7.5. Optimizing container images to a smaller version

You can use the bootc-base-imagectl rechunk subcommand to optimize an input container image into a new image with the same filesystem tree, but split into content-addressed reproducible layers, with precomputed SELinux labeling.

This provides better network efficiency (for both pushes and pulls) since layers that did not change across an image build can be reused without causing a transfer.

The rechunk operation works on an image produced by the default mode of creating new images FROM <rhel-bootc>, but is especially useful in combination with the scratch builds that output only a single large tar layer. Without rechunk every change to the input, for example a kernel update, will result in a new layer including the entire contents of the bootc image. This new layer must then be pushed, stored by registries, and pulled by clients.

The bootc-base-imagectl is shipped as part of the bootc images and is intended to be run inside a container, but requires mapping the host containers-storage into the container to execute.