Cli: Implement import & export functionality for volumes

[rcdailey]

I would like the ability to import and export volumes in Docker. I will explain each of these scenarios below. The motivation for this is that sometimes I like to be able to easily transition between bind mounts on the host and volumes managed by docker. If I previously started a container with a bind mount (which requires a directory on the host), I want to easily create a new volume with the contents of that directory, and run the container next time using a volume instead of a bind mount. The vice-versa of this scenario should also be supported. At the end of the day, the container doesn't know the difference and still sees the same data, this is just a management task for the host machine.

I also feel docker doing this is safer since it already has root privileges, and sudo won't be needed to copy in/out of /var/lib/docker which at the moment is locked down for even viewing.

Import Volumes

Use case: I have a directory that I want to recursively box into a docker volume. The end result of this operation should be:

docker volume create newvolume
cp -r /some/directory /var/lib/docker/volumes/newvolume/_data

Proposed docker command line usage for such a feature:

docker volume import /some/directory newvolume

This recursively copies the contents of /some/directory into a new volume named newvolume. Permissions should be retained during the copy (owners & permission bits). This is really just sugar coating over mkdir and cp, as shown in my earlier example.

Export Volumes

Use case: I have a volume that I want to convert to a directory, where that directory contains an exact recursive replica of the data in the volume. The end result of this operation should effectively be:

cp -r /var/lib/docker/volumes/existingvolume/_data/* /some/export/path

Example docker command line for such feature:

docker volume export existingvolume /some/export/path

pre & post conditions:

• Destination directory must exist
• The destination directory must be empty
• The destination directory will receive the contents of the volume (recursively)
• The destination directory contents will retain the permission bits & owner/group that was there in the volume itself

[cpuguy83]

import and export are pretty ambiguously used here.
I would expect import and export to deal with actual volumes instead of just the data of a volume.

Just copying data from some directory to another is also something that is easily performed with a docker run, e.g. docker run -v /some/directory:/tmp/src -v someVolume:/tmp/dst busybox cp -r /tmp/src/* /tmp/dst/

And this could be optimized with other tools like rsync, or some storage specific optimization, which is something that a command like export and import could possibly optimize for.

[rcdailey]

Granted I don't know the ins and outs of docker, or its cli design principles, but really using run for this just feels hacky. It's more of a workaround. I prefer a polished and simple solution like with import and export. Opinions on naming ambiguity aside, I think it's much simpler and more trivial than what you've suggested. I don't like using run for this at all.

I think we could have a very broad debate on what "import" and "export" mean in various contexts, but I think part of this feature is about picking one and defining it clearly: Later you could expand these to import/export volumes to tarballs or something, to make it a little closer to what you're probably thinking (maybe some backup mechanism?). But I think at the end of the day, since volumes really do deal in data, the terms aren't that ambiguous.

Don't forget that part of this is about keeping things simple.

[thaJeztah]

For reference; earlier discussions around this feature (or similar);

• https://github.com/moby/moby/issues/18718 Add support for importing or exporting from volumes
• https://github.com/moby/moby/issues/25245 How to copy files into a named volume without an attached container?
• https://github.com/moby/moby/issues/31417 how can i export an docker volume to a zip or tar file
• also https://stackoverflow.com/questions/21597463/how-to-port-data-only-volumes-from-one-host-to-another/23778599#23778599