Storage, Execution & Recovery Internals
Write-Ahead Logging and Checkpoints
Write-ahead logging decouples transaction durability from random data-page writes: log records describing a change become durable before the changed page, and commit acknowledgment waits for the commit record’s required durability. Checkpoints bound recovery work without necessarily making every page clean.
- WAL-before-data is the ordering invariant.
- Commit durability is a log boundary.
- LSNs order log positions and page progress.
- Group commit amortizes synchronization.
- A sharp checkpoint is disruptive.
- A fuzzy checkpoint records a moving frontier.
| Model | Concurrent updates | Recovery knowledge | Operational consequence |
|---|---|---|---|
| Sharp/quiescent | Stopped or tightly constrained | All prior pages known durable | Simple restart; large pause/write burst |
| Fuzzy | Continue | Checkpoint plus transaction/dirty-page metadata | More recovery logic; smoother foreground work |