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.
4. Compress and Encrypt in the Right Place
Often times host machines compress information prior to transmission. This is meant to improve bandwidth utilization in a virtual environment. However, compression obfuscates visibility into the actual data, which makes it difficult for downstream WAN optimization devices to provide their full value. Therefore, it may be a better choice to turn off compression functionality in the virtual host (if possible), and instead enable it in the WAN optimization device.
Moving compression into the WAN optimization device has another added benefit: it frees up CPU cycles within the host machine. This can lead to better performance and scalability throughout a virtual environment.
IT staff should also consider where encryption takes place in a virtual infrastructure, since encryption also consumes CPU cycles in the host.
5. Go With the Flows
Network scalability can have an important impact on the performance of virtual applications and VDI. The average thin client machine has 10 to15 TCP flows open at any given time. If thousands of clients are accessing host machines in the same centralized facility, that location must be equipped to handle tens of thousands of simultaneous sessions.
When it comes to supporting large numbers of flows, there are two “best practice” recommendations. First, as discussed above, it is recommended that compression and encryption be moved off the host machine to free up CPU cycles. Second, make sure your WAN acceleration device supports the right amount of flows for your environment. The last thing you want to do is create an artificial bottleneck within the very devices deployed to remove your WAN’s bottlenecks.