vmware

DRS affinity rules are used in DRS clusters to control the placement of virtual machines. Three types of rules exist:

affinity rules – DRS will try to keep certain VMs together on the same host. These rules are often used in multi-virtual machine systems to localize the traffic between virtual machines.anti-affinity rules – DRS will try to keep certain VMs are not on the same host. These rules are often used to keep the VMs separated for availability reasons.VM to host rules – specify whether VMs can or can’t be run on a host. They can be preferential or required. These rules are used in conjunction with DRS groups for ease of administration. A DRS group can either consist of one or more VMs or one or more ESXi hosts.NOTE – if two rules are in conflict with each other, they will no be enabled.

The VM to host DRS rules can either be preferential or required. A preferential rule is softly enforced and can be violated if necessary, for example to ensure the proper functioning of DRS, HA, or DPM. Consider the following example (image source: VMware):

As you can see from the picture above, we’ve created two DRS groups for virtual machines (Group A and Group B) and two DRS groups for ESXi hosts (Blade Chassis A and Blade Chassis B).

The goal of this design is to force the VMs in Group A to run on the hosts in Blade Chassis A and to force the VMs in Group B to run on the hosts in Blade Chassis B. But if the hosts in Blade Chassis A fail, the VMs from Group A will be moved to hosts in Blade Chassis B.

The VM to host DRS rules can either be preferential or required. A required rule is stricty enforced and can never be violated, unlike a preferential rule. Required rules are often used to enforce host-based licensing. For example, if the software that is running in your VMs have licensing restrictions, you can use a required rule to run those VMs only on hosts that have the required licenses.

Here is an example (image source: VMware):

In the picture above you can see that we’ve created a DRS group for virtual machines named Group A and a DRS group for hosts named ISV-Licensed. The goal of this design is to force the VMs from Group A to run only on hosts in the ISV-Licensed DRS group because these hosts have the required licenses. But if the hosts in the ISV-Licensed group fail, VMs from Group A will not be moved to other host DRS groups.