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THE GREAT CULLING: Remove all OverlayFS and delta caching logic. After extensive investigation and testing, we determined that OverlayFS file-level layering is fundamentally incompatible with ccache's access patterns: - ccache opens files with O_RDWR → kernel must provide writable file handle - OverlayFS must copy files to upper layer immediately (can't wait) - Even with metacopy=on, metadata-only files still appear in upper layer - Result: ~366MB deltas instead of tiny incremental diffs The fundamental constraint: cannot have all three of: 1. Read-only lower layer (for base sharing) 2. Writable file handles (for O_RDWR) 3. Minimal deltas (for efficient caching) Changes: - Removed all OverlayFS mounting/unmounting logic - Removed workspace and registry tracking - Removed delta creation and restoration - Removed use-deltas parameter - Simplified to direct tar/extract workflow Before: 726 lines across cache actions After: 321 lines (-55% reduction) Benefits: - ✅ Simpler architecture (direct tar/extract) - ✅ More maintainable (less code, less complexity) - ✅ More reliable (fewer moving parts) - ✅ Same performance (base-only was already used) - ✅ Clear path forward (restic/borg for future optimization) Current state works great: - Build times: 20-30 min → 2-5 min (80% improvement) - Cache sizes: ~323-609 MB per branch (with zst compression) - S3 costs: acceptable for current volume If bandwidth costs become problematic, migrate to restic/borg for chunk-level deduplication (completely different architecture).