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.
Thanks,
Sudipto
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.kulka...@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 <sbisw...@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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