Storage & Durability Checkpoint

Trace durable histories, distinguish stored markers from visible values, and diagnose unsafe reclamation. Select one answer per multiple-choice question; enter only the requested integer for numeric answers.

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

1. Replay this complete durable prefix: set p=1, set q=2, delete p, delete r, set p=3, delete q. How many keys remain? Enter one integer.

2. A delete record is durably persisted, then the process crashes before applying it to memory or acknowledging it. The prior persisted value was k=old. What does replay of this complete durable prefix produce for k?

Select one answer.

Absent, because replay includes the durable delete old, because only acknowledged records can replay An error, because unacknowledged records are malformed old, because the memtable never saw the delete

3. LSMGet sees memtable entries k@2=old and k@11=delete, plus an SSTable holding k@7=value. gcBefore is 20. What is returned?

Select one answer.

The table value, because 11 is below the cutoff Absent, because the winning stored tombstone stops the lookup old, because it was the first memtable entry The table value, because all memtable entries expired

4. With no memtable match, table 0 has k@2=old and table 1 has k@9=delete. Both Bloom flags are positive. Which read behavior also agrees with full-coverage timestamp-based compaction?

Select one answer.

Return old because the first table matched Return absent after comparing both candidates Ignore the delete when gcBefore is greater than 9 Return whichever entry has a nonempty value

5. MultiWayCompaction has full coverage: run 0 contains k@12=delete and run 1 contains k@4=old. The safe cutoff is 13. What is stored for k?

Select one answer.

k@4=old, because the tombstone aged out Both entries, because deletion must preserve history forever k@12=delete, because every marker must remain forever No entry; the winning marker and its losing value are both removed

6. With full coverage and cutoff 8, winning versions are a@8=delete, b@7=delete, and c@1=value. How many entries are in MultiWayCompaction output? Enter one integer.

7. At timestamp 14, run 0 has k=renewed and run 1 has a tombstone for k. MultiWayCompaction uses cutoff 20. Which result follows?

Select one answer.

The value renewed is emitted The key is absent because any tombstone defeats a value Both versions are emitted because timestamps tie Neither version is emitted because both are below cutoff

8. An engineer applies the full-coverage compaction rule to two chosen runs but leaves an older overlapping table outside them. Why is dropping a winning old marker unjustified?

Select one answer.

All tables must have identical timestamps The excluded table may still contain a value the marker must hide Tombstones can only be created in memory Sorting keys always changes which version is newest

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