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Would I get more users on physical or virtual servers?

Physical servers or virtual servers? Ron Oglesby shares his thoughts in this expert response.

I have a question on scalability of Terminal Services on large physical servers vs. multiple Terminal Server virtual machines (VMs) on the same hardware.

Basically, we have standard hardware which we use for VMware ESX servers (4x Dual CPU Opterons with 48GB RAM). We want to get as many users as possible onto this hardware using Citrix PS 4.5.

From Ron's presentations on the subject I gather that you only get half as many users on a Virtual terminal Server as a Physical Terminal Server. But the testing only went as far a two CPU physical servers, not eight CPU servers.

Our servers are eight CPU and the only document I can find on scaling up above two CPU's is HP's document from 2003, and that says it tails off even at four CPU's.

However, the document is old, doesn't even go near eight CPU's, doesn't consider dual core, 64-bit or virtualisation technology built into the chips themselves like IntelVT or Vanderpool.

So the question is, can we run an eight CPU Citrix servers with more users than if we ran eight single CPU virtual machines running on exactly the same hardware.

VMware ESX is our preferred choice, but I hear that SWSoft's Virtuozzo doesn't incurr the overhead that ESX does, so you actually get the same number of users on a virtual machine as on a physical machine. Can you confirm this?

For the most part, 64-bit Terminal Services fixes issues with limits on memory scalability. If you are running out of memory, then newer 64-bit operating systems (by themselves) will fix that issue. But if you attempt to run it on VMware or another virtual machine platform the issue is processor overhead.

The CPU use in a terminal server environment is very different from that of a standard server. In all of my previous tests, the bottleneck or greatest performance degradation was at the processor level. If you are running out of memory and want to go 64-bit you might as well go physical instead of virtual (from a performance perspective).

If you have other reasons to go virtual, and memory is your bottleneck, then performance will probably be fine, albeit at a higher cost as you now have to pay for the virtualization layer and more hardware than if you ran physical. Memory is expensive above 16GB per server.

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