Note: Licensing Optimization by cluster will only apply to larger organizations that have multiple hypervisor clusters running in their environment, but for those that do this is could be a real money saver.
At least for Windows and some variations of Linux, you can have a requirement to license the OS by the number of potential Cores or Sockets on which it might need to run.
For Windows, the minimum number of cores that can be licensed on machine are 16 and would require a Datacenter license. In a three node cluster, this would mean a minimum of 48 licensed cores.
This might seem too obvious, but for that reason it’s often overlooked. Here is an illustration of how to read this AIR report graph and put it to use towards saving licensing fees in your environment.
Overtime people will create new VMs and likely won’t consider the licensing impact associated with where they place that VM. This can create an almost "OS Fragmentation" like effect.
Consider this scenario:
In this environment there are two clusters, each cluster is comprised of 3 nodes and each node has a 2 socket / 4 core per socket hardware spec.
Example A (left graph):
Here we can see that each cluster is hosting approximately 50% licensable Linux and 50% licensable Windows Operating Systems.
The net effect is that each Operating System could have the potentially to cause an over licensing scenario whereby 2 clusters need Windows licensing instead of just one. In a true-up scenario this could be subject to heavy unforeseen fees.
Example B (right graph):
By attempting to concentrate all licensable OSes to a single cluster, the net effect is that it’s possible to reduce licensing overhead to only 1 cluster, thus reducing licensing overhead by 50% (and perhaps 50% associated with two different OS vendors).
The larger the environment, the more potential for OS licensing fragmentation and the greater the value of calling attention to the disbursement of OSes between clusters.
Here is another before and after way to look at this just to drive the point home.
In reality this might force the assumption that all VMs are equal and swapping a VM from one cluster to another is an even trade. This will rarely be the case. There are many other factors like function, location and failover that will factor into VM placement and licensing requirements.
While creating 100% homogenous clusters to an OS might not always be achievable, it’s still worth paying attention to as licensing is expensive and any recoup of overage is pure benefit.