Docker – Creating Docker volumes

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There are a few ways to create a Docker volume. One way is to use the volume create command. The syntax for that command is as follows:

# Syntax for the volume create command
Usage: docker volume create [OPTIONS] [VOLUME]

In addition to the optional volume name parameter, the create command allows for these options:

# The options available to the volume create command:
-d, --driver string         # Specify volume driver name (default "local")
--label list                # Set metadata for a volume
-o, --opt map               # Set driver specific options (default map[])

Let’s start with the simplest example:

# Using the volume create command with no optional parameters
docker volume create

Executing the preceding command will create a new Docker volume and assign it a random name. The volume will be created using the built-in local driver (by default). Using the volume ls command, you can see what random name the Docker daemon assigned our new volume. It will look something like this:

Stepping it up a notch, let’s create another volume, this time supplying an optional volume name with the command. The command will look something like this:

# Create a volume with a fancy name
docker volume create my-vol-02

This time, the volume is created and is given the name my-vol-02, as requested:

This volume still uses the default local driver. Using the local driver simply means that the actual location for the folder this volume represents can be found locally on the Docker host. We can use the volume inspect subcommand to see where that folder can actually be found:

As you can see in the preceding screenshot, the volume’s mount point is on the Docker host’s filesystem at /var/lib/docker/volumes/my-vol-02/_data. Notice that the folder path is owned by root, which means you need elevated permissions to access the location from the host. Notice also that this example was run on a Linux host.

If you are using OS X, you need to remember that your Docker install is actually using a mostly seamless virtual machine. One of the areas where the seams do show up is with the use of the Docker volumes. The mount point that is created when you create a Docker volume on an OS X host is stored in the filesystem of the virtual machine, not on your OS X filesystem. When you use the docker volume inspect command and see the path to the mount point of your volume, it is not a path on your OS X filesystem, but rather the path on the filesystem of the hidden virtual machine.

There is a way to view the filesystem (and other features) of that hidden virtual machine. With a command, often referred to as the Magic Screen command, you can access the running Docker VM. That command looks like this:

# The Magic Screen command
screen ~/Library/Containers/com.docker.docker/Data
/com.docker.driver.amd64-linux/tty
# or if you are using Mac OS High Sierra
screen ~/Library/Containers/com.docker.docker/Data/vms/0/tty
Use Ctrl + AK to kill the screen session.
You can detach with Ctrl + A Ctrl + D, then use screen -r to reconnect, but don’t detach and then start a new screen session. Running more than one screen to the VM will give you tty garbage.

Here is an example of accessing the mount point for a volume created on an OS X host. Here is the setup:

# Start by creating a new volume
docker volume create my-osx-volume
# Now find the Mountpoint
docker volume inspect my-osx-volume -f "{{json .Mountpoint}}"
# Try to view the contents of the Mountpoint's folder
sudo ls -l /var/lib/docker/volumes/my-osx-volume
# "No such file or directory" because the directory does not exist on the OS X host

And here is what the setup looks like:

Now, here is how to use the magic screen command to accomplish what we want, which is access to the volume mountpoint:

# Now issue the Magic Screen command and hit <enter> to get a prompt
screen ~/Library/Containers/com.docker.docker/Data/vms/0/tty
# You are now root in the VM, and can issue the following command
ls -l /var/lib/docker/volumes/my-osx-volume
# The directory exists and you will see the actual Mountpoint sub folder "_data"
# Now hit control-a followed by lower case k to kill the screen session
<CTRL-a>k

And voila…

Now is a good time to point out that we have created these volumes without ever creating or using a Docker container. This is an indication that a Docker volume is outside of the realm of the normal container-union filesystem.

We saw in Chapter 3, Creating Docker Images, that we can also create volumes using a parameter on the container run command, or by adding a VOLUME instruction in the Dockerfile. And, as you might expect, you are able to mount volumes pre-created using the Docker volume create command into containers by using a container run parameter, namely the --mount parameter, for example, as follows:

# mount a pre-created volume with --mount parameter
docker container run --rm -d \
--mount source=my-vol-02,target=/myvol \
--name vol-demo2 \
volume-demo2:1.0 tail -f /dev/null

This example will run a new container that will mount the existing volume named my-vol-02. It will mount that volume in the container at /myvol. Note that the preceding example could also have been run without pre-creating the my-vol-02:volume, and the act of running the container with the --mount parameter would create the volume as part of the process of starting up the container. Note that any contents defined in the image’s mount point folder will be added to the volume when the volume is mounted. However, if a file exists in the image’s mount point folder, it also exists in the host’s mount point, and the contents of the host’s file will be what ends up being in the file. Using an image from this Dockerfile, here is what that looks like:

# VOLUME instruction Dockerfile for Docker Quick Start
FROM alpine
RUN mkdir /myvol
RUN echo "Data from image" > /myvol/both-places.txt
CMD ["sh"]

Note the Data from image line. Now, using a pre-created volume that contains a file with the matching name of both-places.txt, but has the Data from volume contents in the file, we will run a container based on the image. Here is what happens:

As you can see, even though the Dockerfile created a file with the Data from image contents, when we ran a container from that image and mounted a volume that had the same file, the contents from the volume (Data from volume) prevailed and is what was found in the running container.

Remember that you cannot mount a pre-created volume via a VOLUME instruction in a Dockerfile. There is no such thing as a Dockerfile VOLUME instruction named volume. The reason for this is that the Dockerfile cannot dictate the location on the host that a volume is mounted from. Allowing that would be bad for a few reasons. First, since the Dockerfile creates an image, every container that was run from that image would be trying to mount the same host location. That could get real bad real fast. Second, since a container image can be run on different host operating systems, it is quite possible that the definition of the host path for one OS would not even work on another OS. Again, bad. Third, defining the volumes host path would open up all kinds of security holes. Bad, bad, bad! Because of this, running a container from an image build with a Dockerfile that has a VOLUME instruction will always create a new, uniquely-named mount point on the host. Using the VOLUME instruction in a Dockerfile has somewhat limited use cases, such as when a container will run an application that will always need to read or write data that is expected at a specific location in the filesystem but should not be a part of the Union File System.

It is also possible to create a one-to-one mapping of a file on the host to a file in a container. To accomplish this, add a -v parameter to the container run command. You will need to provide the path and filename to the file to be shared from the host and the fully-qualified path to the file in the container. The container run command might look like this:

# Map a single file from the host to a container
echo "important data" > /tmp/data-file.txt
docker container run --rm -d \
   -v /tmp/data-file.txt:/myvol/data-file.txt \
   --name vol-demo \
   volume-demo2:1.0 tail -f /dev/null
# Prove it
docker exec vol-demo cat /myvol/data-file.txt

Here is what that might look like:

There are a few different ways to define the volume in the container run command. To illustrate this point, look at the following run commands, each of which will accomplish the same thing:

# Using --mount with source and target
docker container run --rm -d \
   --mount source=my-volume,target=/myvol,readonly \
   --name vol-demo1 \
   volume-demo:latest tail -f /dev/null

# Using --mount with source and destination
docker container run --rm -d \
   --mount source=my-volume,destination=/myvol,readonly \
   --name vol-demo2 \
   volume-demo:latest tail -f /dev/null

# Using -v 
docker container run --rm -d \
   -v my-volume:/myvol:ro \
   --name vol-demo3 \
   volume-demo:latest tail -f /dev/null

All three of the preceding container run commands will create a container that has mounted the same volume, in read-only mode. This can be verified with the following command:

# Check which container have mounted a volume by name
docker ps -a --filter volume=in-use-volume

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