Why `copying /nix/store/xxxxx-source to store` with the same `flake.lock`

[phaeseeKe5Ee]

I have NixOS setup with flake.
For example, I do a nix flake update && nixos-rebuild switch --flake . a week ago.
Then one week later I do a nixos-rebuild switch --flake . with the same flake.lock generated a week ago, it would produce a lot of these message:

copying '/nix/store/xxxxx-source' to store...
copying '/nix/store/yyyyy-source' to store...
copying '/nix/store/zzzzz-source' to store...

Like this

Nix copying a store-path into the store

I noticed some curious behavior of nix run after rebuilding the system from a flake. Specifically it spends some significant amount of time copying a store-path into the store (which from my understanding should result in the same store-path as its essentially a fixed input derivation). The store-path in question is nixpkgs’s source. Either i am mistaken about how the input hash is actually calculated or this is simply a bug. Either way it’s quite expensive to copy 400ish MiB after every rebuild…

And then it pull some new packages and update the system.

But why it pull some new package even though the flake.lock remain unchanged?

[tejing]

Probably because garbage collection deleted the local copies of those flakes in the meantime.

[phaeseeKe5Ee]

It totally make sense!