The VM-Host affinity rule allows you to control which hosts in the cluster can run which VMs and requires that at least one VM DRS group and at least one host DRS group are created before managing host affinity rules.

The typical use case is about licensing, where only a subset of ESXi hosts are licensed for a particular software that the VM consumes; Oracle, for example. Without DRS, you would need either a dedicated cluster for Oracle VMs (because you need to assign an Oracle license to each physical server where the VM might run), or, by using DRS affinity rules, you can specify the subset of hosts that will be used by such VMs.

To create a VM-Host affinity rule, proceed as follows:

1. From vSphere Client, right-click the cluster to configure and select the Settings option.
2. Under Configuration, select VM/Host group and click on the Add button to create a VM group and a host group.
3. Specify a name and select from the Type drop-down menu the VM group. Click the Add button to add members to this group, and then click OK to save the configuration.
4. Repeat steps 2 and 3 to create a host group.
5. Now, under Configuration, select VM/Host Rules and click the Add button.

6. Enter a name and select from the Type drop-down menu the Virtual Machines to Hosts. Specify VM Group and the rule (for example, Must run on hosts in a group), select the Host Group to associate, and click OK to save the rule:

The options available for the rule can be one of the following:

• Must run on hosts in group: VMs in the selected VM group must run on host members of the specified host group. DRS will never break the rule, nor will vSphere HA. If there is only one ESXi host in the group, HA will not restart the server on the other nodes.
• Should run on hosts in group: VMs in the VM group should run on hosts of the specified host group, but it is not required. DRS will try its best to satisfy the rule, but in some cases, the rule might be broken.
• Must not run on hosts in group: VMs in the VM group must never run on host members of the specified host group. DRS will never break the rule.

• Should not run on hosts in group: VMs in the VM Group should not, but might, run on hosts of the specified host group. DRS will try its best to satisfy the rule, but in some cases, the rule might be broken.