I am evaluating a system architecture where the application is split on two virtual servers on the same hardware.
The reason is that the overall system will perform better if a certain part of it runs on its own virtual server. Is this correct?
I would think that if the processes run on the same hardware, a split on two servers will give an overhead to communication compared with installation on one virtual server.
To make sure, it sounds like you're asking, more or less, about how 2 (or more) virtual machines might "compete" with one another, and how might separating the processes run on them affect the overall performance?
Quick Answer: In short, the good news is that you can control how the VM's "fight" over resources very tightly if you wish. This will keep VM's from competing with each other over things like RAM, CPU, etc. and can improve the overall performance of your application. I think you're interested in knowing about two main things: VM reservations/limits/shares and resource pools. Links are included below.
In-depth Answer: In general, it's a great idea to separate the functionality of your application. A web server and a DB server running on different machines is a perfect example of this. I don't know about your application in particular, but if it's not leveraging multi-threading (to enable the use of multiple processors) already, separating your application onto two servers might really help performance. Here's why:
VM's understand, in a sense, what HW they're running on. This means they know the CPU/RAM/disk space that's available to them. So let's say your ESX server has 4 CPU's and 16 GB of RAM. When you create a VM, you're free to give 2 CPU's and 8 GB of RAM to each server, or you can alter the setting to meet your needs. In VMware, you can guarantee a certain resource level by using something called: limits, shares, and reservations. Documentation on them can be found here http://pubs.vmware.com/vsphere-4-esx-vcenter/index.jsp?topic=/com.vmware.vsphere.vmadmin.doc_41/vsp_vm_guide/configuring_virtual_machines/t_allocate_cpu_resources.html, among other places. These reservations will help you guarantee that a certain VM always has access to a certain level of resources and will keep VM's from causing contention over RAM, CPU, etc. VMware also offers another extension of this idea using something called "resource pools", which are pools of RAM, CPU, etc. that can be set aside for certain machines. You can read about them here: http://pubs.vmware.com/vsphere-4-esx-vcenter/index.jsp?topic=/com.vmware.vsphere.resourcemanagement.doc_41/managing_resource_pools/c_why_use_resource_pools.html.