What happens when you violate affinity or anti-affinity rules?

The effects of rule conflicts

Rule conflicts and other unintended violations can cause functional or performance problems -- DRS, HA and DPM might not operate as intended. For example, DRS won't migrate VMs to enable a host maintenance mode if the migration violates a must or must not rule. Similarly, DRS won't spin up VMs, DRS won't migrate VMs for load balancing, vSphere HA won't fail over VMs and vSphere DPM won't migrate VMs to put hosts into standby mode if these operations result in a must or must not rule violation.

The easiest way to prevent functional problems due to rule violations is to use should or should not (preferential) rules rather than must or must not (required) rules. You should review rules, compare them against other rules and then test in a nonproduction environment before you deploy them in production. In addition, allocate enough hosts to the DRS group to ensure that rule snafus don't cause computing shortfalls. An administrator can employ alarms to report rule violations. For example, add an alarm to a VM using the VM is violating VM-host affinity rule as an event trigger.

Affinity rules can't read minds

It's important to point out that VM-host and VM-VM rules do not understand why those rules are implemented in the first place. That's beyond the scope of affinity and anti-affinity rules. For example, if you implement VM-host affinity rules to ensure that specific VMs only run on hosts with properly licensed software, the rules do not monitor whether those hosts are actually licensed properly or not. It's up to the business to correlate the rules to the reasons that provoked those rules. This might require a higher level of policymaking or periodic review to ensure that rules are still necessary and appropriate.

VM placement and migration are incredibly flexible -- that's one of the attributes that makes virtualization so rewarding for the enterprise. But flexible VM placement doesn't mean random VM placement.

VM placement and migration are incredibly flexible -- these attributes contribute to why virtualization is so rewarding for the enterprise. But flexible VM placement doesn't mean random VM placement. New and failover instances should never be placed entirely by chance. A virtualization administrator can take advantage of VMware's DRS and HA affinity and anti-affinity rules to govern the way that VMs are placed with respect to hosts and other VMs. Still, there are limitations and issues that administrators must consider when working with affinity.