It was a fault-tolerant, fully distributed relational database which was compatible with MySQL's variant of SQL. There were no key-value stores involved.

Tables (and indexes) were automatically partitioned and replicated as needed, completely under the covers.

Queries (reads and writes) were distributed to the nodes where the data resided, in parallel.

Scaling the system was as simple as adding new nodes. Data was automatically rebalanced to take advantage of the new capacity.

Failure recovery was automatic too. If a disk or node failed, the data involved would be reconstructed from replicas and moved elsewhere with no interruption in service and no failed transactions.

It was a pretty impressive system, which predated Google Spanner. But, in the early days, you had to run their custom hardware to get it. There was no cloud version.