A photo-sharing app has 50 million daily active users; 10% upload two photos a day at ~2 MB each. Estimate daily and five-year storage, including thumbnails and replication.
Key points
- Uploads per day, 50M × 10% × 2 = 10 million photos per dayIn plain words: Multiply users by the share who upload by photos each to get photos per day
- Raw storage, 10M × 2 MB = 20 TB per day, about 7 PB per yearIn plain words: Multiply photos by their size for a day's storage, then by 365 for a year
- Derivatives add ~10-20% (a few thumbnails at 20-200 KB each)In plain words: Small preview versions of each photo add a little on top
- Replication factor of 3 (or erasure coding ~1.5x) multiplies the total, so plan roughly 25-60 PB over five yearsIn plain words: Keeping several copies for safety multiplies everything, so the five-year total runs to tens of petabytes
- Metadata is negligible by comparison, 10M × ~500 bytes ≈ 5 GB per dayIn plain words: The facts about each photo take almost no room next to the photos themselves
Model answer
Storage estimates are about finding the one term that dominates and then applying multipliers honestly.
Uploads per day 50 × 10^6 DAU × 0.10 uploaders × 2 photos = 10 × 10^6 = 10 million photos/day, about 115 per second on average, maybe 400/s at peak.
Raw bytes per day 10^7 × 2 MB = 2 × 10^7 MB = 20 TB/day.
Per year and five years 20 TB × 365 ≈ 7.3 PB/year → ~36 PB in five years, raw, assuming flat growth (real products grow, so this is a floor).
Derivatives Each photo gets, say, three resized versions: 20 KB, 100 KB and 400 KB ≈ 0.5 MB total, about 25% of the original. Call derivatives +25% → 25 TB/day, ~45 PB over five years.
Replication and durability Three full replicas: ×3 → ~135 PB. Erasure coding (e.g. 10+4 Reed–Solomon) gives similar durability at ~1.4×, so ~63 PB. Managed object stores hide this but you pay for it in price per GB.
Metadata 10 million rows/day × ~500 bytes (ids, owner, timestamps, dimensions, object key, a few indexes) ≈ 5 GB/day, ~9 TB over five years. Trivial next to the blobs, but it lives in a database that must handle ~400 inserts/s and billions of rows, so it still needs sharding by user or by photo id eventually.
Bandwidth check 20 TB/day ≈ 230 MB/s average inbound, ~2 Gbps; peak perhaps 8 Gbps. Easily absorbed by object storage, but a reminder that uploads should go straight to the bucket via presigned URLs rather than through your API servers.
Reading the result. Blobs dominate by three orders of magnitude, so the design effort is in object storage, CDN and lifecycle tiering (photos older than a year are rarely viewed and can move to cheaper storage), not in the database.
Likely follow-ups
- How would lifecycle tiering to cold storage change the cost estimate?
- What does the per-day write bandwidth come to, and does it stress the network?