WAL Recovery Checkpoint

Reason through crash snapshots and distinguish redo from restartable undo.

Get every question correct to pass this checkpoint. You can review feedback, revise your answers, and submit again.

1. A loser update at LSN 12 is already present on a page with pageLSN=12. During the one-pass exercise:

Select one answer.

Redo is skipped but undo still restores its before-image Both redo and undo are skipped because the LSN matches Redo reapplies it and undo skips it The transaction is inferred to have committed

2. A loser writes one byte 4→7 at LSN 5, then 7→9 at LSN 6. Which undo sequence restores the original byte?

Select one answer.

Write 4, then 7 Write 9, then 4 Skip both because redo was done Write 7, then 4

3. P has pageLSN=20 and Q has pageLSN=3. A committed update at LSN 10 targets Q. Redo should:

Select one answer.

Skip because another page already reached 20 Apply because 10 is newer than Q's 3 Skip because 10 is less than the latest log LSN Apply only if P also has a pending update

4. A committed transaction changed a page that was never flushed. What makes recovery possible under no-force?

Select one answer.

The buffer pin count survives the crash The in-memory index still points to the new value The transaction's redo records and commit record are durable The page is automatically durable at commit

5. T2 aborts, T3 later commits a new value on the same byte, and another crash occurs. Why is blindly undoing T2 again unsafe?

Select one answer.

T2's old before-image can overwrite T3's committed value A committed transaction cannot follow an aborted transaction Page LSNs are transaction IDs Redo is unnecessary whenever an abort exists

6. Which claim accurately describes the standalone Recover exercise?

Select one answer.

It supports arbitrary crashes during undo without extra log records A commit record is enough to recover a torn page Its output LSN after undo follows the complete ARIES protocol It models one recovery pass; the capstone adds completed rollback history

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