There is an ongoing debate about what type of storage is best to use to support desktop virtualization solutions, especially in small to medium sized implementations. Storage is one of the most expensive parts of a desktop virtualization project and as a result anything you can do to drive cost out of the storage purchase is going to make desktop virtualization economics work better. This leads some to advocate local storage.The theory goes that local storage is going to be a less expensive and an easier to implement solution for the virtual desktop project. At the surface those are valid points but when you weigh what you give up by not going to shared storage and how you have to configure local storage to provide users with a respectable desktop experience I'm not sure if local storage is quite the deal it sounds like.

First let's look at what you have to do to local storage to get it ready for desktop virtualization. From a performance perspective you are going to need enough drives to generate enough IOPs to provide adequate performance to those now virtualized desktops. While the typical working IOPs requirement of a virtual desktop is relatively light, typically less than 5 IOPs. However the per virtual desktop IOP during boot up, logon/logoff, software update operations that number can increase substantially, as much as 5X. As we discuss in our recent article "Solving Boot Storms With High Performance NAS" these activities are the real challenge in the environment and something that needs to be planned for when designing the storage system.

Providing your virtual desktop environment with high performance and highly reliable storage is not as simple as running down to your local computer store and picking up that $99 2TB hard drive. You're going to want something a little more enterprise class with a 15K RPM speed. Most environments will either use RAID 1 or RAID 5 for data protection so that will require a more expensive controller to be purchased and the protection overhead will eat into performance. The need for performance and reliability is typically going to require an eight to ten drive RAID configuration. This drive count is going to be beyond the internal drive capability of most servers, which means an external storage system.

The combination of faster drives and an external chassis erodes some of the price advantage compared to mid-range storage systems but not all of it. Its the limits of locally attached systems in this type of configuration that become the real challenge. Most price competitive external systems can only be expanded so far. As you add virtual desktops you may need additional external systems, which adds to the cost and to complexity.

As we discussed in our webinar "Making Sure Desktop Virtualization Won't Break Storage" there is some planning required vs local storage. That planning though is often worth what you gain from shared storage. The big give up with local storage is that you loose much of what desktop virtualization brings like virtual machine migration and server balancing. You need shared storage to be able to migrate machines and balance load. You also give up the ability to offload from the hypervisor all the things that shared storage does well like scalability, snapshots, cloning, deduplication and replication. While its true that some of these functions can be performed via software all of those come at an added cost of not only dollars but also server resources. Finally shared storage can be leveraged for other storage uses, like server virtualization, as well so the cost of the shared storage investment can be allocated across several projects.

Local storage may have a roll to play in desktop virtualization but you have to weigh all the odds. Is desktop virtualization without shared storage really going to give you a return on the investment? If you factor everything in, you may be better off getting shared storage first and then deploying virtual desktop later than you would be to live with an virtual desktop project that under achieves due to poor storage performance. One thing we have seen consistently is once users get a bad taste for virtual desktop, they rarely will give it a second chance.

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George Crump is lead analyst of Storage Switzerland, an IT analyst firm focused on the storage and virtualization segments. Find Storage Switzerland's disclosure statement here.