Nothing but Nix

Transform your GitHub Actions runner into a Nix ❄️ powerhouse by ruthlessly slashing pre-installed bloat.

GitHub Actions runners come with meager disk space for Nix - a mere ~20GB. Nothing but Nix brutally purges unnecessary software, giving you 65GB to 130GB for your Nix store! 💪

Usage 🔧

Add this action before installing Nix in your workflow:

jobs:
  build:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      id-token: write
    steps:
      - uses: actions/checkout@v4
      - uses: wimpysworld/nothing-but-nix@main
      - name: Install Nix
        uses: DeterminateSystems/nix-installer-action@main
      - name: Run Nix
        run: |
          nix --version
          # Your Nix-powered steps here...

Requirements ️✔️

• Only supports official Ubuntu GitHub Actions runners
• Must run before Nix is installed
• macOS/Darwin runners: This action will gracefully skip with a warning if run on macOS. macOS runners already provide sufficient space for Nix and do not require this action
• Windows runners: This action will gracefully skip with a warning if run on Windows. Windows runners have different filesystem layouts and constraints

The Problem: Making Room for Nix to Thrive 🌱

Standard GitHub Actions runners are stuffed with "bloatware" you'll never use in a Nix workflow:

• 🌍 Web browsers. Lots of them. Gotta have 'em all!
• 🐳 Docker images consuming gigabytes of precious disk space
• 💻 Unnecessary language runtimes (.NET, Ruby, PHP, Java...)
• 📦 Package managers gathering digital dust
• 📚 Documentation no one will ever read

This bloat leaves only ~20GB for your Nix store - barely enough for serious Nix builds! 😞

The Solution: Nothing but Nix ️❄️

Nothing but Nix takes a scorched-earth approach to GitHub Actions runners and mercilessly reclaims disk space using a two-phase attack:

1. Initial Slash: Instantly creates a large /nix volume (~65GB) by claiming free space from /mnt
2. Background Rampage: While your workflow continues, we ruthlessly eliminate unnecessary software to expand your /nix volume up to ~130GB
   • Web browsers? Nope ⛔
   • Docker images? Gone 🗑️
   • Language runtimes? Obliterated 💥
   • Package managers? Annihilated 💣
   • Documentation? Vaporized ️👻

The file system purge is powered by rmz (from the Fast Unix Commands (FUC) project) - a high-performance alternative to rm that makes space reclamation blazing fast! ⚡

• Outperforms standard rm by an order of magnitude
• Parallel-processes deletions for maximum efficiency
• Reclaims disk space in seconds rather than minutes! ️⏱️

The end result? A GitHub Actions runner with 65GB to 130GB of Nix-ready space! 😁

Dynamic Volume Growth

Unlike other solutions, Nothing but Nix grows your /nix volume dynamically:

1. Initial Volume Creation (1-10 seconds): (depending on Hatchet Protocol)

   • Creates a loop device from free space on /mnt
   • Sets up a BTRFS filesystem in RAID0 configuration
   • Mounts with compression and performance tuning
   • Provides a 65GB /nix immediately, even before the purge begins

2. Background Expansion (30-180 seconds): (depending on Hatchet Protocol)

   • Executes purging operations
   • Monitors for newly freed space as bloat is eliminated
   • Dynamically adds an expansion disk to the /nix volume
   • Rebalances the filesystem to incorporate new space

The /nix volume automatically grows during workflow execution 🎩🪄

Choose Your Weapon: The Hatchet Protocol 🪓

Control the level of annihilation 💥 with the hatchet-protocol input:

- uses: wimpysworld/nothing-but-nix@main
  with:
    hatchet-protocol: 'cleave'  # Options: holster, carve, cleave (default), rampage

Protocol Comparison ⚖️

Choose wisely:

• Holster when you need the runner's tools to remain fully functional
• Carve to preserve functional runner tooling but claim all free space for Nix
• Cleave (default) for a good balance of space and functionality
• Rampage when you need maximum Nix space and don't care what breaks #nix-is-life

Witness The Carnage 🩸

By default, the purging process executes silently in the background while your workflow continues. But if you want to watch the slaughter in real-time:

- uses: wimpysworld/nothing-but-nix@main
  with:
    ️hatchet-protocol: 'cleave'
    witness-carnage: true  # Default: false

Customize Safe Havens 🛡️

Control how much space to spare from the Nix store land grab with custom safe haven sizes:

- uses: wimpysworld/nothing-but-nix@main
  with:
    ️hatchet-protocol: 'cleave'
    root-safe-haven: '3072'   # Reserve 3GB on the / filesystem
    mnt-safe-haven: '2048'    # Reserve 2GB on the /mnt filesystem

These safe havens define how much space (in MB) will be mercifully spared during space reclamation:

• Default root-safe-haven is 2048MB (2GB)
• Default mnt-safe-haven is 1024MB (1GB)

Increase these values if you need more breathing room on your filesystems, or decrease them to show no mercy! 😈

Grant User Ownership of /nix (Nix Permission Edict) 🧑‍⚖️

Some Nix installers or configurations expect the /nix directory to be writable by the current user. By default, /nix is owned by root. If you need user ownership (e.g., for certain Nix installer scripts that don't use sudo for all operations within /nix), you can enable the nix-permission-edict:

- uses: wimpysworld/nothing-but-nix@main
  with:
    nix-permission-edict: true  # Default: false

When nix-permission-edict is set to true, the action will run sudo chown -R "$(id --user)":"$(id --group)" /nix after mounting /nix.

Configure Nix to use /nix/build

This action creates /nix/build for Nix derivation builds to use the reclaimed space. Add build-dir to your Nix configuration:

- uses: cachix/install-nix-action@v31
  with:
    extra_nix_config: |
      build-dir = /nix/build

Or with DeterminateSystems:

- uses: DeterminateSystems/nix-installer-action@main
  with:
    extra-conf: |
      build-dir = /nix/build

This directs Nix to perform builds on the large BTRFS volume rather than the system's default temporary directory.

Troubleshooting 🔍

"No space left on device" during large builds

If your build runs out of space despite using Nothing but Nix, it's likely because the background purging hasn't completed before your build consumes the available space. This commonly affects:

• NixOS VM tests that assemble large disk images
• Builds with many dependencies that aren't cached
• Rust toolchains and other large compilations

Solution: Use witness-carnage: true to force synchronous purging. This ensures all space is reclaimed before your build starts:

- uses: wimpysworld/nothing-but-nix@main
  with:
    hatchet-protocol: 'rampage'
    witness-carnage: true

This adds 30-180 seconds to your workflow setup, but guarantees maximum space is available when your build begins.

Now go and build something amazing with all that glorious Nix store space! ❄️