Citrix at a high load can be a demanding application due to high kernel-resource utilization and a high level of context switches associated with its underlying Terminal Server architecture. But in many environments this high utilization often doesn't occur. Many times, application conflicts -- or the need for security isolation among applications or users -- force a horizontal scaling of Citrix servers. These kinds of applications or users that can't cohabitate on a single server can result in an organization buying new Presentation Servers even though existing server utilization is low.
In a virtualized environment, multiple server instances are enabled to run on the same physical server chassis. When application conflicts or security requirements force additional servers to be brought online, virtualization can enable it to be done with relative ease. You can copy and paste new servers to your heart's content until the hardware resources of your physical chassis max out.
VMware recently completed scalability tests that involved initiating a series of increasing user log-ons to a virtualized Citrix server, followed by a pre-recorded series of actions using Microsoft Word. The test simulated users logging in, opening a Word document and typing for up to 15 minutes. CPU resources were measured so as to identify the number of users capable of being supported by the hardware chassis when running at up to 80 percent of CPU utilization.
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