I was listening to some VMworld talks during the weekend and something caught my attention which I hadn’t realized before. The talk I was listening to was VSP2122″VMware vMotion in vSphere 5.0, Architecture and Performance”. Now this probably doesn’t apply to most of the people reading this so let me set the scenario first:
- Different hosts from a CPU/Memory perspective in a single cluster (different NUMA topology)
- VMs with more than 8 vCPUs
Now the thing is that the vNUMA topology is set for a given VM during the power-on. This is based on the NUMA topology of the physical host that has received the power-on request. When you move a VM to a host which has a different NUMA topology then it could result in reduced performance. This is also described in the Performance Best Practices whitepaper for vSphere 5.0. A nice example of how you can benefit from vNUMA is explained in the recently released academic paper “Performance Evaluation of HPC Benchmarks on VMware’s ESXi Server“.
I’ve never been a huge fan of mixed clusters due to complications it adds around resource management and availability, but this is definitely another argument to try to avoid it where and when possible.