# 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!