What is the role of the write-ahead log in a journaling file system?

• A. It records intended changes after applying to storage.
• B. It logs user access patterns.
• C. It records intended changes before applying; upon crash, logs are replayed to restore consistency.
• D. It stores encryption keys for data blocks.

Answer: (C)

The main idea is crash resilience: a journaling file system uses a write-ahead log to record what will be changed before those changes actually go to disk, so the system can recover to a consistent state after a crash. When a write is issued, the filesystem first writes a log entry describing the pending modifications (which blocks or metadata will be updated and how) and ensures that this log is safely stored on disk. Only after the log is durable does the system apply the actual data or metadata changes. If the system crashes, the log is replayed on reboot to finish any pending updates or roll back incomplete ones, bringing the filesystem to a consistent state. This approach provides atomicity: every operation either fully happens or is undone, preventing partial, inconsistent updates. Some systems journal only metadata, while others journal both metadata and data for stronger guarantees. Choices about logging user access patterns or storing encryption keys don’t fit this crash-recovery role, and logging changes after applying wouldn’t protect against crashes in the middle of an update.