Nix Garbage Collection¶

This repository has automatic garbage collection configured for all hosts.

Configuration¶

Setting	Value	Location
Automatic	Yes	`modules/common/nix.nix`
Retention	30 days	`modules/common/nix.nix`
Schedule (NixOS)	Weekly	`modules/nixos/nix.nix`
Schedule (Darwin)	Sunday 2am	`modules/darwin/nix.nix`

How It Works¶

The garbage collector removes: - Store paths not reachable from any GC root - System generations older than 30 days

GC roots include: - Current system profile - User profiles - Running processes - Nix build results

Manual Operations¶

Check garbage amount¶

nix-store --gc --print-dead | wc -l

Trigger GC manually¶

# On NixOS
sudo systemctl start nix-gc.service

# On Darwin
sudo launchctl kickstart system/org.nixos.nix-gc

Check timer status¶

# NixOS
systemctl list-timers | grep gc

# Darwin
launchctl list | grep nix-gc

Delete specific generations¶

# List generations
nix-env --list-generations

# Delete specific generation
nix-env --delete-generations 42

# Delete all but last N
nix-env --delete-generations +5

Rollback¶

Even with GC enabled, you can rollback to any generation within the 30-day window:

# List available generations
nixos-rebuild list-generations

# Rollback to previous
sudo nixos-rebuild switch --rollback

# Boot into specific generation (from GRUB menu)
# Or switch to specific generation
sudo nix-env --switch-generation 42 -p /nix/var/nix/profiles/system
sudo /nix/var/nix/profiles/system/bin/switch-to-configuration switch

Disk Space Settings¶

Additional disk protection in modules/common/nix.nix:

nix.settings = {
  max-free = 1000000000;  # 1GB - trigger GC when free space drops
  min-free = 128000000;   # 128MB - minimum to maintain during builds
};

This ensures builds fail gracefully rather than filling the disk completely.