Real-world Docker Series: Bind Mounting Persistent Storage & Ports

Bind Mounting Storage:

Docker containers do not retain data once they've stopped running. If you want to keep any data generated by a container, you must bind storage from the docker host to the docker container.

To accomplish this, all you need is a valid path on the docker host. When running the container, you will specify where to bind this path.  If you are already familiar with virtualization technology, think of this as assigning a new virtual disk to a virtual machine, and then mounting it on a particular path.  With bind mounting, we're simply mounting a path on the host to a path on the container.

When using selinux, you must specify a :z option for a path shared by many containers (ex: web application's www root.) If the data is specific to a single container, (think log files in /var) you will use the :Z option. You can have multiple volume bind mounts.

Run:

docker run --name Cont1 --rm -v /root/cont1:/usr/share/nginx/html:Z nginx

Command Breakdown:

–name Cont1 is the unique run-time name we've assigned to (in this example) the nginx container (tag)

--rm Remove the container (after it's work is done i.e. process it performs ends)

-v Volume, /root/cont1 (path on docker host) /usr/share/nginx/html (where to mount on the container)

From this example you see that we bind mounted /root/cont1 to the Cont1 container with tag nginx.

The --volumes-from=<container id> (obtain container id from 'docker ps')  This will bind mount all the mounted volumes from a running container to another container.  You can specify :ro (read-only) or :rw (read-write) to override the container's mounting settings.  The default is to inherit the settings of the volumes from the container you're bind mounting from.

Bind Mounting Ports:

Docker containers ports are not automatically exposed.  Furthermore, docker containers run on a private docker network bridge.  Furthermore, based on the configuration of the docker container's process, you will need to map a docker host port to the container's port where the process is running.  On top of that, docker will automatically create iptables rules for you, based upon your port binding command.

Port binding is completed with the -p switch.  See your container's documentation to configure a particular service port.  In this example we're using the nginx container from hub.docker.com, and the default port on the container is 80.

We'll add on to what we did in the storage bind mounting section.

Run: docker run --name Cont1 --rm -v /root/cont1:/usr/share/nginx/html:Z -p 80:80 nginx

Command Breakdown:

Notice the added on switch -p:

-p Port bind mounting. The first 80 is the docker host port (where the service will be accessible) the second 80 is the container's available port.  We're mapping port 80 from the container to the host to open the service up from outside the docker host.

Now to check that you can access what the container is serving, visit your docker host's IP on port 80.

ex: 192.168.122.5 in a web browser will work just fine.  If foloowing this example, make sure you have some content (index.html) in /root/cont1 on your docker host.

Next:  Working with NFS Mounts