A while back I had discussion with someone and he asked me if it was possible to limit the amount of eggs in a single basket, in other words limit the amount of VMs per host. The reason this customer wanted to do this was to limit the impact of a failure. They had roughly 1500 VMs in their cluster and some hosts carried 50 VMs while other had 20 or 80. This is the nature of DRS though and totally expected.
If one of these hosts would fail, and lets say they had 80 VMs the impact of that would be substantial. To minimize the risk they wanted to limit the amount of VMs per host. I had thought about this before and had already asked the HA and DRS team if they could do anything around this. The DRS team started looking in to it and to my surprise they managed to get it in quick.
In VMworld 2012 session “VSP2825: DRS: Advanced Concepts, Best Practices and Future Directions” by Ajay Gulati and Aashish Parikh a solution is presented. (You can watch this session for free on youtube, highly recommended!) This solution is a new vSphere DRS advanced setting which is introduced in vSphere 5.1.
Note that when you configure this setting it might impact the performance of your virtual machines as it could limit the load balancing mechanism of your cluster. If you have no requirements to limit the amount of VMs per ESXi host, don’t do it. When this setting is configured, vSphere DRS will not allow migrations to a host which has reached the threshold and will also not admit new VMs to the host if it has reached the threshold.