Desktop virtualization comparison: VDI vs. Remote Desktop Services

For desktop virtualization, you have a choice between the paths of VDI and Remote Desktop Services. Robert Frost took the road less travelled, but you don't have to choose just one.

For desktop virtualization in your organization, you can take the VDI route or the Remote Desktop Services route -- or you could go with a combo.

Microsoft Remote Desktop Services (RDS), previously called Terminal Services, has been with us for 15 years. Virtual desktop infrastructure (VDI), on the other hand, is still getting its engine going in many organizations.

To help you choose the right path for your environment, see how VDI and RDS stack up against one another in this desktop virtualization comparison.

What's the difference?

Microsoft Remote Desktop Services and VDI are each suited to different scenarios.

VDI has a separate virtual machine (VM) for each user and uses a desktop operating system in that VM. It puts isolation between users, so VDI is better for highly regulated or secured environments where information disclosure is a big issue. That also means you have lots of copies of Windows to keep clean and safe. Plus, you have to deal with mass VM boots and updates, which can drain VDI storage performance.

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Of course, VDI environments don't have to be locked down. You can allow users full administration and local application install rights in their personal VM. However, this attaches the user to the specific VM and often means the VM cannot be updated using a single master image update.

RDS, on the other hand, has users sharing a VM running a server OS. That makes Microsoft Remote Desktop Services more suited to task-oriented and collaborative uses. Since there are fewer Windows instances, the storage load for updates and reboots is less. There's also less CPU and RAM overhead, so you can usually run more users per unit of hardware.

RDS environments are always locked down because they're a shared resource and need to provide a uniform level of service to every user. Microsoft Remote Desktop Services also provides high availability through clusters or farms of servers. That means RDS VMs are effectively disposable: You just need enough available for the users who are connected at any given moment. Since you don't need specific VMs to be constantly available, you don't need expensive shared storage and can place RDS VMs on local storage in the host.

As you can see from our desktop virtualization comparison, these two technologies address very different uses. So, many organizations deploy both. Different groups of users benefit from VDI and Microsoft Remote Desktop Services, as they're really complimentary solutions. To make things easier, you can use the same Connection broker and the same client to run both VDI and RDS.

Where vendors can improve RDS support

To choose your desktop virtualization path, it's important to know a little more about the major vendors. There are lots of VDI offerings out there, but here's how Citrix Systems Inc. and VMware Inc. provide VDI and RDS support:

Citrix has been doing data center-based user desktops for around 20 years and hitched onto the Windows wave nice and early. In fact, what is now called RDS was originally Citrix Multi-Win. When Microsoft first released Terminal Services as a special edition of Windows NT 4.0, it was Citrix developers that wrote the code, licensed to Microsoft.

Citrix held onto its very efficient HDX protocol, an important part of VDI. It also has Citrix Provisioning Server, which streams the OS to a VM as it boots and enables the single image OS maintenance nirvana -- and works with both RDS and VDI environments. Citrix XenApp also works with RDS and makes it a more scalable and WAN-optimized solution. Making things easier for customers that use both RDS and VDI, Citrix provides a license that covers both XenDesktop and XenApp.

I'm an old-school Citrix guy and I always used to say Citrix is very easy to do... badly. Citrix has been tarred with a lot of poor implementations, but the VDI wave allowed the company to revitalize itself.

VMware could do a much better job of supporting RDS in its View product. The ability to use View to broker and secure a connection to RDS has been available since version 3, but you won't often hear about that. The big restriction with View and RDS is the lack of VMware's PC-over-IP (PCoIP) protocol support. It only supports Microsoft's own Remote Desktop Protocol. Teradici, however, recently announced that it will add PCoIP support to Remote Desktop Session Host.

I suspect the reason is that the software PCoIP rendering engine in View is quite CPU intensive, meaning two or three PCoIP users could redline an RDS server by watching YouTube videos. Now that Teradici released its PCoIP server offload card, the Apex 2800 that removes the CPU load for the PCoIP encode, I'd love to see it used to enable PCoIP from RDS.

The other thing VMware could do to improve RDS support is bring its linked clone technology to RDS hosts. This would allow a single master image to be patched and maintained, and then you could use it to automatically update dozens of RDS VMs.

Keeping the caveats from this desktop virtualization comparison in mind, you can determine where VDI or RDS might solve problems you have in your business. Neither solution is right for all uses, and few organizations use only one approach. Both paths can help you cope with the potential end of the Windows desktop.

Alastair Cooke
is a freelance trainer, consultant and blogger specializing in server and desktop virtualization. Known in Australia and New Zealand for the APAC virtualization podcast and regional community events, Cooke was awarded VMware's vExpert status for his 2010 efforts.

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