VM /ETC

There were 2 messages from the Citrix Summit 2008 conference in Orlando this week that really stuck with me. I’ve already posted about the first, “XenServer 5 is ready for the enterprise“. The other message was “Use XenServer to host XenApp”.

Virtualizing XenApp is a concept to get on board with in general, but it’s also logical to assume Citrix can best support and optimize their products when they are used together.

The message is that virtualizing existing and future implementations of XenApp (or Presentation Server) on XenServer can

• Reduce physical server count
• Increase availability
• Increase flexibility
• Improve performance

Let me explain how it was explained to me during a hands on lab.

Consider that there are a lot of Citrix implementations today that are 32 bit. Next understand that 32 bit Windows can only address 4GB of RAM. The point is that virtualizing these 32 bit Citrix servers on to today’s 64 bit hardware (required by XenServer) will not sacrifice the hardware resources dedicated to silo-ed XenApp physical servers. This is because XenServer can utilize more than 4 GB of RAM (now up to 128GB tested) and bigger and faster CPUs. Of course you always have the option to migrate to 64 bit Windows and 64 bit XenApp, but let’s just assume you want to stay status quo for now. Here’s the example presented to me this week that really illustrates the point:

A physical 32 bit Citrix XenApp (Presentation) Server with 4GB of RAM and two 2.0 Ghz CPUs can support up to 100 users. A 64 bit XenServer with 16GB of RAM and 2 Quad core CPUs can support over 300 users across four 32 bit VMs. Assume each VM uses 4GB of RAM and 2 vCPUs, and you still are not even over utilizing your host resources. You have reduced your physical server count by 3 or 4x, however. That’s just for a single, stand alone host.

Now consider a 500 concurrent user XenApp farm that uses 20 physical servers. This is a more conservative design so you average 20 to 50 users per XenApp server. Your physical server hardware is the same dual 2.0 Ghz CPU and 4 GB of RAM set up. This is a very common configuration today. Virtualizing these 20 servers can be accomplished on 6 XenServer hosts each with dual socket 3.0 Ghz quad core CPUs and 16 GB of RAM. Using the same four 32 bit VM per host model as before (2 vCPUs and 4 GB RAM each) you’ve consolidated your Farm as is without compromising performance or available resources. Doing the math you should have enough room for 24 VMs, but we’ve left that reserve to provide for the overhead needed for virtualizing 32 bit XenApp.

Okay, if you are skeptical, at this point you are thinking “but now I have as many as 200 users trying to access applications from one host and that’s not a good thing.” That’s true, but put 4 to 6 NIC ports in your host and you’ve still got plenty of bandwidth for your user connections.

When you consider using shared storage and Citrix Provisioning Server it really gets interesting. There’s always the obvious high availability and live migration features for the increased availability, but now you have added flexibility too. You most likely can standardize your 20 XenApp VMs in to 3 or 4 standard application sets or configurations, and then create an image for each. Provisioning Server can then dynamically stream those images to new “virtual bare metal” VMs from your 4 templates. Now you are using significantly less storage, and you can scale your Citrix Farm up and down as needed. When it’s time to patch or upgrade your applications you only have to modify the templates instead of all 20 servers. By the way, Provisioning Server is included when you by XenServer and XenApp Platinum Editions.

Finally, Citrix claims that their specialized settings for XenApp VMs result in better performance on XenServer than if XenApp was running on other vendor’s virtualization hosts. I don’t want to try to prove or dispute this claim (I saw graphs and I listened to the testing methodology), but I will say it makes sense to me that Citrix would know best how to virtualize their own products.

For additional reading about XenApp on XenServer check out these links.

• Shedding some light on XenApp on XenServer performance tuning – http://community.citrix.com/blogs/citrite/anilma/2008/08/04/Shedding+some+light+on+XenApp+on+XenServer+performance+tuning
• You want to virtualize XenApp. Really? – http://community.citrix.com/pages/viewpage.action?pageId=30933014
• XenApp on XenServer – Round 2 (Ding, Ding) – http://community.citrix.com/blogs/citrite/danielf/2008/07/01/XenApp+on+XenServer+-+Round+2+%28Ding%2C+Ding%29
• TechTalk: Make Server Virtualization Work for Citrix Presentation Server – https://www1.gotomeeting.com/register/912651626

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