You can use the vSphere 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 affinity rule establishes an affinity (or anti-affinity) 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, each MSCS virtual machine must be in a different virtual machine DRS group, linked to a different host DRS group with 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.