The advantages of a VDI infrastructure are that virtual desktops are hardware independent and can be accessed from any common OS. It's also much easier to deploy virtual desktops and to facilitate the freedom that the users require of them. And because of the single-user OS, application compatibility is much less of an issue than it is with terminal servers.

However, when implementing a VDI infrastructure, certain points need to be addressed. First of all, the TCO/ROI calculation may not be as rosy as some people suggest. Secondly, the performance impact on applications--specifically multimedia and 3D applications--needs to be investigated. And finally, you have to deal with the licensing aspects, as this can be a very significant factor in VDI infrastructure.

While centralized desktop computing provides important advantages, all resources come together in the datacenter. That means that the CPU resources, memory resources, networking and disk resources all need to be facilitated from a single point--the virtual infrastructure.

The advantage of a central infrastructure is that when sized properly, it's more flexible in terms of resource consumption than decentralized computing. It's also more capable of handling a certain amount of peak loads, as these only occur once in a while on a small number of systems in an average datacenter.

But what if the peak loads are sustained and the averages are so high that the cost of facilitating them is disproportionate to that of decentralized computing?
As it turns out, there is a hidden danger to VDI. There’s a killer named “IOPS”.

It should be obvious by now that calculating the amount of storage needed in order to properly host VDI is not to be taken lightly. The main bottleneck at the moment is the IOPS. The read/write ratio of the IOPS that we see in practice in most of the reference cases demonstrate figures of 40/60 percent, sometimes even as skewed as 10/90 percent. The fact is that they all demonstrate more writes than reads. And because writes are more costly than reads - on any storage system - the number of disks required increases accordingly, depending on the exact usage of the users and the application.

Some questions remain:

• What is the impact of application virtualization on the R/W IOPS?
• What exactly is the underlying cause of the huge difference in read/write ratios between lab tests and actual production environments? 
• What if all the write IOs only need to be written to a small part of the total dataset (such as temporary files and profile data)? Could all the data, or at least most of it, be captured in a large write cache?

These questions will be investigated as an increasing number of VDI projects are launched.

Source: http://www.brianmadden.com/blogs/rubenspruijt/archive/2009/12/10/vdi-and-storage-deep-impact.aspx