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Docker In-depth: Volumes

One of the most common roadblocks for people using Docker, and indeed easily the most common questions I see on on various Docker support channels, is regarding the use of volumes.

So let's take a closer look at how volumes work in Docker.

First, let's dispell the most common and first misconception:

Docker volumes are for persistence.

This likely comes from the idea that container's are not persitant, which is indeed not true. Container's persist until you remove them, and you can only do that by doing

docker rm my_container

If you did not type this command then your container still exists and will continue to exist, can be started, stopped, etc. If you do not see your container, you should see this:

docker ps -a

docker ps only ever shows you running containers, but a container can be in a stopped state, in which case the above command would show you all containers regardless of state. docker run ... is actually a multi-part command, it creates a new container, then starts it.

So, again, volumes are not for persitance.

What is a volume

Volumes decouple the life of the data being stored in them from the life of the container that created them. This makes it so you can docker rm my_container and your data will not be removed.

A volume can be created in two ways:

  1. Specifying VOLUME /some/dir in a Dockerfile
  2. Specying it as part of your run command as docker run -v /some/dir

Either way, these two things do exactly the same thing. It tells Docker to create a directory on the host, within the docker root path (by default /var/lib/docker), and mount it to the path you've specified (/some/dir above). When you remove the container using this volume, the volume itself continues to live on.

If the path specified does not exist within the container, a directory will be automatically created.

You can tell docker to remove a volume along with the container:

docker rm -v my_container

Sometimes you've already got a directory on your host that you want to use in the container, so the CLI has an extra option for specifying this:

docker run -v /host/path:/some/path ...

This tells docker to use the specified host path specifically, instead of creating one itself within the docker root, and mount that to the specified path within the container (/some/path above). Note, that this can also be a file instead of a directory. This is commonly referred to as a bind-mount within docker terminology (though technically speaking, all volumes are bind-mounts in the sense of what is actually happening). If the path on the host does not exist, a directory will be automatically be created at the given path.

Bind-mount volumes are treated a little differently than a "normal" volume, with the preference of not modfying things on the host that Docker did not itself create:

  1. With a "normal" volume, docker will automatically copy data at the specified volume path (e.g. /some/path, above) into the new directory that was created by docker, with a "bind-mount" volume this does not happen.
  2. When you docker rm -v my_container a container with "bind-mount" volumes, the "bind-mount" volumes will not be removed.

You can share volumes with another container.

docker run --name my_container -v /some/path ...
docker run --volumes-from my_container --name my_container2 ...

The command above will tell docker to mount the same volumes from the first container into the 2nd container. This effectively allows you to share data between two containers.

If you docker rm -v my_container, if the 2nd container above still exists, the volumes will not be removed, and indeed will not ever be removed unless you remove the second container with the same docker rm -v my_container2.

VOLUME in Dockerfiles

As mentioned earlier, the VOLUME declaration in a Dockerfile does the same exact thing as the -v flag on the docker run command (except you can't specify a host path in a Dockerfile). It just so happens that because of this, there can be suprising effects when building your image.

Each command in a Dockerfile creates a new container which runs the specified command and commits the container back to an image, each step building off the previous one. So ENV FOO=bar in a dockerfile is the equivelant of:

cid=$(docker run -e FOO=bar <image>)
docker commit $cid

So let's look at what happens with this example Dockerfile

FROM debian:jessie
VOLUME /foo/bar
RUN touch /foo/bar/baz
docker build -t my_debian .

What we expect to happen here is docker to create an image called my_debian with a volume at /foo/bar and put an empty file at /foo/bar/baz, but let's look at the equivelant CLI commands actually do:

cid=$(docker run -v /foo/bar debian:jessie)
image_id=$(docker commit $cid)
cid=$(docker run $image_id touch /foo/bar/baz)
docker commit $(cid) my_debian

Now, this isn't exactly what happens, but it is a very close approximation.

So, what happened here is the volume is created before anything is actually in /foo/bar, and as such every time we start a container from this image we will have an emtpy directory at /foo/bar. This happens because as stated earlier, each Dockerfile command is creating a new container. This means a new volume is also created. Since in the example Dockerfile the volume is specified before anything existed in that directory, when the container that was created to run the touch /foo/bar/baz command, it did so with a volume mounted in for /foo/bar, so baz was written to the volume mounted at /foo/bar, not the actual container/image filesystem.

So, keep in mind the placement of your VOLUME declarations in your Dockerfile as it does create essentially immutable directories in your image.

docker cp(#8509), docker commit, and docker export do not support volumes (yet).

Currently, the only way to manage volumes (create/destroy) is during container creation/descruction, which is a little odd since volumes are meant to decouple the data contained within them from the life of the container. This is something being worked on but is not yet merged (#8484).

If you want this sort of functionality, checkout docker-volumes

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