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.
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.