Commits on Feb 14, 2023

1. create_disk: Create image layer refs by default …

   Fedora CoreOS is not yet using containers by default for updates;
   xref coreos/fedora-coreos-tracker#1263
   etc.
   
   Consequently, when one boots a FCOS system and wants
   to rebase to a custom image, one ends up downloading the entire image,
   including the parts of FCOS that you already have.
   
   This changes things so that when we generate disk images by default,
   we write the *layer refs* of the component parts - but we
   delete the "merged" container image ref.
   
   The semantics here will be:
   
   - Only a tiny amount of additional data used by default;
     the layer refs are just metadata, the bulk of the data still
     lives in regular file content.
   - When a FCOS system auto-updates to its by-default usage of
     an ostree commit, the unused layer refs will be garbage collected.
   - But, as noted above when rebasing to a container image instead,
     if the target container image reuses some of those layers (as
     we expect when rebasing FCOS to a FCOS-derived container) then
     we don't need to redownload them - we only download what the user
     provided.
   
   Hence, this significantly improves rebasing to container images,
   with basically no downsides.
   
   The alternative code path to actually deploy *as a container*
   remains off by default.  When that is enabled, `rpm-ostree upgrade`
   fetches a container by default, which is a distinct thing.

   

   cgwalters committed Feb 14, 2023
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