A few questions about bootc and composefs

[scotttrakker]

@siosm

We are already producing official but not recommended and unofficial but recommended bootable container images for the Atomic Desktops. See the " Compose Methods and Outputs" in Making sure you're not a bot! .

And how is this related to the Renewing the Fedora bootc initiative? How would you describe what the goal of this initiative is? The plan is the have an official and recommended version of Fedora Atomic Desktops available with bootc and composefs at the launch of Fedora 44? As far as I know there is no change request for this, but this is the plan?

@siosm

There are no plans to change the name of any Atomic Desktops.

This can be confusing, because you have now two competing names: Fedora Atomic Desktop(-s) and e.g. Fedora Silverblue. Maybe the name Silverblue can be dropped in the future and you only call it Fedora Atomic Desktop GNOME.

@siosm

Bootable Container images can (and usually are) split into multiple layers. When a new image is built, the layers that did not change are shared and are thus not re-downloaded on updates.

So there is some kind of protocol that compares the local installed image with the new online image and then decides which layers to download?

@siosm

The layers are container image layers. Each layer is essentially a tarball of files + some JSON metadata. See the OCI image format: The OpenContainers Image Spec. How they are stored on the disk depends on the tool. bootc currently imports those into an ostree repository but soon we will use a native composefs repo instead.

Clear, thanks!

@siosm

The term “image” is ambiguous. Bootable Container systems use OCI container images, which are files in tarballs plus JSON metadata. If you use dm-verity for example then this is a disk image because it’s an image of a filesystem written to a block device.

I read that dm-verity uses a hash tree, so that makes the image an image: a collection of files cryptographically hashed together.

@siosm

Bootc currently uses ostree with composefs and soon pure composefs as backends. systemd-sysupdate is another way to do updates from the systemd projects and is unrelated.

But there is a bootc version of GNOME OS (which uses systemd-sysupdate), so I guess bootc can be combined with either composefs or systemd-sysupdate.

@siosm

Currently, bootc reads the content of the container images and imports it into the ostree repo. It then generate the composefs image that will be used at boot time for the system. i would recommend watching the following talks for more info:

Thanks! I actually watched those two videos at that time!

@siosm

Currently, everything is still in the ostree repo (/sysroot/ostree). Soon, once we move to the native composefs backend, it will be in (/sysroot/composefs).

Clear!

Thanks for answering the questions!