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I'm using libvirt with KVM/QEMU. I found a great PPA, uvtool (http://s3hh.wordpress.com/2013/12/12/quickly-run-ubuntu-cloud-images-locally-using-uvtool/), that manages images and metadata in a volume storage pool. I can quickly create new VM guests from the command line. This makes it easy to work with shell scripts to create/destroy using uvt-kvm (uvtool's wrapper) and start/stop using virsh (from libvirt package).

For some reason, guests have only one cpu/core and I need to set the number from the command line. I found references to virsh's setvcpu option, but no examples of how to use it. Here's what I tried with the error messages:

Start existing guest with 2 CPU's:

tahoar@pttools0:~$ virsh start test-host setvcpus --count 2
error: command 'start' doesn't support option --count

Start existing guest with defaults:

tahoar@pttools0:~$ virsh start test-host
Domain test-host started

Set CPU's on running guest:

tahoar@pttools0:~$ virsh setvcpus test-host --count 2
error: invalid argument: requested vcpus is greater than max allowable vcpus for the domain: 2 > 1

Thanks for any help.

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When creating VMs with uvt-kvm create, you can use the --cpu cores option to set the number of CPUs in the VM.

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For setvpcus to work you need to have "free" vcpus for the VM configured already. An example xml config extract where the cpu count is 4 and maximum 16

<vcpu placement='static' current='4'>16</vcpu>

I only tested this on CentOS 6 but it should be the same in Ubuntu. Some more info here:

https://serverfault.com/questions/617797/kvm-how-to-update-amount-of-cores-procs-during-runtime/617798#617798

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