Docker, like virtually everything else, needs a way to persist files and have local configuration and data. Docker provides mechanisms to not only store container specific information but to also share it between containers running in the same Docker daemon process.
Docker volumes are a way to provide a file system for the container with data that isn’t part of the docker image. This is significant as it gives a way for an application to have transient data and configuration files that are not part of the probably shared and hopefully reuseable image.
Volumes at build-time
When you use ‘docker commit’, volume contents are not saved. Using ‘docker commit’ is generally discouraged, but regardless of this there is nothing to be gained by writing to your volumes when the image is built as it will be discarded anyway. Using docker build eliminates the temptation to populate the volume with data.
Volumes are read-write
Normally docker volumes are mounted as read-write, but if you want to mount a volume for read-only, you need to explicitly set a :ro flag.
docker run -v /data/host:/container/readonly:ro ubuntu /bin/bash
Volumes are separate from containers
Docker volumes are stored outside of the container, so long as at least one container is using the volume. When the last reference to the volume is lost, then the volume itself is destroyed. Volumes can be shared between containers using the –volumes-from option.
docker run -d --volumes-from dbdata --name db1 ubuntu /bin/bash
This will run an instance of Ubuntu, with the volumes available to the ‘dbdata’ container, and given the container name ‘db1’. It will detach from the console, which in this example is perhaps not terribly useful but we can attach to it later if we need to.
Commands to manage Docker storage
Remove all Docker images
You can remove all Docker images from the daemon with these commands - I’ve provided three ways to do this with different shell syntax, choose the one you like best. You can force removal with ‘docker rmi -f’ but it’s not recommmended - shut down the container first.
docker rmi `docker images -q`
docker images -q |xargs docker rmi
docker rmi \$(docker images -q)
Of course, if you are using boot2docker you may find it’s quicker to delete the virtual machine and start again.
Remove Docker containers that are not running
If you need to remove containers that are not running to reclaim disk space or as part of housekeeping duties then any of these three commands will do it for you.
docker ps -aq |xargs docker rm
docker rm \$(docker ps -aq)
docker rm `docker ps -aq`
Stop all running Docker containers
Any of these commands will ask Docker to shutdown all containers.
docker ps -aq |xargs docker stop
docker stop \$(docker ps -aq)
docker stop `docker ps -aq`
The first one pipes it to docker stop via xargs. The second and third use shell expansion to achieve the same result. Different system configurations may mean you have to use one style to get this to work, and the same applies to the other examples in the chapter. If it doesn’t work, it’ll give you a command-not-found or some syntatic error so it should be fairly obvious as well as benign.