Relationship between QPS and System Design--QPS determined the choice of storage system

So what storage of system we can choose?
Three:
SQL – MySQL, PostgreSQL -1K QPS
NoSQL based on Disk – MongoDB, Cassandra - 10K QPS
NoSQL based on Memory – Redis, Memcached - 100K-1M QPS

based on different QPS volume, we need to choose suitable database for different services(functions)

use user system as an example:
what is a user system? what services does it provide?
login, register, info change request, info query request
for a user system, we need to know that it has high request to DB for read, and less for write.
and remember this:
for s system read more and write less, we need to use Cache to optimize it.

Feature questions:
read more and write less: use memcached to optimize them.
read more and write more: still we need to read so cache is good, but since more write, then we can use more server or use Redis.