I have seen a lot of scalability reports lately around desktop virtualization. This is good in that we can start to see how the different things we do can provide better capacity. However, one thing that does trouble me is when I see tests only allocating 512 or 768MB of RAM to a Windows 7 VM. Sure it works. And yes it does successfully complete the scalability test, but remember what the scalability test is testing. It is not telling you how many users YOU will get. It is telling you how well the infrastructure can scale and what bottlenecks we might experience when the hardware is stressed. Unfortunately, because of these tests, too many people believe that they too can roll out a virtual Windows 7 desktop on 512MB of RAM. I wish that was the case. In fact, I bet Microsoft wishes that was the case as well. But I’m sorry to say, but sadly it’s true that it is not.
I wanted to provide you with what we (myself, Nicholas Rintalan, Doug Demskis and Dan Allen) figure is a reasonable estimation for resource allocation for Windows 7 and Windows XP desktops when delivered in the hosted VM-Based virtual desktop model (or VDI for short).
|User Group||Operating System||vCPU Allocation||Memory Allocation||Avg IOPS (Steady State)||Estimate Users/Core|
|Light||Windows XP||1||768MB-1 GB||3-5||10-12|
|Windows 7||1||1-1.5 GB||4-6||8-10|
|Normal||Windows XP||1||1-1.5 GB||6-10||8-10|
|Windows 7||1||1.5-2 GB||8-12||6-8|
|Power||Windows XP||1||1.5-2 GB||12-16||6-8|
|Windows 7||1-2||2-3 GB||15-25||4-6|
|Heavy||Windows XP||1||2 GB||20-40||4-6|
|Windows 7||2||4 GB||25-50||2-4|
See anything shocking? How about 1.5 GB of RAM for light Windows 7 users? Remember, we are talking about the typical implementation that we have seen. That means the desktop image includes antivirus agents, malware agents, monitoring agents and line-of-business applications. These agents and applications add up (especially Line-of-Business apps). Even though the user is a light user, that means they only run 1 or 2 applications. However, those applications are more than Microsoft Word. They are the main Line of Business application. So even though they don’t hit the CPU hard, they still consume a lot RAM (of course these implementations could just put the line of business app on XenApp and not worry about providing a true Windows 7 desktop for light users).
Are you wondering what defines the four groups of users? Here is how we define them:
|Light||One or two applications no browser-based activity|
|Normal||Multiple applications with limited browser-based activity|
|Power||Many simultaneous applications with extensive browser-based activity and Internet-based applications.|
|Heavy||Few applications but have heavy system resource requirements. Data processing, compiling, or graphics manipulation are common applications.|
I’m hopeful that as you start planning your XenDesktop environment, you use realistic approximations on your virtual desktop specifications.
If you want to know more about resource allocation as well as many other areas for planning a XenDesktop environment, then sign up for the XenDesktop Design Handbook. This helps guarantee that you have the latest and greatest design information available.
Daniel – Lead Architect