Gcroot for partial nixos-rebuild

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1

Hello, I was wondering if there’s any way to create a root to prevent the garbage collector from cleaning up the packages downloaded by the latest nixos-rebuild in cases where the rebuild does not succeed?

As I’m running unstable and some other flakes, so it’s not uncommon for a rebuild to fail. Often these can be fixed by switching a package to my input for nixpkgs-stable or nixpkgs-master, but sometimes end up in the ‘check on this later’ pile.

I enabled nix.gc.automatic. If the garbage collector runs before a new rebuild generation completes it will delete the downloaded packages, resulting in them all being downloaded again next rebuild.

Any ideas on how that could be avoided?

2

you could make the garbage collector less aggressive using max-free and min-free maybe?

3

Make sure you have

{
  nix.settings.keep-outputs = true;
}

In your configuration.nix (and rebuild).

Then if a nixos-rebuild fails (assuming non-flakes), you can run

nix-instantiate '<nixpkgs/nixos>' -A config.system.build.toplevel --add-root system

to prevent garbage collection of already-downloaded/built paths for the system.

Ideally nixos-rebuild would have this functionality built-in

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4

I suppose the way with flakes would be (replace MACHINE-NAME with flake output name)?

nix-instantiate $(nix path-info --derivation .#nixosConfigurations.MACHINE-NAME.config.system.build.toplevel) --add-root system
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