Lately I have been seeing more and more people recommending to limit clusters to eight hosts. I guess I might be more or less responsible for this “myth“, unintentionally of course as I would never make a recommendation like that.
My article was based on the maximum amount of VMs per host in a HA cluster with 9 hosts or more. The current limit is 40 VMs per host when there are 9 hosts or more in a cluster. With a maximum of 1280 VMs per cluster. (32 hosts x 40 VMs)
So why this post? I want to stress that you don’t need to limit your cluster based on these “limitations”. Just think about it for a second, how many environments do you know where they have 40+ VMs running on every single host? I don’t know many environments where they do exceed these limits, I guess exceptions are VDI environments…
So why would you want to “risk” exceeding these limits? Simple answer: TCO. Having two clusters is more expensive than a single a cluster. For those who don’t understand what I am trying to say: N+1. In the case of a single cluster you will have 1 spare host. In the case of two clusters you will have two spare hosts in total.
Another justification for a single cluster is DRS. More hosts in a cluster leads to more opportunities for DRS to balance the cluster. A positive “side effect” is also that the chances of resource congestion are reduced because there are more VM placement combinations possible.
Is there a recommendation? What is the VMware Best Practice? There simply isn’t one that dictates the cluster size. Although the maximums should be taken into consideration for support you should calculate your cluster size based on customer requirements and not on a max config sheet.