[openstack-dev] [nova][rfc] Booting docker images using nova libvirt
sbiswas7 at linux.vnet.ibm.com
Wed Jul 27 14:17:00 UTC 2016
While working with customers, we have realized:
- They want to use containers but are wary of using the same host kernel
for multiple containers.
- They already have a significant investment (including skills) in
OpenStack's Virtual Machine workflow and would like to re-use it as much
- They are very interested in using docker images.
There are some existing approaches like Hyper, Secure Containers
workflows which already tries to address the first point. But we wanted
to arrive at an approach that addresses all the above three in context
of OpenStack Nova with minimalist changes.
We tried a few experiments with the present libvirt driver in nova to
accomplish a work flow to deploy containers inside virtual machines in
OpenStack via Nova.
The fundamental premise of our approach is to run a single container
encapsulated in a single VM. This VM image just has a bare minimum
operating system required to run it.
The container filesystem comes from the docker image.
We would like to get the feedback on the below approaches from the
community before proposing this as a spec or blueprint.
1. The docker image is obtained in the form of a tar file.
2. Upload this tar file in glance. This support is already there in
glance were a container-type of docker is supported.
3. Use this image along with nova libvirt driver to deploy a virtual
Following are some of the changes to the OpenStack code that implements
1. Define a new conf parameter in nova called –
This option is used to specify the base VM image.
2. define a new /sub_virt_type/ = container in nova conf. Setting this
parameter will ensure mounting of the container filesystem inside the VM.
Unless qemu and kvm are used as virt_type – this workflow will not work
at this moment.
3. In the virt/libvirt/driver.py we do the following based on the
sub_virt_type = container:
- We create a qcow2 disk from the /base_vm_image/ and expose that 'disk'
as the boot disk for the virtual machine.
Note – this is very similar to a regular virtual machine boot minus
the fact that the image is not downloaded from
glance but instead it is present on the host.
- We download the docker image into the //var/lib/nova/instances/_base
directory/ and then for each new virtual machine boot – we create a new
directory //var/lib/nova/instances/<instance_uuid>/ as it's and copy the
docker filesystem to it. Note – there are subsequent improvements to
this idea that could be performed around the lines of using a union
- The step above allows each virtual machine to have a different copy of
- We create a '/passthrough/' mount of the filesystem via libvirt. This
code is also present in the nova libvirt driver and we just trigger it
based on our sub_virt_type parameter.
4. A cloud init – userdata is provided that looks somewhat like this:
// - mount -t 9p -o trans=virtio share_dir /mnt//
// - chroot /mnt /bin/<command_to_run>/
The /command_to_run /is usually the entrypoint to for the docker image.
There could be better approaches to determine the entrypoint as well
(say from docker image metadata).
In this approach, the workflow remains the same as the first one with
the exception that the
docker image is changed into a qcow2 image using a tool like
virt-make-fs before uploading it to glance, instead of a tar file.
A tool like virt-make-fs can convert a tar file to a qcow2 image very
This image is then downloaded on the compute node and a qcow2 disk is
created/attached to the virtual machine that boots using the
A custom qcow2 image is created using kernel, initramfs and the docker
image and uploaded to glance. No changes are needed in openstack nova.
It boots as a regular VM.
Changes will be needed in image generation tools and will involve few
additional tasks from an operator point of view.
I look forward to your comments/suggestions on the above.
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