I think everyone knows the three admissions control policies which can be enforced on a VMware HA Cluster. If you are using the default “Host Failures Allowed” policy, you must keep in mind that the largest virtual machine reservation will decide how big your cluster slot size is going to be. In most cases when you are using reservations that differ, I would prefer to use the “Percentage of cluster resources reserved as failover spare capacity”. But be careful, I’ve pulled two quotes which warn us for scattered resources and the need to set restart priority on large virtual machines. Duncan Epping over at Yellow-Bricks has written a real great HA-Deepdive article and says :
Basic design principle: Do the math, verify that a single host has enough resources to boot your largest VM. Also take restart priority into account for this/these VM(s). Make sure you select the highest restart priority for this VM to ensure it will be able to boot.
When looking at the slides from the VMware Design Workshop I’ve found a similar suggestion.
To avoid not being able to access a large portion of host resources in a smaller cluster, you can configure the Percentage of Cluster Resources Reserved policy. However, due to the way resources are divided across hosts, this policy cannot always guarantee that all virtual machines can be restarted.
So what does it mean? I’ve created a PowerPoint slideshow with heavy animation :-) which shows the three policies and explains the caveat of using the “Percentage of cluster resources reserved as failover spare capacity” policy.
