Contact

Subscribe via Email

Subscribe via RSS/JSON

Categories

Creative Commons Attribution 4.0 International License
© Rakhesh Sasidharan

Docker volumes & bind-mounts and existing data

Just putting out into a blog post some of the things I understand about Docker volumes and bind-mounts. Been doing a bit of reading on these and taking notes. 

We have two types of locations that can be mounted into a Docker container. One is where we give the absolute path to a folder or file, the other is where we let Docker manage the location. The first is called a bind mount, the second is volumes. With volumes we just create a volume by name and Docker puts it in a location managed by it. For instance:

As you can see Docker created a folder under /var/lib/docker/volumes and that’s what will get mounted into the container. (Note: this is in the case of Linux. For macOS Docker runs inside a Linux VM on macOS, so the volume path shown won’t be a path on the macOS file system but will be something in the Linux VM and we can’t access that directly. Not sure how it’s on Windows as I haven’t tried Docker on Windows yet).

There’s two ways to mount a bind-mount or volume into a container – using a -v (or --volume) switch or using a --mount switch. The former is the old way, the latter is the new and preferred way. With the --mount switch one can be more explicit. 

The two switches behave similarly except for one difference when it comes to bind-mounts. If the source file or directory does not exist, the -v switch silently creates a new directory while the --mount switch throws an error. This is because traditionally the -v switch created a directory and that behaviour can’t be changed now. In the case of volumes either switch simply creates the volume if it does not exist. 

Between volumes and bind-mounts there’s one more difference to keep in mind (irrespective of the -v or --mount switches). If one were to mount an empty volume to a path which has data in a container, the data in that path is copied into this empty volume (and any further changes are of course stored in this volume). 

On the other hand if I were to do the same for a bind-mount, the existing contents of that path are hidden and only any changes are visible. 

The three files in there are what Docker added for name resolution. And only these three files get copied into the path we mapped.

Something else I found interesting:

This seems useful. I am still playing with Docker so I haven’t figured out how it would be useful, but it seems to be. :)

Docker volumes & bind-mounts and existing data by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.