Virtualization Is Not Cloud Computing’ (post focued on Microsoft solutions, but with interesting points)
- Virtualization isn’t cloud computing (ZDNet post)