Since Microsoft changed pricing models from single licenses to a per-core model, many administrators have experienced...
Windows Server 2016 licensing cost sticker shock. Refusing to upgrade and sticking with Windows Server 2012 is, unfortunately, not an option for many, and paying the full price can be staggering. If the costs are currently too much, a split environment is a useful method to temporarily defray the damage.
Prepare for potential restrictions
Host clusters are normally split as different generations of hardware equipment are brought on board. VM rules are put in place due to application resiliency or dependency, but the same tools and concepts can be used to help with licensing. No matter how you split your environment, however, you have to do so carefully because the additional layer of management can bring new restrictions. These restrictions can alter traditional performance controls, which could affect your performance or failover ability in the event of host issues.
Pin hosts to limit licenses
One of the easiest things to do to soften the Windows Server 2016 licensing cost is to use Distributed Resource Scheduler (DRS) and High Availability (HA) rules to control where VMs can run. These rules allow you to pin certain VMs to specific hosts and limit their ability to use vMotion. You can then determine which hosts to license for 2016 and which you want to license for 2012 or Linux. On the surface, this seems ideal because it's a software-based strategy, and in terms of hard costs, it's free if you already have DRS and HA. Of course, there's always a downside, and in this case, it's all about what happens under load or failure.
DRS and HA work best when you can spread the load to as many hosts as possible in the event of host failure or performance issues. By artificially restricting those options, you handcuff your environment -- a risk that will come to light during a failure. The new DRS and HA rules will shrink the pool of hosts that can absorb VMs, which increases your restart time and resource contention. As a result, some hosts will strain for resources, while others without licenses for those VMs might have plenty of resources.
Technically, it's possible to softly enforce VM host rules, but if you allow VMs to run on hosts where they aren't licensed, you will violate licensing terms. While this might seem minor, vMotion logs will show the violation, so it might not be worth the risk.
Dedicate different licenses to split farms
The other method to mitigate the Windows Server 2016 licensing cost is to physically split your environment. This can be especially practical if you already have split farms due to multiple generations of host hardware. This would allow you to dedicate one farm for 2012 and one for 2016. Again, this seems ideal on the surface, but it works much better if you have enough hosts in each farm to allow for failures. You also have to account for the number of hosts in each farm and then match which farm has more performance capacity to the OS that has the greater need. All of this is doable but comes at a cost of time and effort.
Determine what your environment can do
The decision to split your environment comes down to a single but complex question: Can your split environment handle a failure or performance issue? Mixed production and test environments are already useful because they give more room to squeeze test servers in the event of contention. The same principle applies to splitting due to license restrictions.
Splitting your environment seems ideal until something breaks, as the damage can show itself quickly. Using DRS and HA rules to migrate and delay the Windows Server 2016 licensing cost shouldn't be considered a permanent or even long-term solution. It will prove too restrictive when failure arrives, and you need your environment to be as flexible as possible.
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