A marketplace has 100 million DAU; 5% place an order on a given day. A full order record — line items, address, payment reference, and its entire status-update history — averages 4 KB.
Use: 1 day = 86,400 sec ≈ 10^5 sec. Powers of 10: 1 KB = 10^3 B, 1 MB = 10^6 B, 1 GB = 10^9 B, 1 TB = 10^12 B, 1 PB = 10^15 B.
Compute daily orders, write QPS, 5-year storage, and Black Friday peak.