About how much slower is a random 4 KB read from an SSD than a read from main memory?

Options

• ✗ About the same; both are electronic
• ✗ About 10× slower
• ✓ About 1,000× slower (roughly 100 µs vs 100 ns) Correct
• ✗ About 1,000,000× slower

Why

Rough numbers worth knowing: main memory about 100 ns, an SSD random read tens to a hundred-odd microseconds, a spinning-disk seek about 10 ms. That gap is why hot data lives in a memory cache such as Redis instead of being read from disk on every request.

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