ssh-ng:// excessive `QueryPathInfo` on `nix copy --from`

README.md

ssh-ng:// excessive QueryPathInfo on nix copy --from

The NixOS config in this repos is just used to produce a non-trivial derivation graph that can demonstrate the slowness introduced by the many sequential QueryPathInfo ops that nix copy --from ssh-ng://... results in.

$ nix path-info --derivation .#nixosConfigurations.test.config.system.build.toplevel
/nix/store/q4bjy0j6jfs2ydp3d8xqx5g8giv5v9mq-nixos-system-test-26.11.20260531.331800d.drv

$ nix --version
nix (Nix) 2.31.5

$ ssh wisteria nix --version
nix (Nix) 2.31.5



$ nix copy --to ssh-ng://wisteria /nix/store/q4bjy0j6jfs2ydp3d8xqx5g8giv5v9mq-nixos-system-test-26.11.20260531.331800d.drv

$ time nix copy --from ssh-ng://wisteria /nix/store/q4bjy0j6jfs2ydp3d8xqx5g8giv5v9mq-nixos-system-test-26.11.20260531.331800d.drv

real	0m9.988s
user	0m0.763s
sys	0m0.599s

$ time nix copy --from ssh-ng://wisteria /nix/store/q4bjy0j6jfs2ydp3d8xqx5g8giv5v9mq-nixos-system-test-26.11.20260531.331800d.drv

real	0m11.494s
user	0m0.874s
sys	0m0.601s

$ time nix copy --from ssh://wisteria /nix/store/q4bjy0j6jfs2ydp3d8xqx5g8giv5v9mq-nixos-system-test-26.11.20260531.331800d.drv

real	0m0.546s
user	0m0.073s
sys	0m0.056s

flake.lock

{
  "nodes": {
    "nixpkgs": {
      "locked": {
        "lastModified": 1780243769,
        "narHash": "sha256-x5UQuRsH3MqI0U9afaXSNqzTPSeZlRLvFAav2Ux1pNw=",
        "owner": "NixOS",
        "repo": "nixpkgs",
        "rev": "331800de5053fcebacf6813adb5db9c9dca22a0c",
        "type": "github"
      },
      "original": {
        "owner": "NixOS",
        "ref": "nixos-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "nixpkgs": "nixpkgs"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  description = "Simple NixOS configuration for testing large derivation evaluation";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs = { self, nixpkgs, ... }: {
    nixosConfigurations.test = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ({ lib, pkgs, modulesPath, ... }: {
          imports = [
            (modulesPath + "/profiles/qemu-guest.nix")
          ];

          # Minimal bootloader/filesystem so the config evaluates as a buildable system.
          boot.loader.grub.device = "/dev/sda";
          fileSystems."/" = {
            device = "/dev/sda1";
            fsType = "ext4";
          };

          networking.hostName = "test";

          # Pull in a reasonably large derivation tree so evaluation has some weight.
          environment.systemPackages = with pkgs; [
            git
            vim
            curl
            wget
            htop
            tmux
            python3
            gcc
            gnumake
          ];

          system.stateVersion = "24.05";
        })
      ];
    };
  };
}