BLOG: "Only 16 nodes per cluster?! - but VMware..." limitations and rightsizing of failover clusters

Dear readers,

I have actually forgot to mention a very important point, which just came into my mind very recently when a customer with a 20 nodes large cluster and SAN attached flagged issues. (without SAN the 16 nodes limit applies)

Processor compatibility mode in Hyper-V based virtualization - a phyrric victory as of today 

The larger the cluster, especially when not using S2D and Azure Stack HCI, the more likely it is one expanding the cluster with hardware of different CPU generations. 

This is something very common as one CSV using a SAN cannot be presented across different clusters. 

Then with different CPU generations customers often enable the CPU compatibility flag in the VMs so they can still leverage seamless LiveMigration between hosts / nodes with different CPU generations.

Without the compat flag only Quick Migration would be possible, causing the VM to restart, then negotiating existing CPU instructions. 

This is because Intel and AMD offer new instruction sets to gain perfomance. Some popular to name are AVX256, AVX512. 

For detailed information on Intel CPUs, head to ark.intel.com and the compare feature, to see the CPU instructions available and if you need this compat flag. 

If you think CPU compat flag is the solution, let me warn you: It's a phyrric victory! 

Other than VMware ESXi, all Hyper-V versions before Windows Server 2025 and before Azure Stack HCI 22H2 will severely degrade the CPU performance causing lower pCPU to vCPU ratios and higher CPU load.

This is because at the moment, this CPU compat flag sets the CPU to a technical state back to "Sandy Bridge" like CPU instructions.

So enabling it gives you peace of mind but effectively you are downgrading your investment. 

The dynamic CPU feature negotiation is available starting with Azure Stack HCI 22H2 and Windows Server 2025. 

Learn more about it in the documentation.