A notification service covers 500 million users; 20% receive at least one notification daily, each fanning out to the user's 2 devices. Every delivery writes a 500-byte log entry, stored with 3× replication for 90 days.
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 fan-out volume, write QPS, peak QPS, and the 90-day log footprint.