You should choose a VMware HA admission control policy based on your availability needs and the characteristics of your cluster.

When choosing an admission control policy, you should consider a number of factors.

Resource fragmentation occurs when there are enough resources in aggregate for a virtual machine to be failed over. However, those resources are located on multiple hosts and are unusable because a virtual machine can run on one ESX/ESXi host at a time. The Host Failures Cluster Tolerates policy avoids resource fragmentation by defining a slot as the maximum virtual machine reservation. The Percentage of Cluster Resources policy does not address the problem of resource fragmentation. With the Specify a Failover Host policy, resources are not fragmented because a single host is reserved for failover.

Admission control policies differ in the granularity of control they give you when reserving cluster resources for failover protection. The Host Failures Cluster Tolerates policy allows you to set the failover level from one to four hosts. The Percentage of Cluster Resources policy allows you to designate up to 50% of cluster resources for failover. The Specify a Failover Host policy only allows you to specify a single failover host.

Clusters can be heterogeneous in terms of virtual machine resource reservations and host total resource capacities. In a heterogeneous cluster, the Host Failures Cluster Tolerates policy can be too conservative because it only considers the largest virtual machine reservations when defining slot size and assumes the largest hosts fail when computing the Current Failover Capacity. The other two admission control policies are not affected by cluster heterogeneity.