Q: What is the Windows Server 2012 NUMA Spanning option and should it be enabled or disabled?

A: Non Uniform Memory Access (NUMA) relates to machines with multiple processors with each processor controlling and directly connected to specific blocks of memory via a bus known as local memory. The best performance comes from processes running on processor cores using local memory, rather than having to "span" NUMA nodes. This means the memory required is connected to another processor, known as foreign memory or remote memory and has a higher latency than using local memory.

By default, Windows Server 2012 enables NUMA spanning, which provides the most flexibility as virtual machines (VMs) can access and use memory in any NUMA node. But it may result in lower performance compared to forcing VMs to use memory on the same NUMA node as the processor cores.

By disabling NUMA spanning, you ensure that VMs use memory and processor cores in the same NUMA node, giving the best performance. However,  it could also mean there is a potential that VMs may not be able to start if the required amount of memory for the VM isn't available on a single NUMA node.

This should only be changed once, if, as an administrator, you feel comfortable with NUMA and the implications of disabling and also if you have some additional management suite that can help ensure best configuration.

To configure NUMA spanning, open the Hyper-V Settings and select the NUMA Spanning option (see screen shot below).

Windows Server Hyper-V NUMA Spanning Option