ASR Replication Appliance install

Hello Gavin Dack,

You’ve run into an issue during your Azure Site Recovery (ASR) replication appliance setup: although you provisioned a Standard D8s_v3 VM with 8 vCPUs, the Appliance Configuration Manager only sees 4 cores. Common causes include:

• Windows boot settings limiting the number of processors
• Azure’s NUMA or CPU-group configuration causing the guest OS to report fewer cores
• VM resizing or configuration changes not properly applied—double-check in the Azure portal that your VM is still a Standard D8s_v3 (8 vCPUs)
• On-premises hypervisor settings provisioning fewer vCPUs—ensure your host assigns all 8 cores to the VM

Review and correct these areas so that Windows can detect and use all 8 vCPUs. 

The replication appliance prerequisites check doesn’t just look at the VM SKU, it queries Windows to read available cores. Microsoft ASP installers perform a strict hardware check, it will not proceed it detects fewer than 8 CPU cores, regardless of RAM or other specification, this is by design and cannot be overridden for production scenarios

Deploy the configuration server in Azure Site Recovery - Azure Site Recovery | Microsoft Learn

Support requirements for Azure Site Recovery replication appliance - Azure Site Recovery | Microsoft Learn

To verify CPU count in Windows, Run the following PowerShell command inside the replication appliance VM:

Get-WmiObject -Class Win32_Processor | Select-Object NumberOfCores, NumberOfLogicalProcessors

For D8s v3 you should see 8 logical processors. If you see 4, the issue is OS configuration.

Ensuring Windows Isn’t Capped to Four Processors: Sometimes Windows is forced to use fewer processors; you can verify by following steps.

• Press Win + R, run msconfig
• Go to Boot -> Advanced options.
• See if Number of processors is checked.
  • If it is and limited to 4 → uncheck it (Windows will use all available).
• Reboot the VM and rerun Appliance Configuration Manager.

To resolve the issue:

• Resize or reconfigure the VM: Ensure the machine is deployed as a Standard D8s v3 (or another SKU with at least 8 vCPUs). If you change the size, perform a full power cycle (stop/deallocate, then start) to apply the new configuration.
• Verify inside the OS: Confirm that Windows recognizes all 8 vCPUs before proceeding.
• Retry the setup: Once the VM shows the correct core count, rerun the Appliance Configuration Manager installation.

Please let us know if you have any further questions. Thank you!