Global photo store
Design a global photo store for a service holding hundreds of billions of photos (Facebook-Haystack scale). Constraints: photos are small (tens of KB to a few MB), the workload is overwhelmingly read-heavy with a long tail, the killer problem is metadata/inode overhead and disk seeks at this object count, you serve globally with low latency, and storage cost per photo must be tiny. Cover the storage layout, the read path, and replication.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.