When should I virtualise a Citrix server?

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This is a good question which goes on to ask in which cases is it right to virtualise Citrix?.
Even if in principle nothing stops you from Virtualising Citrix servers, there are in fact several "best practice" guidelines you should adhere to.

  • Let's start with the easiest: yes you can virtualise your Citrix servers

The example we shall give represents the vast majority of Citrix infrastructures installed in companies.

If you have 2 dual core servers for example each hosting 50 users and the users are satisfied with the quality of service, i.e. there are no abnormal behaviours (extremely slow network, blocking of ICA screens, or server CPU running at 100% capacity) then you can virtualise them.

In this intentionally simplistic example, there are no major difficulties, you carry out the PtoVs (it is however recommended where possible to completely reinstall the VMS in the case of Citrix) and return the service to your users. Nothing out of the ordinary since this is the simplest case which does not need any real adjustments as the service is provided but will not necessarily be the most efficient one.
I will deal with the issue of sizing the VM below.

  • And now for a slightly more complex case: yes you can, but be careful.

Let's go back to our example - you have 2 dual core servers hosting 50 users except that this time your applications use a lot of CPU but as we are on a physical architecture, the slowness is smoothed out and very rarely experienced by the users.

You are therefore going to virtualise your two servers as described above except that the physical servers are already heavily used in normal working times.

After virtualisation, you will inevitably and by definition have added a layer which will consume hardware resources between the Citrix VM and hypervisor hardware in a way that will be experienced to a greater or lesser extent by each editor, which in turn implies loss of throughput compared with an identical physical server.

Logically you would want to reproduce the system identically (2 VMs with 2 CPUs) but this is exactly what you mustn't do.

Indeed, numerous benchmarks have shown that you obtain the greatest efficiency on Citrix VMs by configuring only one CPU per VM. We end up with 2 single-CPU VMs (the equivalent of 2 physical dual core servers).
I say minimum as in the final analysis and as regards the Virtualisation architecture put in place, the number of users per CPU will be, in spite of everything, less in a virtual machine in a multi-user environment (TSE or Citrix).
It will sometimes be necessary to add additional single-CPU VMs in order to obtain an average load per server/CPU which is satisfactory for the quality of service expected.

In conclusion, it's important to modify the way of thought the transformation of a physical infrastructure towards the virtual.

Nicolas Jacquey
Cedric Moula

Cube Pre-Sales