vSphere Cluster Size

I’ve pondered over this question before, “how big should I make my cluster”. At that point you have to take a step back and think about the tradeoffs with the potential impacts that your next decision is going to bring.

So you have a design decision: Should you design a vertical or horizontal vSphere HA cluster?

At this point you have some design choices to make. Let’s assume that you have an already established cluster of 5 nodes with HA and DRS enabled. There is money in the budget to buy additional servers to accommodate future growth.

There is a considerable amount of information in order for you to consider in order to make the most appropriate decision, such as cost, power, cooling, floor space and max cluster limitations to name a few. There are advantages and disadvantages to designing a vertical or horizontal cluster.

Scale-up cluster

Advantage Disadvantage
Managing fewer host reduces administrative cost HA failover potential takes longer to complete
Less hardware to provide redundancy vice splitting the cluster in two and needing more host for failover Need to be careful to stay within the cluster VM pert Host maximums. Potentially resulting in VMs not being restarted after a failure.
More cluster resources reserved for failure
For DRS, fewer migration choices available to balance out the cluster
Patching large clusters can take longer

Scale-out cluster

Advantage Disadvantage
A host failure affects fewer VMs Potential affects the maximum size that a VM can be configured
HA failover takes less time More data center floor space
Fewer resources reserved for failover Increase costs, i.e. power, cooling
Less of a concern with staying within the cluster VM per Host maximums
For DRS, it provides a greater migration choice and more opportunities for a better workload balance

On top of these advantages/disadvantages then you have to decide if your current design is meeting the demand of the business to achieve the performance, scalability and the return on investments. Gathering a current state assessment will aid the decision making process to guide you towards a design. Getting back to the scenario above it definitely depends on a multitude of factors, but if you can identify all of these different interdependencies. There is not enough information to make a decision up front, but you can start to formalize a game plan to get you in the right direction.

With these choices being taken into account, then one must decide whether you’re meeting the requirement of the business needs. I definitely enjoy making those decisions and figuring out the best possible solution available.

Useful resources

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