[Docs]: More Differentiation between save/load and import/export

[stellarpower]

Feature request description

I find the documentation is a bit sparse about the differences between saving and loading images, and exporting and importing containers. From the wording, it implies to me that the main difference is that save/load are used for images, export is used for containers. However, if I understand correctly:

• save/load save an image using all the layers, to an OCI/etc. format
• export/import just use regular tarballs of the container's filesystem, that I could extract and have a look at. Layers are therefore not preserved.
• export creates the full tarball, not only changes that have occurred over the top of the last image layer (?)
• Export works on containers, but import will then create an image from this(?)

For example:

The image of the container exported by podman export can be imported by podman import. To export image(s) with parent layers, use podman save.
It's not totally clear to me whether the emphasis here is on the fact that the parent layers are included when saveing, or on the fact that it is used on images, rather than containers.

I think a few sentences in the documentation for all four commands more clearly differentiating the use of each command pair and contrasting when you would use which would be really useful.

As an aside, perhaps a command that could do a more full system migration might be useful. I had to system reset to ensure I got off the FUSE drivers and start again with kernel namespace mounting, so my reason for needing this is trying to get myself set back up again to where I was. I somewhat expected export to export the full container, but it seems it may be best to commit the running containers and then change the create command from inspecting them in order to use the new image with any working changes. EDIT: I guess ihs may be what checkpoint does; I'm running rootless so didn't have that option.

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Have you considered any alternatives?

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Additional context

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[baude]

@stellarpower would you be willing to submit a PR for the changes you think are needed?

[stellarpower]

Potentially, although I'm not totally sure if my understanding above is correct to start with.

[mheon]

Your understanding is correct. Save/load work about the way you'd expect (put an existing image in an archive format so you can move it around easily). Import and export work with containers, not images, and as such are a lot more complicated. We do not provide a way to turn an archive directly into a container, for one, so import creates images. Export throws everything in a tarball, no layering, which has some applications, but honestly in most cases podman commit + podman save on that image feels more correct...

[github-actions]

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