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

Sudipta Biswas sbiswas7 at linux.vnet.ibm.com
Mon Aug 1 06:06:44 UTC 2016

Thanks Devdatta/Maxime for your comments.

I am definitely not rigid about implementing the workflow in Nova and 
it's well known that there can be multiple integration points for this 
work including that in docker itself. However, there are two prime 
reasons why we chose Nova as integration point in OpenStack:

1. Minimal changes to a VM boot workflow. No need to depend on Swift or 
any other service.
2. Faster boot up times - since the downloading of the virtual machine 
image is negated. Downloading the docker filesystems should be more or 
less easier.

Some comments inline.



On 27/07/16 11:59 PM, Maxime Belanger wrote:
> +1 on this,
> Still you loose all the great stuff about the containers but it is a 
> first step towards native container orchestration platform.
IMHO, it is not about just losing stuff. We are not emulating a docker 
workflow. The expectation is to have the ability to run a container 
inside a virtual machine and then take that filesystem out and run it 
natively on the hardware as desired. You can debate on whether it's 
really needed in Nova or elsewhere and I think that's a fair debate.

I am sure there are further technical challenges to overcome, if we want 
to think in this direction.
> ------------------------------------------------------------------------
> *From:* Devdatta Kulkarni <devdatta.kulkarni at RACKSPACE.COM>
> *Sent:* July 27, 2016 12:21:30 PM
> *To:* OpenStack Development Mailing List (not for usage questions)
> *Subject:* Re: [openstack-dev] [nova][rfc] Booting docker images using 
> nova libvirt
> Hi Sudipta,
> There is another approach you can consider which does not need any 
> changes to Nova.
> The approach works as follows:
> - Save the container image tar in Swift
> - Generate a Swift tempURL for the container file
> - Boot Nova vm and pass instructions for following steps through cloud 
> init / user data
>   - download the container file from Swift (wget)
I believe this has to be carried out for every docker image? That is if 
i have a nginx image and it's provisioned twice, a fresh copy of such 
has to be wget'ed every time? IF the nova workflow is acceptable, then 
there can be optimizations thought around this. At this moment, my 
implementation copies the cached image for each of the containers - 
atleast making further boots faster.
Also, how do you tackle the problem with snapshoting a container?
>   - load it (docker load)
>   - run it (docker run)
Do you run the docker native commands inside the virtual machine? In 
such a case, do you actually install docker
as a part of the cloud-init scripts? Do you have numbers w.r.t the boot 
time of the container image in this case?
> We have implemented this approach in Solum (where we use Heat for 
> deploying a VM and
> then run application container on it  by providing above instructions 
> through user_data of the HOT).
> Thanks,
> Devdatta
> -----
> From: Sudipta Biswas <sbiswas7 at linux.vnet.ibm.com>
> Sent: Wednesday, July 27, 2016 9:17 AM
> To: OpenStack Development Mailing List (not for usage questions)
> Subject: [openstack-dev] [nova][rfc] Booting docker images using nova 
> libvirt
> Premise:
> 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 
> machine.
> Following are some of the changes to the OpenStack code that 
> implements this approach:
> 1. Define a new conf parameter in nova called – 
> base_vm_image=/var/lib/libvirt/images/baseimage.qcow2
> 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:
> runcmd:
>   - 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 
> easily.
> This image is then downloaded on the compute node and a qcow2 disk is 
> created/attached to the virtual machine that boots using the 
> base_vm_image.
> 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.
> Thanks,
> Sudipto
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