Personal disk images versus shared images--which is better for VDI? This is one of the biggest decisions that companies need to think about when designing VDI solutions.
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Each has pros and cons. Conventional wisdom is that:
Shared / non-persistent / 1-to-many images are easier to manage (since you only have to update a single master) and you can handle applications with app virtualization (App-V, ThinApp, InstallFree, Endeavors, Symantec) and the user settings with user virtualization (AppSense, RES, Scense, triCerat, Liquidware, etc.). Shared images is also where we think about "layering" (Unidesk, Wanova, MokaFive, RingCube, Atlantis)
Personal / persistent / 1-to-1 images, on the other hand, are "easier" because you just keep in doing your desktops the old way. But the downside is that they're harder to manage (you need something like Microsoft SCCM or Symantec Altiris Client Management Suite or BigFix to handle the patching and updating). Also, personal images take up more disk space which translates to more IOPS and a higher overall storage bill (because you literally have a separate VMDK or VHD for every user).
So that's conventional wisdom when it comes to VDI storage: Shared is lighter on the resources (but maybe not as personal), while 1-to-1 is great for personalization and easier to implement, but it comes at the cost of higher resource requirements.
Is this still true in 2011?
Consider the following:
(1) The "expense" in storage today is IOPS, not raw capacity. So yeah, maybe 1000 users would need 20TB of storage with personal images where you'd be able to get away with only 1TB if you were using shared images, but your 1000 users would need 50k IOPS regardless of whether they were using shared or not. And conventional sharing mechanisms (linked clones, thin provisioning, etc.) are actually WORSE for IOPS because every user accesses the same physical base image. This means that you can't "automagically" just share a single master without getting some advanced storage involved (like caching, acceleration, tiered storage, SSD, or a combination of all). And if you have to do that, then why are you messing around with all this layering in the first place?
(2) This brings up the second issue which is that shared images and layering are complex. I've written previously about something I'm calling "Madden's Paradox" which states that if your desktop environment is simple enough that today's layering technologies work, then you can probably just use Remote Desktop Session Host (Terminal Server). The consequence of this is that most of today's VDI environments actually do NOT use shared images (which is kind of backwards to what we all thought it would be). Now to be fair, layering is evolving, but so are storage technologies.
(3) Desktop-specific storage technologies are getting really awesome now. It's taken a few years for storage vendors to get their products tuned for desktop workloads. (Remember most of them came from the server world where we really cared about capacity and read optimizations.) But now we're seeing stuff from DataCore, Xiotech, NetApp, Fusion-io, Virsto, Atlantis, and about a hundred other companies who are doing everything they can to provide high-speed, high-IOPS, cost-effective storage (things like block-level single-instance storage, block-sharing across LUNs, block-level tiered storage and caching, etc.).
(4) And the final irony of this all is that even with personal disk images, most companies still store the most personalized stuff (user data, home folders, profile) outside of the user's desktop VM. So that means that all these desktop disk images -- even fully personal 1-to-1 images -- are almost completely identical anyway! (I mean they have the same OS, same patches, same basic apps...) So even when different, they're the same, which means our advanced storage technologies (which we have to have regardless) can kick in and let them scream.
To be clear, I am not trying to suggest that personal images are better than shared images. Of course this is something you need to evaluate on a case-by-case basis. What I am saying is that while the conventional wisdom has been that shared images consume fewer resources than personal images, I don't think that's true in 2011 anymore.