Increase a VM’s vcpu count with virsh

You have a virtual machine you created with virsh. You want to increase the number of vcpus in the virtual machine, so you use the obvious command:

virsh setvcpus --count 8 <vm name>

… and get the irritating error message:

error: invalid argument: requested vcpus is greater than max allowable vcpus for the domain: 8 > 2

This is virsh telling you that you can’t increase the number of vcpus to a number larger than what you started with.

Although virsh doesn’t support increasing the number of vcpus while the VM is running, you can change the number of vcpus if you’re willing to reboot the VM. All you need to to is to edit the virsh XML file with:

virsh edit <vm name>

Look for the line “vcpu placement” and increase the value to the number of vcpus that you want. I changed the vcpus from 2 to 8 here:

<vcpu placement='static'>8</vcpu>

Save the file.

Shutdown the VM:

virsh shutdown <vm name>

Wait until the VM’s status is “shut down”.

virsh list --all

Destroy the VM:

virsh destroy <vm name>

Start up the VM:

virsh start <vm name>

Once the VM starts you’ll have more vcpus running.

Hope you find this useful.

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