System DesignNumbers Reference Numbers Every Engineer Should Know
Key numbers for back-of-the-envelope calculations in system design interviews.
Latency
| L1 cache reference | 0.5 ns |
| L2 cache reference | 7 ns |
| Main memory reference | 100 ns |
| SSD random read | 150 µs |
| HDD seek | 10 ms |
| Round trip within same datacenter | 0.5 ms |
| Round trip CA to Netherlands | 150 ms |
Throughput
| Sequential read from SSD | 1 GB/s |
| Sequential read from HDD | 30 MB/s |
| 1 Gbps network | 100 MB/s |
| Compress 1KB with Snappy | 3 µs |
Scale
| QPS a single MySQL server handles | ~1K |
| QPS a single Redis server handles | ~100K |
| QPS a single web server handles | ~10K-50K |
| Messages Twitter processes per day | ~500M |
| Google searches per day | ~8.5B |
Storage
| 1 ASCII character | 1 byte |
| UUID | 16 bytes |
| Average tweet | ~300 bytes |
| Average web page | ~2 MB |
| Average photo | ~200 KB |
| 1 minute of 1080p video | ~100 MB |
Availability (per year)
| 99% (two nines) | ~3.65 days downtime |
| 99.9% (three nines) | ~8.77 hours downtime |
| 99.99% (four nines) | ~52.6 min downtime |
| 99.999% (five nines) | ~5.26 min downtime |