I’ve written about HA admission control recently, in this post, so there will be a little bit of overlap here. When choosing an admission control policy for a particular cluster, there are a number of things to consider. These are covered in more detail in the vSphere Availability Guide, so I will just cover them briefly here.
The first thing to decide is whether to enabled admission control or not:
Generally speaking, admission control should be enabled, as it helps ensure that virtual machines will be able to be powered on in the event of a host failure. Once enabled, a decision needs to be made on which admission control policy to use.
One of the issues to consider is resource fragmentation. A cluster is considered to be suffering from resource fragmentation when it has enough total/aggregate resources to provide for the cluster’s failover requirements, but those resources are spread across the hosts in the cluster. This could result in the inability to power on certain VMs, with larger resource requirements, as there may not be enough available resource on a single host to satisfy those requirements. This can be a problem when ‘Percentage of Cluster Resources’ is used as the admission control policy, though DRS will help by moving virtual machines around to free up resource. Resource fragmentation isn’t an issue if you are using the ‘Host failures cluster tolerates’ admission control policy, however the downside of that is that you may end up reserving too much resource on your hosts, resulting in poor consolidation ratios. The ‘percentage’ policy is more flexible and will result better consolidation ratios when a range of virtual machine reservations are in use.
Another thing to be aware of when deciding upon which admission control policy to use is the hardware specifications of the hosts in the cluster. If the hosts have different amounts of CPU or physical memory resource then the ‘percentage’ policy will be the best choice.