| CompactionReport | What one compaction pass did. |
| Compactor | Makes stored history cheaper without making it less restorable. |
| LevelPolicy | The retention ladder: how long each granularity of history is kept. |
| PruneReceipt | The written record of what a prune removed, and what that cost. |
| Reachability | Decides whether an object can be removed without breaking something that still works. |
| Recompressor | Rewrites stored objects at a denser compression setting. |
| Rollup | Folds the segments of one retention level into the coarser level above it. |