6.0.1

Choosing an Admission Control Policy
You should choose a vSphere 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.
Avoiding Resource Fragmentation
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 ESXi host at a time. The default configuration of 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 Failover Hosts policy, resources are not fragmented because hosts are reserved for failover.
Flexibility of Failover Resource Reservation
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 as a number of hosts. The Percentage of Cluster Resources policy allows you to designate up to 100%
of cluster CPU or memory resources for failover. The Specify Failover Hosts policy allows you to specify a
set of failover hosts.
Heterogeneity of Cluster
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.
NOTE vSphere HA includes the resource usage of Fault Tolerance Secondary VMs when it performs
admission control calculations. For the Host Failures Cluster Tolerates policy, a Secondary VM is assigned a
slot, and for the Percentage of Cluster Resources policy, the Secondary VM's resource usage is accounted for
when computing the usable capacity of the cluster.
vSphere HA Interoperability
vSphere HA can interoperate with many other features, such as DRS and Virtual SAN.
Before configuring vSphere HA, you should be aware of the limitations of its interoperability with these
other features or products.
Using vSphere HA with Virtual SAN
You can use Virtual SAN as the shared storage for a vSphere HA cluster. When enabled, Virtual SAN
aggregates the specified local storage disks available on the hosts into a single datastore shared by all hosts.
To use vSphere HA with Virtual SAN, you must be aware of certain considerations and limitations for the
interoperability of these two features.
For information about Virtual SAN, see VMware Virtual SAN.
ESXi Host Requirements
You can use Virtual SAN with a vSphere HA cluster only if the following conditions are met:
n
The cluster's ESXi hosts all must be version 5.5 or later.
Chapter 2 Creating and Using vSphere HA Clusters
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