The Tool Desk
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Set the reservation with PowerShell
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On the Hyper-V host, open PowerShell as an administrator.
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Run the command below, replacing
Web01with the VM’s name and20with the percentage you want to reserve:Set-VMProcessor -VMName "Web01" -Reserve 20The documented
-Reserverange is 0 through 100. See Microsoft’s Set-VMProcessor documentation.Do these 3 things before closing this tab:
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Check the setting with:
Get-VMProcessor -VMName "Web01"Inspect the returned
Reserveproperty. The processor settings apply uniformly to all virtual processors assigned to that VM.
What a CPU reservation does
A reserve is a minimum-allocation control: it specifies the percentage of processor resources reserved for the VM. Microsoft’s System Center VMM 2019 documentation describes the percentage in relation to one logical processor and says a value of zero means no specific percentage is reserved. This is a scheduling control, not a guarantee of application throughput; the cited documentation does not establish a universal relationship between a chosen percentage and workload performance. See VMM VM settings.
Choose the right CPU control
| Control | Purpose |
|---|---|
| Reserve | Sets the minimum CPU resources to allocate to the VM. |
| Maximum or limit | Caps the CPU resources the VM may consume. |
| Relative weight | Sets the VM’s priority when the host distributes CPU among VMs competing for resources. |
Use a reserve when the goal is a minimum allocation, not a cap or a higher relative priority. Microsoft describes these as separate per-VM resource controls in its Virtual Machine Resource Controls guidance.
Can you set it in a graphical interface?
System Center Virtual Machine Manager (VMM) 2019 documents a VM processor setting labeled “Reserve CPU cycles (%)”. That label and interface belong to VMM; they should not be assumed to match every version of Hyper-V Manager. The PowerShell cmdlet above is the clearest documented route when the management-tool version is uncertain.
Is CPU Groups required?
No. A per-VM CPU reserve is an individual VM setting. CPU Groups are a separate mechanism for grouping VMs and assigning shared allocations, caps, or processor affinity. Microsoft says CPU Groups are managed through the Host Compute Service and are not managed through Hyper-V Manager, WMI, or PowerShell management interfaces. For one VM’s reservation, use its per-VM processor setting; CPU Groups are unnecessary.
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