how do I add a vcpu to one of my KVMs in QEMU?

Question

I want to add a vcpu to a virtual server in QEMU. I tried editing the XML file in /etc/libvirt/qemu and starting the server, but that does not seem to work.

Answer 1

I assume you're using libvirt based on you trying to edit its configuration. You can't just edit the configuration directly because the libvirt daemon won't notice the changes. You need to use a libvirt client to make the change.

1. Stop the VM

2. Edit the configuration with a libvirt client, e.g.:

   # virsh edit server0
   

   change the number of CPUs, save and quit.

3. Start the VM again, e.g.:

   # virsh start server0