On twitter Mike Laverick asked a question around DRS Affinity Rules and if HA would respect these. In this particular instance the question was around VM-Host affinity rules and I noticed multiple tweeps responding and figured it would not hut to repeat this.
There are two different types of VM-Host affinity rules:
- Must aka mandatory
- Should aka preferential
The difference between these two with regards to HA is that HA will always respect a must rule. These are mandatory, even if that results in downtime for the VM. The should rule is also known as the preferential rule. In others words it would be nice if this rule can be respected, but if it can’t… no harm.
How does HA know which VM belongs to which host with regards to DRS rules? Well that is fairly straight forward. HA keeps track of which VM is compatible with which hosts. This “VM to Host compatibility list” is used for portgroups and datastores but also for DRS rules. Check the screenshot below for a hint…