Docker

Volumes

In order to be able to save (persist) data and share data between containers, Docker came up with the concept of volumes. Quite simply, volumes are directories (or files) that are outside of the default Union File System and exist as normal directories and files on the host file system.

In a DockerFile

VOLUME ["/etc/openvpn"]

At Runtime

There are two ways to initialise volumes, with some subtle differences that are important to understand. We can declare a volume at run-time with the -v flag:

$ docker run -it --name container-test -h CONTAINER -v /data debian /bin/bash
root@CONTAINER:/# ls /data
root@CONTAINER:/# 

This will make the directory /data inside the container live outside the Union File System and directly accessible on the host. Any files that the image held inside the /data directory will be copied into the volume. We can find out where the volume lives on the host by using the docker inspect command on the host (open a new terminal and leave the previous container running if you’re following along):

docker inspect -f Template:.Volumes container-test

And you should see something like:

map[/data:/var/lib/docker/vfs/dir/cde167197ccc3e138a14f1a4f7c0d65b32cecbada822b0db4cc92e79059437a9]

Telling us that Docker has mounted /data inside the container as a directory somewhere under /var/lib/docker. Let’s add a file to the directory from the host:

sudo touch /var/lib/docker/vfs/dir/cde167197ccc3e138a14f1a4f7c0d65b32cecbada822b0db4cc92e79059437a9/test-file

Then switch back to our container and have a look:

$ root@CONTAINER:/# ls /data
test-file

Changes are reflected immediately as the directory on the container is simply a mount of the directory on the host. We can achieve exactly the same effect by using VOLUME instruction in a Dockerfile:

FROM debian:wheezy
VOLUME /data

But there’s one more thing the -v argument to docker run can do and that can’t be done from a Dockerfile, and that’s mount a specific directory on the host to the container. For example:

docker run -v /home/adrian/data:/data debian ls /data

Will mount the directory /home/adrian/data on the host as /data inside the container. Any files already existing in the /home/adrian/data directory will be available inside the container. This is very useful for sharing files between the host and the container, for example mounting source code to be compiled. The host directory for a volume cannot be specified from a Dockerfile, in order to preserve portability (the host directory may not be available on all systems). When this form of the -v argument is used any files in the image under the directory are not copied into the volume.

Between Containers

To give another container access to a container’s volumes, we can simply give the –volumes-from argument to docker run. For example:

$ docker run -it -h NEWCONTAINER --volumes-from container-test debian /bin/bash
root@NEWCONTAINER:/# ls /data
test-file
root@NEWCONTAINER:/#

It’s important to note that this works whether container-test is running or not. A volume will never be deleted as long as a container is linked to it.