Lately I have received many questions about the interoperability between HA and affinity rules of DRS and Storage DRS. I’ve created a table listing the (anti-) affinity rules available in a vSphere 5.0 environment.

Technology Type Affinity Anti-Affinity Respected by VMware HA
DRS VM-VM Keep virtual machines together Separate virtual machines No
VM-Host Should run on hosts in group Should not run on hosts in group No
Must run on hosts in group Must not run on hosts in group Yes
SDRS Intra-VM VMDK affinity VMDK anti-affinity N/A
VM-VM Not available VM Anti-Affinity N/A

As the table shows, HA will ignore most of the (anti-) affinity rules in its placement operations after a host failure except the “Virtual Machine to Host – Must rules”. Every type of rule is part of the DRS ecosystem and exists in the vCenter database only. A restart of a virtual machine performed by HA is a host-level operation and HA does not consult the vCenter database before powering-on a virtual machine.

Virtual machine compatibility list
The reason why HA respect the “must-rules” is because of DRS’s interaction with the host-local “compatlist” file. This file contains a compatibility info matrix for every HA protected virtual machine and lists all the hosts with which the virtual machine is compatible. This means that HA will only restart a virtual machine on hosts listed in the compatlist file.

DRS Virtual machine to host rule
A “virtual machine to hosts” rule requires the creation of a Host DRS Group, this cluster host group is usually a subset of hosts that are member of the HA and DRS cluster. Because of the intended use-case for must-rules, such as honoring ISV licensing models, the cluster host group associated with a must-rule is directly pushed down in the compatlist.

Please be aware that the compatibility list file is used by all types of power-on operations and load-balancing operations. When a virtual machine is powered-on, whether manual (admin) or by HA, the compatibility list is checked. When DRS performs a load-balancing operation or maintenance mode operation, it checks the compatibility list. This means that no type of operation can override must- type affinity rules. For more information about when to use must and should rules, please read this article: Should or Must VM-Host affinity rules.

Contraint violations
After HA powers-on a virtual machine, it might violate any VM-VM or VM-host should (anti-) affinity rule. DRS will correct this constraint violation in the first following invocation and restore “peace” to the cluster.

Storage DRS (anti-) affinity rules
When HA restarts a virtual machine, it will not move the virtual machine files. Therefore creation of Storage DRS (anti-) affinity rules do not affect virtual machine placement after a host failure.