  The increasing need for managing big data has led the emergence of advanced
database management systems. There has been increased efforts aimed at
evaluating the performance and scalability of NoSQL and Relational databases
hosted by either private or public cloud datacenters. However, there has been
little work on evaluating the performance and scalability of these databases in
hybrid clouds, where the distance between private and public cloud datacenters
can be one of the key factors that can affect their performance. Hence, in this
paper, we present a detailed evaluation of throughput, scalability, and VMs
size vs. VMs number for six modern databases in a hybrid cloud, consisting of a
private cloud in Adelaide and Azure based datacenter in Sydney, Mumbai, and
Virginia regions. Based on results, as the distance between private and public
clouds increases, the throughput performance of most databases reduces. Second,
MongoDB obtains the best throughput performance, followed by MySQL C luster,
whilst Cassandra exposes the most fluctuation in through performance. Third,
vertical scalability improves the throughput of databases more than the
horizontal scalability. Forth, exploiting bigger VMs rather than more VMs with
less cores can increase throughput performance for Cassandra, Riak, and Redis.
