We have to be concerned with the definition of file virtualization. On one hand, we define it as a technology that sits across different NAS devices, file servers and file sharing systems.
But, there is also another view that defines file virtualization within an appliance or filer to support global namespace and global clustered file systems within a storage system. Other definitions involve the ability to transfer data transparently across different tiers of storage within a NAS device. But, there is even another element of file virtualization where some NAS devices actually support the idea of a virtual NAS server -- similar to the VMware environment where an ESX server runs multiple virtual machines with their guest operating system. Some NAS appliances and storage systems support a similar feature where you have a physical server or NAS storage device along with a virtual filer that lets you isolate different workloads, customers or applications and move those around across different physical resources.
So, the key is in your definition of file virtualization and where those virtualization features are implemented. Another concern is the use of proprietary versus open implementations.
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File Virtualization FAQ Guide