The pros and cons of using storage virtualization for virtual desktops

Deploying virtual desktops tends to be expensive, and achieving adequate performance can be a challenge. Since both of these issues are often related to the underlying storage infrastructure, some administrators address them by using storage virtualization.

Here, we'll cover the pros and cons of using storage virtualization in a virtual desktop environment. Before I delve into them, however, note that storage virtualization isn't based on a single product or technology. Many vendors offer their own storage virtualization solutions. For the purposes of this article, I will define storage virtualization as a technology that places a layer of abstraction between the virtual desktop (or the underlying hypervisor) and the physical storage.

The benefits of storage virtualization for VDI

The actual benefits achieved by using storage virtualization vary depending on which technology is being used. Some of the most common are reduced storage costs, higher performance and easier manageability. Of course, way in which such benefits are achieved varies widely from one product to the next.

One method to reduce costs by decreasing the virtual machine storage requirements is deduplication. Deduplication is well suited to virtual desktop infrastructure (VDI) because all of the virtual hard disk files used by the individual virtual machines (VMs) are typically identical, although an organization may maintain several sets of virtual hard disks.

There are a number of different deduplication products on the market, and although they are all designed to conserve storage space, most do nothing to improve storage performance.

A company named Atlantis Computing Inc. has received a lot of press for Atlantis ILIO, a software-based deduplication product specifically for virtual desktop environments. The software sits between the hypervisor and the storage and performs deduplication in real time before write operations are sent to disk.

Another product that is designed to adapt virtual storage to VDI environments is Virsto VDI. Like AtlantisILIO, Virsto VDI improves storage performance in virtualized environments, but its approach is completely unique.

Virsto VDI uses a proprietary virtual hard disk (VHD) type, rather than the VHD architecture that is natively used by the hypervisor. The end result is a thinly provisioned (dynamically expanding) virtual hard disk with performance that, according to the company, exceeds that of fixed-size VHDs. This performance boost is achieved with a transactional storage mechanism.

Since write operations are so random in VDI environments, Virsto VDI seeks to smooth out the pattern of write I/O requests through the use of logged transactions, which are then staged to be written to physical storage. The end result is that I/O operations are streamed in a way that delivers optimal performance.

The disadvantages of storage virtualization

In spite of the benefits that can be achieved through storage virtualization, there are some disadvantages to virtualizing your storage. First, even though most vendors advertise that their products can help decrease storage costs, there is an acquisition cost associated with the storage virtualization product, as well as long-term support costs. These costs may or may not eventually be offset by savings associated with reduced storage consumption.

Another potential disadvantage of implementing storage virtualization is that it adds a layer of complexity to the storage architecture. This additional complexity can make troubleshooting problems more difficult. In addition, you might find that hypervisor manufacturers are reluctant to provide technical support once storage virtualization software is in place. For example, Virsto VDI uses a completely different type of virtual hard drive file than what is natively used by the hypervisor. While I don't know of any actual cases in which a hypervisor vendor has refused technical support to a customer using Virsto VDI, I can certainly see the potential for such an occurrence.

Finally, storage virtualization can lead to situations in which your physical storage resources are overcommitted. Thin-provisioning systems create virtual hard disk files that start out very small and dynamically expand as space is needed. The overcommitment of storage resources makes it possible to increase virtual machine density without adding any storage, but an organization can run out of physical storage space if the virtual hard disk files expand beyond the underlying physical storage capacity.

So if you are contemplating using virtual storage for your virtual desktops, it is important to weigh the pros and cons. In most cases, you will find that the benefits outweigh any potential disadvantages, but it is still important to carefully consider both sides.

ABOUT THE AUTHOR:
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. As a freelance technical writer, Posey has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies.