Virtualization in Vista vs SVS; What's the Difference?

Q:
Kevin asked, "I understand that Windows Vista is capable of virtualization. What's the difference between Vista Virtualization and SVS?"

A:
Timely question, Kevin. See if this helps.

The SVS virtualization technology is based on file system and registry redirection. This snippet from my blog explains:

Redirection can occur at the file-by-file level (like with SVS), at the directory level (e.g., UnionFS) or at the volume level (like the old DOS SUBST command). So it's nothing new and it certainly is not specific to Altiris. In fact, Microsoft is doing file system redirection in Vista to better support legacy applications that want to write to protected system areas; see this. [Unfortunately, they are calling the feature "file system and registry virtualization," which is causing some customer confusion. But it is "virtualization" by my own definition, so we'll just have to do our best explaining the differences.]

The difference is that with the Vista feature you reference, all legacy apps write to the same "virtual" space (or the same "layer," to use the SVS terminology). As MS explains in their doc, this can cause conflicts (exactly the opposite of SVS), and, since the common redirection area is ACL'd somewhat open, can reduce security (whereas SVS passes through the standard ACL's of the corresponding real locations in the file system and registry, so normal Windows security is maintained).

So the technology is fundamentally different, as well as the purpose. All the Vista feature is concerned with is legacy app compatibility. While SVS can get you that (without the conflicts and the security compromise), the main purpose of SVS is virtualizing an entire app -- any app -- as a discrete unit that can be rapidly and reliably moved around. In a nutshell, app virtualization is all about "faster, more reliable availability and removal of apps; conflict elimination; and state maintenance."