What is the purpose of VOLUME in Dockerfile
I'm trying to go deeper in my understanding of Docker's volume, and I'm having an hard time to figure out the differences / use-case of:
- The docker volume create command
- The docker run -v /path:/host_path
- The VOLUME entry in the Dockerfile file
I particularly don't understand what happens if you combine the VOLUME entry with the -v flag.
If someone understood well, please explain me!
A volume is a persistent data stored in /var/lib/docker/volumes/...
You can either declare it in a Dockerfile, which means each time a container is started from the image, the volume is created (empty), even if you don't have any -v option.
You can declare it on runtime docker run -v [host-dir:]container-dir. combining the two (VOLUME + docker run -v) means that you can mount the content of a host folder into your volume persisted by the container in /var/lib/docker/volumes/...
docker volume create creates a volume without having to define a Dockerfile and build an image and run a container. It is used to quickly allow other containers to mount said volume.
If you had persisted some content in a volume, but since then deleted the container (which by default does not deleted its associated volume, unless you are using docker rm -v), you can re-attach said volume to a new container (declaring the same volume).
See "Docker - How to access a volume not attached to a container?". With docker volume create, this is easy to reattached a named volume to a container.
docker volume create --name aname docker run -v aname:/apath --name acontainer ... # modify data in /apath ... docker rm acontainer # let's mount aname volume again docker run -v aname:/apath --name acontainer ls /apath # you find your data back!