In NixOS parlance, the toplevel is a complete system, and is really "just" a
path in the Nix store. In a NixOS machine, /run/current-system points to the
current toplevel, which is usually built by nixos-rebuild.
In Nixpkgs, the nixos/default.nix file exposes the toplevel as the system
attribute.
This repository contains a Nix expression to define a test toplevel, together with a GitHub action to build it and cache it to Backblaze B2.
In addition to the toplevel content, a file containing the latest toplevel (as a path in the Nix store) is uploaded to https://f003.backblazeb2.com/file/hypered-store/toplevels/test.txt.
The test toplevel contains an update-system script which check the above
file, downloads the corresponding Nix paths, then activate the toplevel.
Building, including downloading from cache.nixos.org, takes about 100 seconds. Uploading everything to B2 took about 14 minutes the first time. Next uploads take about 50 seconds.
Playing with a binary cache may reuse cached data in /root/.cache/nix,
resulting in does not contain a valid signature, even though nix path-info --sigs ... --store ... seems fine. Deleting the cache solves the problem.
Backblaze "Daily Class B Transactions Caps" seems to be impacted by simply uploading the narinfos to it (I guess that some metadata are downloaded to check if the uploads should actually happen).
Downloading the toplevel on a different machine downloads about 500MB, resulting in 2GB once unpacked. If it is a minor change, it can take around 5 seconds for about 5MB. If nothing has changed, it takes less than a second.