podman save question - size differences between formats docker-archive and oci-archive?

[Jared-Sprague]

Hello!

I'm working with saving a large image, and I noticed that when running podman save with the default docker-archive format the image is MUCH larger than when I use --format oci-archive

Example:
podman save -o myimage-docker-archive.tar myimage
du -hs myimage-docker-archive.tar # 14G
podman save --format oci-archive -o myimage-oci-archive.tar myimage
du -hs myimage-oci-archive.tar # 7.4G

The oci-archive was 6.6G smaller!!!

First: go oci-archive format, that's amazing.
Second: why the size difference?

Is oci-archive employing compression during the save process on certain files? If so, what files, and what compression format is is using? I noticed that it compresses even more than gzip -9

Thanks!

[vrothberg]

Thanks for reaching out @Jared-Sprague!

The layers inside the oci-archive are compressed while the layers inside the docker-archive are not compressed. That can make quite a difference.

@mtrmac anything you want to add?

[mtrmac]

Yes, the default of oci-archive is compression; that’s just gzip with no special options (but a not quite default implementation), AFAIK.

docker-archive layers are always uncompressed (but it is customary to compress the output afterwards).

---

Just by a quick grep:

c/image’s types.SystemContext.OCIAcceptUncompressedLayers seems to be provided by libimage, and visible as podman save --uncompressed

So it should be possible to set the two on a level footing.

[Jared-Sprague]

Thanks!