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From Dave Cahill <>
Subject QEMU support in CloudStack
Date Fri, 08 Feb 2013 09:34:18 GMT

Recently I encountered two "nested virtualization" use cases which made me
want QEMU hypervisor support in CloudStack. I'm interested to hear if
anyone else is interested in this feature, and any notes on how it should
be implemented.

Here is a good explanation from OpenStack docs [2] on why they support QEMU:
"From the perspective of the Compute service, the QEMU hypervisor is very
similar to the KVM hypervisor. Both are controlled through libvirt, both
support the same feature set, and all virtual machine images that are
compatible with KVM are also compatible with QEMU. The main difference is
that QEMU does not support native virtualization. Consequently, QEMU has
worse performance than KVM and is a poor choice for a production

Here are the use cases I encountered:

[Use case: Dev environment]
    Wanted to use Vagrant [1] to create a portable multi-node dev
environment; however Vagrant uses VirtualBox, which doesn't support KVM.
    Also, devcloud uses VirtualBox and devcloud-kvm uses kvm-within-kvm. I
imagine maintenance of devcloud and devcloud-kvm would be easier if
devcloud-kvm could use VirtualBox too.
    Note: Of course, I'm aware of devcloud-kvm as an alternative for this
use case, and I'll be looking into that next.

[Use case: Demo environment]
    We may want to spin up a multi-node CloudStack install in Amazon AWS
for demo purposes.
    Again, AWS instances don't support KVM, so this is not possible without
QEMU support.

[Implementation ideas]
    The management server currently does a check for KVM support ("kvm-ok")
on the host, and refuses to add the host if that fails. I think this check
could be removed, as the agent setup scripts will fail anyway if the user
is trying to setup a certain hypervisor on a machine which doesn't support
    Create a new setting in like "use_qemu", with a
default of "false". If the person deploying CloudStack agent sets this to
"true", cloud-setup-agent and other setup scripts would ignore lack of KVM
support as long as QEMU support was available.
    Lastly, when creating the libvirt XML file for a VM, set hypervisor to
QEMU rather than KVM in the XML file depending on the config setting.

Thanks for reading,


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