Citrix release XenApp Planning Guide – Virtualization Best Practices

Desktop virtualization comprises of many different types of virtual desktops. One option is to use a Hosted Shared Desktop model, which consists of a published desktop running on a Citrix XenApp server.
One of the goals when creating a design for Hosted Shared Desktops is to try and maximize scalability while still providing an adequate use experience.
Hosted Shared Desktops provide an advantage over other desktop virtualization techniques by only requiring the use of a single operating system, which significantly reduces user resource requirements and helps improve scalability numbers.

However, in order to get the most users, making correct design decisions as to the resource allocation is important.

Creating too many virtual machines or too few might negatively impact scalability.
This planning guide provides resource allocation recommendations for users running on a Hosted Shared Desktop on either Windows Server 2003 or Windows Server 2008.

Physical servers have continued to expand with more processor cores and RAM, but many older operating systems, like Windows Server 2003, are not able to take full advantage of these improvements. There are some options that can be used to try and extend the scalability of the system, like using the /PAE option for Windows 2003. Although these options do help increase scalability of a single server, they often do not provide enough value to fully utilize a server.
Virtualizing Windows Server 2003 with XenApp will often allow a greater number of user sessions on the hardware. However, creating too many virtual servers requires more operating system installations, which uses system resources while creating too few virtual servers could also result in wasted resources as the operating system is unable to scale.

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