Is your feature request related to a problem? Please describe.

Currently, nix copy will copy whole closure. For many users who self-host cache server, it is unnecessary to upload paths that have been cached in cache.nixos.org to a self-host server. This default behavior wastes time and storage, especially when nix copy a nixosConfiguration to self-host cache server.

Describe the solution you'd like

I use traefik to reverse proxy minio server. Therefore, I make a router rule which will forward GET http://MYHOST/BUCKET/HASH.narinfo to a proxy server as a workaround. This proxy will GET http://cache.nixos.org/HASH.narinfo first. If status code is 200(already in official cache), it just pipes cache.nixos.org response to client so that nix copy will not upload this path. If status code is 404, it pipes MYHOST response to client.

Test Result

1. nix copy whole closure
   Full log: https://github.com/mlyxshi/flake/actions/runs/3798651110/jobs/6460501893
   Time: 16min 58s
   Storage: 383.5MiB

2. nix copy exclude paths which are already in cache.nixos.org
   Full log: https://github.com/mlyxshi/flake/actions/runs/3798783525/jobs/6460750038
   Time: 27s
   Storage: 23.6MiB

All in all, nix copy add a flag that supports to exclude paths already in cache.nixos.org will benefit a lot for self-hosted users. Although, I have found a workaround, It would be more convenient to have official support and the implementation is not complex

Describe alternatives you've considered
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Additional context
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