Tue, November 18, 2008 — CIO — IT departments can reap enormous benefits from virtualizing applications and implementing Virtual Desktop Infrastructures (VDI). However, the management and cost savings of virtualization can be lost if performance is so bad that it hampers productivity, as can happen when virtual applications and desktops are delivered across a Wide Area Network (WAN).

For an in-depth look at a WAN revamp, see CIO.com's related article, "How to Make Your WAN a Fast Lane: One Company's Story."

How can enterprises overcome poor performance to reap the rewards of virtualization?

Jeff Aaron, VP of marketing at Silver Peak Systems, suggests these five tips.

1. Understand The Network Issues

For starters, it makes sense to understand why your virtualized applications and virtual desktops perform poorly across the WAN. It's typically not due to the application or VDI components, but due to the network. More specifically, virtualized environments are sensitive to the following WAN characteristics:

• Latency: the time it takes for data to travel from one location to one another.
• Packet loss: when packets get dropped or delivered out of order due to network congestion they must be re-transmitted across the WAN. This can turn a 200 millisecond roundtrip into one second. To end users, the virtual application or desktop seems unresponsive when packets are being re-transmitted. They start to re-hit the keys on their client machines, which compounds the problem.
• Bandwidth: WAN bandwidth may or may not be an issue depending on the type of traffic being sent. While most virtualized applications are fairly efficient when it comes to bandwidth consumption, some activities (such as file transfers and print jobs) consume significant bandwidth, which can present a performance challenge.

2. Examine WAN Optimization Techniques

WAN optimization devices can be deployed on both ends of a WAN link to improve the performance of all enterprise applications. The following WAN optimization techniques are used by these devices to improve the performance of virtual applications and desktops:

Latency can be overcome by mitigating the "chattiness" of TCP, the transport protocol used to by virtual applications for communication across the WAN. More specifically, WAN optimization devices can be configured to send more data within specific windows, and minimize the number of back and forth acknowledgements required prior to sending data. This improves the responsiveness of keystrokes in a virtual environment.

Loss can be mitigated by rebuilding dropped packets on the far end of a WAN link, and re-sequencing packets that are delivered out of order in real-time. This eliminates the need to re-transmit packets every time they are dropped or delivered out-of-order. By avoiding re-transmissions, virtual applications and desktops appear much more responsive across the WAN.

Bandwidth can be reduced using WAN deduplication. By monitoring all data sent across the WAN, repetitive information can be detected and delivered locally rather than resent across the network. This significantly improves bandwidth utilization in some (but not all) virtualized environments.

3. Set Application Priorities

The average enterprise has more than 80 applications that are accessed across the WAN. That means that critical applications, including terminal services and VDI, are vying for the same resources as less important traffic, such as Internet browsing. Because virtual applications and desktops are sensitive to latency, it often makes sense to prioritize this traffic over other applications using Quality of Service (QoS) techniques. In addition, QoS can guarantee bandwidth for VDI and virtual applications.