VMs Failing to Start? Here's What Changed in Windows Server 2025's NUMA Settings!

Hi, I'm Shiichan! Today I've got a story that anyone running Hyper-V should know about: a quiet but important behavior change. Let's make sure nobody gets surprised by VMs that suddenly won't start after an upgrade.

What was announced?

This comes from the JP Windows Support blog (Microsoft Japan's Windows Support team). They explain that in Windows Server 2025's Hyper-V, the virtual processor allocation check performed when starting a VM with NUMA spanning disabled has become stricter than it was in Windows Server 2022 and earlier.

The story so far

In Windows Server 2022 and earlier, even with NUMA spanning disabled, a VM could still start even if the total logical processors assigned to running VMs overcommitted a single physical NUMA node.

In the blog's test environment (2 physical NUMA nodes, 16 logical cores per node, NUMA spanning disabled), starting three 24-vCPU VMs (12 vCPUs placed on each node) succeeded for all three VMs on Windows Server 2022 and earlier.

What changes

Running the same test on Windows Server 2025, the first VM starts fine, but the second and later VMs fail to start because they overcommit a physical NUMA node. In other words, a configuration that used to work can now fail to boot after nothing more than an OS upgrade.

On the other hand, a configuration where the VM's virtual NUMA node count and max processors per node are set explicitly (24 vCPUs, 2 virtual NUMA nodes, 16 max processors per node) starts successfully on both Windows Server 2022 and earlier and on 2025. And a VM configured with 24 vCPUs and a single virtual NUMA node, with no explicit NUMA settings, fails to start on both versions.

Dive Deep

The blog recommends considering enabling NUMA spanning for configurations that overcommit vCPUs at the physical NUMA node level. With NUMA spanning enabled, a VM's vCPUs can be allocated across multiple physical NUMA nodes, which should avoid tripping this stricter check.

Wrap-up

• Windows Server 2025's Hyper-V tightens the vCPU allocation check when NUMA spanning is disabled
• Configurations that overcommit a physical NUMA node could start fine on Windows Server 2022 and earlier, but can now fail to boot on 2025
• Configurations with explicit virtual NUMA node counts and max processors per node are less affected
• The recommended workaround is enabling NUMA spanning

If you're running many VMs densely packed on a NUMA-aware Hyper-V host, it's worth checking your configuration before moving to Windows Server 2025!