Scalability is driven by one of the 4 V’s — Velocity, aka throughput.
Disambiguation: having many machines to store the data as readonly isn’t “scalability”. Any non-scalable solution could achieve that without effort.
Big data often requires higher throughput than RDBMS could support. The solution is horizontal rather than vertical scalability.
I guess gmail is one example. Requires massive horizontal scalability. I believe RDBMS also has similar features such as partitioning, but not sure if is economical. See posts on “inexpensive hardware”.
The Oracle nosql book suggests noSQL compared to RDBMS, is more scalable — 10 times or more.
RDBMS can also scale out — PWM used partitions.
