Over the last couple of weeks I’ve been asked this question by various field people and potential customers: does VMware support non-uniform / heterogeneous VSAN clusters? The question typically comes during the pre-sales stage and by the sounds of it is one of those myths that competitors use to give their product the edge in a bake-off.
For those who just care about the facts I can be short: Yes we support non-uniform / heterogeneous VSAN clusters. Our very first datasheet has the following statement in it:
Hardware independence – Virtual SAN can be deployed on hardware from any server manufacturer. This gives you the flexibility to build out customized storage systems in heterogeneous hardware environments.
Anyone telling you VMware VSAN does not support non-uniform / heterogenous environments is badly informed, or not being honest. Some of you may say, hold on… I’ve seen posts where people are making a recommendation for uniform clusters. A strong recommendation even in some cases, does that mean VSAN doesn’t function properly when you use a non-uniform cluster? Again, I can be short: No it does not. But why did they make this recommendation?
The reason is simple: lower operational complexity. Just like with vSphere HA and vSphere DRS, balancing resources and doing calculations to see if your cluster has sufficient spare resources to reprotect VMs after a failure is easier when you have a uniform cluster. Not that it is rocket science when the cluster is non-uniform, as you will just need to assume that if something fails it will be the host providing the most storage capacity. Luckily with VSAN the whole cluster acts as “rebuild capacity”, which means that mitigating this risk is relatively easy.
As an architect I usually prefer to avoid risks though, but also understand that situations like these occur and there is no reason to fear them. I spoke with a customer who is managing a rather large non-uniform environment last week. They started with 1U hosts, but for various reasons decided to go with 2U hosts more recently. Using the same cache to capacity ratio per host they’ve had no challenges whatsoever in this environment (they even went with more cache as SSD prices lowered). Performance and management has been a breeze, they are very keen on standardization but the ability to be flexible when needed is actually what they love about VSAN.