You can use the vSphere Web Client to set up two types of DRS groups: virtual machine DRS groups, which contain at least one virtual machine, and host DRS groups, which contain at least one host. A VM-Host rule establishes an relationship between a virtual machine DRS group and a host DRS group.

You must use VM-Host affinity rules because vSphere HA does not obey VM-VM affinity rules. This means that if a host fails, vSphere HA might separate clustered virtual machines that are meant to stay together, or vSphere HA might put clustered virtual machines that are meant to stay apart on the same host. You can avoid this problem by setting up DRS groups and using VM-Host affinity rules, which are obeyed by vSphere HA.

For a cluster of virtual machines on one physical host, all MSCS virtual machines must be in the same virtual machine DRS group, linked to the same host DRS group with the affinity rule "Must run on hosts in group."

For a cluster of virtual machines across physical hosts, all MSCS virtual machines must be in the same virtual machine DRS group and all hosts must be in the same host DRS group. The virtual machine and host DRS groups must be linked the affinity rule "Must run on hosts in group."

Caution:

Limit the number of hosts to two when you define host DRS group rules for a cluster of virtual machines on one physical host. (This does not apply to clusters of virtual machines across physical hosts.) Since vSphere HA does not obey VM-VM affinity rules, virtual machines in the configuration could be spread across hosts during a vSphere HA recovery from host failure if more than two hosts are included in a host DRS group rule.