Setup automatic garbage collection for NixOS

[JPyke3]

Add daily nix garbage collection with store optimization:

• Conservative 14-day retention for most systems
• Aggressive 7-day retention for china (125GB drive constraint)

[claude]

Pull Request Review: Automatic Garbage Collection for NixOS

Overview

This PR adds automatic garbage collection and store optimization to all NixOS systems with system-specific overrides. The changes are well-structured and address a real need for disk space management.

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✅ Strengths

1. Good separation of concerns: Default config in shared configuration.nix, china-specific override in china/configuration.nix
2. Clear documentation: Comments explain the rationale ("conservative defaults", "aggressive for 125GB drive")
3. Store optimization: Including nix.optimise.automatic = true is excellent for long-term space savings
4. Conservative defaults: 14-day retention is reasonable for most systems

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🔍 Code Quality & Best Practices

systems/nixos/configuration.nix:86-95

• ✅ Configuration is clear and well-commented
• ✅ Uses nix.gc.dates = "daily" which is appropriate for automated cleanup

systems/nixos/china/configuration.nix:9-10

• ✅ Override pattern is correct (nix.gc.options only)
• ✅ 7-day retention makes sense for 125GB constraint

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💡 Suggestions & Considerations

1. Timing Coordination (Minor)

The garbage collection runs "daily" but does not specify a time. Consider setting explicit timing to avoid conflicts with system.autoUpgrade which runs at 03:00.

2. Store Optimization Timing (Minor)

nix.optimise.automatic runs after every build. Consider using scheduled optimization instead to reduce I/O during builds, especially on china.

3. Steam Deck (japan) Consideration (Question)

Does japan (Steam Deck OLED) have storage constraints? SD cards can fill quickly with game data. Consider if it needs a china-style override.

4. Darwin System (Info)

This PR only affects NixOS systems. The darwin (Mac Mini) system does not get these settings. Consider adding GC configuration there as well if needed.

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🔒 Security & Performance

• ✅ Security: No security concerns - GC is a safe operation
• ✅ Performance: Minimal impact - runs during low-activity hours
• ⚠️ Disk I/O: nix.optimise.automatic can be I/O-intensive on china during builds

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🧪 Test Coverage

Recommended manual testing before merge:

• Verify systemd timers are created correctly
• Check GC options are applied per-system
• Test on china to ensure 7-day retention works

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📊 Impact Assessment

System	Before	After	Notes
china	No auto-GC	7-day GC	Critical - fixes 125GB constraint
norway	No auto-GC	14-day GC	Good preventative maintenance
austria	No auto-GC	14-day GC	Helpful for Apple Silicon storage
japan	No auto-GC	14-day GC	May need shorter retention for SD card
darwin	No auto-GC	No change	Consider adding later

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✅ Verdict

LGTM with minor suggestions - This PR is ready to merge as-is. The suggestions above are optional improvements, not blockers.

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Great work on proactively managing disk space! This will prevent future "out of space" issues on china.