There may be situations where you require horizontal scaling and Cloud SQL will not fit these requirements. Enter Cloud Spanner. Cloud Spanner is a cloud-native, fully managed offering that is designed specifically to combine relational database features such as support for ACID transactions and SQL queries with the horizontal scaling of a non-relational database. We should look to use Cloud Spanner when we require storage capacity requirements above 10 TB, as well as if we have requirements for high queries per second or to deliver over multiple regions. Unlike most databases, Cloud Spanner is globally distributed and provides a strongly consistent database service with high performance.
It also offers an availability SLA of 99.999% when you're using a multi-regional instance and is capable of providing up to 10,000 queries...