docker-flexvol

A FlexVolume driver for kubernetes which allows you to mount Docker volumes to your kubernetes pods.

Status

Proof of concept

Using

This flex volume plugin can start a fresh docker volume from a specified container image and attach it to the kubernetes pod. This is useful for scenarios where you need your pod to access a bunch of files and if already have that data as a docker container image, you can just specify the container image name and the volume name in your kubernetes pod definition and make it available to whatever is running in the pod.

Installing

In order to use the flexvolume driver, you'll need to install it on every node in the kubelet volume-plugin-dir. By default this is /usr/libexec/kubernetes/kubelet-plugins/volume/exec/

You need a directory for the volume driver vendor, so create it:

mkdir -p /usr/libexec/kubernetes/kubelet-plugins/volume/exec/dims.io~docker-flexvol

Then drop the binary in there and set the execute permission:

mv docker-flexvol.sh /usr/libexec/kubernetes/kubelet-plugins/volume/exec/dims.io~docker-flexvol/docker-flexvol
chmod +x /usr/libexec/kubernetes/kubelet-plugins/volume/exec/dims.io~docker-flexvol/docker-flexvol

You can now use Docker volumes as usual!

Pod Config

An example pod config would look like this. Note the image and name parameters. The image is the name of the container that we need to start. name is the name of the volume we need to mount.

Note that image is mandatory, name is optional. If name is not specified all the contents of the container are made available to the pod.

apiVersion: v1
kind: Pod
metadata:
  name: nginx
  namespace: default
spec:
  containers:
  - name: nginx
    image: nginx
    volumeMounts:
    - name: test
      mountPath: /data
    ports:
    - containerPort: 80
  volumes:
  - name: test
    flexVolume:
      driver: "dims.io/docker-flexvol"
      options:
        image: "my-container-image"
        name: "/data-store"

This will use the /data-store volume on the my-container-image and mount it on the nginx pod under /data directory.

Run docker ps -a and find the container with image "my-container-image", that's the one that has the docker volume. this should be in the Created state. Look for the Source directory, it will be something like /var/lib/docker/volumes/2ec90b1efa7b1f51913882d035192a450810419d72b56814a1332b31457aa356/_data

Run kubectl exec -it nginx -- /bin/bash, cd to /data directory and create a file with say a datestamp. Now look under the directory above and you should see the same file and contents.

Container Image with pre-defined volume

To create a Container Image with Volume, save content below into Dockerfile in an empty directory:

FROM alpine
ADD https://raw.githubusercontent.com/kubernetes/kubernetes/master/README.md /data-store/README.md
VOLUME ["/data-store"]
ENTRYPOINT ["/bin/sh"]

Create a docker image using

docker image build -t my-container-image .

Test the image using docker

docker run --name my-container-1 -it my-container-image

Now this container image is ready to be used with this flexvolume driver. When you use this image with the pod config above, you can exec into the pod and see the file

[dims@bigbox 16:15] ~ ⟩ kubectl exec -it nginx -- /bin/bash
root@nginx:/# cd /data
root@nginx:/data# ls -altr
total 12
-rw-------  1 root root 3241 Jan  1  1970 README.md
drwxr-xr-x  2 root root 4096 Aug 18 20:15 .
drwxr-xr-x 32 root root 4096 Aug 18 20:15 ..