How to store key-value pairs?

Question

big data

There is a task to record and access approximately 90-100 million values ​​per day. Search by key (int) - the result is also int. The search time for such a repository is very critical. The volume of records does not exceed 1 million operations, the volume of access is approximately 20-30 million.
Various options were considered from those with which I worked: Pg, MySQL, MongoDB, memcached.
These storages are not suitable. Memkesh is suitable for speed, but not suitable for storage (data must be saved).
What can you advise? Maybe a cassandra or some other "exotic"?

Answer 1

For riak

Answer 2

A write directly to a file? filename = key, content = value.
For redundancy it is possible through NFS, if locally then ssd should work smartly

Answer 3

Memkesh is suitable for speed but not suitable for storage (data must be saved).

A Redis?