Differences between Nix channels

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I’ve managed to glean the following information about the various Nix channels. I haven’t found these facts in one place, so I’m posting them here. Please correct me for any inaccuracies.

Sources:

Differences between channels

  • There are four classes of NixOS channels:

    • nixos-xx.yy, the ‘LTS’ channels, denoted by version number
    • nixos-unstable the rolling-update unstable NixOS
    • nixpkgs-unstable, recommended for Nix-as-a-package-manager (NaaPM)
    • nixos-(xx.yy|unstable)-small, a subset of NixOS curated for minimal servers.

  • Despite all the docs advising nixos-* for NixOS and nixpkgs-* for NaaPM, there is theoretically nothing that prevents you from using nixos-* for NaaPM and nixpkgs-* for NixOS. It is merely assumed that NaaPM users already have a stable system and want cutting-edge packages, whilst NixOS users need to rely on a more stable set. Beware, however, that something may break, because maintainers will assume this convention is followed.

  • nixpkgs-unstable is a statement of fact rather than a variant - there is no nixpkgs-stable (except maybe for Darwin?)

  • The set of packages in nixpkgs-unstable is not related in any way to nixos-unstable. nixos-unstable is only slighty less stable than nixos-stable (and is intended for rolling updates), nixpkgs-unstable is quite a bit looser.

  • All three of them are based on the nixpkgs master branch. Each of them advance only if all of their tests are passing. The only difference is the list of tests that each of them have.

  • It seems that nixos-* runs a ‘tighter’ set of packages - different packages are more likely to depend on the same version of a dependency, so you’ll have less duplicate packages in /nix/store.

  • nixpkgs-* contains a lot of macOS (Darwin)- specific packages, which nixos-*, being Linux-based, doesn’t need.

  • Besides for the above points, and of course package versions, it seems there are very few (no?) packages available on nixpkgs which aren’t on nixos-*.

One takeaway is, if you’re running NaaPM on Linux and can survive not having the latest version of everything, subscribing to nixos-unstable instead of nixpkgs can save you some disk space.

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All the “unstable” channels are a snapshot of master branch at a certain point of time, all the “stable” channels are a snapshot of release-*.* branch. (Think git tags that move along the respective branches.) The difference is how often they advance, or rather what are the criteria for their advancement (tests that need to succeed).

See ~~https://howoldis.herokuapp.com/~~ the comment below for more info about how the channels advance.

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The nixos channel also runs many more nixosTests as part of it jobset as well as constituents (required jobs for a channel release). And the nixos channel is the one which also creates the command database which is used for the command-not-found logic:

[09:15:16] jon@jon-desktop /home/jon/projects/nixpkgs (master)
$ kubectl
The program 'kubectl' is not in your PATH. It is provided by several packages.
You can make it available in an ephemeral shell by typing one of the following:
  nix-shell -p kubectl
  nix-shell -p kubernetes
  nix-shell -p openshift
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Small correction, as the Wiki links here.

According to [1] this is now https://status.nixos.org/

[1] What happend to howoldis? - #2 by acowley

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The site also had this very nice description of the update process by @domenkozar, which is why I continued linking it even after it stopped updating. But I guess I can just quote it here:

How does channel update?

1. Git commit

Anyone with commit access can push changes to either master or one of the release-XX.XX branches.

2. Hydra build

Each channel has an associated jobset on Hydra (Nix CI) that is a collection of individual jobs . A job roughly corresponds to building a particular package, which itself may depend on other packages built by other jobs .

Before a channel can update, its associated jobset must be finished building, though the building of some jobs may fail.

As long as certain special jobs – unstable for nixpkgs, and tested for nixos – build successfully, the channel can update. The role of these special jobs is to ensure that essential functionality is present on each channel update.

There are two major differences between those jobsets:

  • nixpkgs-unstable
    • builds all packages for supported platforms (Linux, Darwin)
    • unstable job that consists of most commonly used packages
  • nixos-*
    • builds all packages and NixOS machinery only for Linux
    • tested job that consists of mostly NixOS tests firing up qemu instances with different kinds of configurations

3. Channel update

For a channel to be updated two conditions need to be satisfied:

  • Particular jobset evaluation needs to be completely built ie. no more queued jobs, even if some jobs may fail
  • Particular jobset evaluation’s tested/unstable job needs to be built succesfully

The nixos.org server has a cronjob for which nixos-channel-scripts are executed and poll for the newest jobset that satisfies the above two conditions and trigger a channel update.

Once triggered, release files such as ISOs are copied. For the NixOS channel command-not-found index is generated, which can take some time since it has to fetch all packages.

nixpkgs is quickly updated since none of the above needs to happen once a channel update is triggered.

Some details are no longer true (e.g. the command-not-found tooling has been replaced by nix-index) but I believe it is still mostly correct.

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thanks for a excellent expiation of channels vs the master branches!