[openstack-dev] [nova][rfc] Booting docker images using nova libvirt

Sudipta Biswas 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 
as possible.
- 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.

**Design Considerations:*

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.

*Approach 1*

User workflow:

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 
this approach:

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 
filesystem approach.

- The step above allows each virtual machine to have a different copy of 
the filesystem.

- 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).

**Approach 2.*

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 

*Approach 3*

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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