Virtual desktop infrastructures (VDIs) place new demands on the storage systems that support them. Ideally, the storage systems should still be shared as they are in server virtualization for maximum flexibility, but the storage needs to deal with even more pressure to be cost effective, as well as handle a few unique problems such as boot storms and high-write traffic. The first challenge is dealing with the cost of implementing shared storage when the competition is the local desktop hard drive.

Overcoming the price difference between shared storage and local desktop storage requires more than just aggressive pricing. It requires smarter use of this expensive asset. The shared storage must be optimized to its fullest extent in order to reach the price point that makes it practical for VDI.

Optimization of the VDI environment is a multi-step process, with each step driving out additional storage capacity demand. The first step is to only allocate storage capacity as it is actually being used. This can be done via thin provisioning. As we discuss in our paper "Thin Provisioning Basics", this technique dynamically allocates additional storage capacity on the fly to the requesting virtual desktop. In many cases this can eliminate upfront capacity demands by 50% or more.

The next step is to use masters and clones. Both the hypervisor and many advanced storage systems have the ability to provide this capability. In either case, a primary virtual desktop is set up and then "cloned" as each user's desktop is created. This clone leverages the master for most of its information--operating system files and applications. Then the user's unique data--settings, data, and unique applications--are stored in his clone.

[ Are you doin virtualization properly? Read 6 Common Desktop Virtualization Mistakes. ]

Clones are often thin provisioned and cloned. As this unique data is created by the user, space for it has to be identified as unique--not in the master. Then space has to be allocated to the clone, and then the write can complete. The final step is deduplication and compression, which might sound redundant to clones but does have added value in the VDI environment. Although there does have to be similar information between files for deduplication to work, it essentially eliminates redundant data that seeps into the environment between two clones. For example, if two users with virtual desktops are working on a similar file but making slightly different changes to that file, there is probably a fair amount of redundancy that can be eliminated. A deduplication ratio of 3:1 to 5:1 on this redundant data is not uncommon.

Compression eliminates the redundant information within a file. Therefore it can work on all files in the environment, regardless if there are similar files within that environment. A compression rate of 2:1 to 3:1 is possible; combined with deduplication, you can realize a rate of 5:1 to 8:1.

There is a case to be made that deduplication and compression could be used by themselves, eliminating the need and efficiency gains of masters and clones but significantly increasing the effective deduplication rate to 40:1 or greater. We have yet to be able to test the comparative performance/efficiency trade-off of the two techniques in our lab and we have not seen a comparative study as of yet. The good news is that if you want to use shared storage in the VDI environment, vendors have added enough efficiency techniques to make it affordable to do so, even when compared to the cost of desktop hard drives.

Like almost anything else in IT, however, there is a compromise to using these techniques that needs to be considered. For one, they can put a considerable load on the storage system or hypervisor. The performance problems can in large part be overcome by using solid state drives, which are faster than traditional hard drives, or faster storage controllers found in enterprise systems. But these two options raise the price of the storage system and so the IT manager has to do the math: Is it cheaper to buy basic locally attached storage to the VDI host, giving up the flexibility that shared storage offers, or is it better to have the flexibility and try to offset the cost of an enterprise system by using the above storage-efficiency techniques?

Cost of the supporting storage is just one of the challenges VDI poses. In our next column we will look at the performance challenges caused by boot storms and all the above storage-efficiency techniques. We'll conclude this series with a review of the products vendors are bringing to market to try to address these cost and performance challenges.

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