MySQL/MariaDB: Scaling 10 millions database in cluster This is for SaaS app (currently in MySQL-InnoDB). Each app contains about 150 tables. The database size is not big, but the number is high.

For each database, InnoDB create directory in /var/lib/mysql. Because too many database, I choose to have individual database per app to prevent file system bottleneck (Performance degrading badly if too many files/directories exist under a directory).

I will upgrade once it reach 5000 databases.

I need to know if MariaDB cluster or MySQL cluster able to deal with 10 millions database or more, or is there any other solution?
10 million databases is too many to have in one cluster and will likely hit a bunch of internal limits and be unable to balance workloads. You should look into containerizing the databases so you have one small DB associated with each client.