Blob store durability and tiering
Design the storage engine for a blob/object store (S3-like) holding 50PB across trillions of objects ranging from 1KB to 5GB, with 11-nines durability, high read throughput, and cost as a primary constraint (mostly cold data, a hot working set of ~2%). You control the data plane down to the disks. Describe how objects are stored and addressed, the durability mechanism, the metadata/index for object lookup, hot/cold tiering, and the central trade-off versus simple 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.
Clarify5:30 left
Estimate5:30 planned
Design16:30 planned
Deep dive13:30 planned
Failure9:00 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.