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## Multiple Databases in DB Systems

FAQs on creating multiple database in Oracle Database Appliance DB systems.

What is the multi-database feature in DB system?

Prior to Oracle Database Appliance release 19.23, one DB system supported one database. Starting with Oracle Database Appliance release 19.23, a DB system can run multiple databases and multiple database homes.

I updated my bare metal system to Oracle Database Appliance release 19.23, what happens to the existing DB systems? How do I know if a DB system has multi-database feature enabled?

After the bare metal system is updated to Oracle Database Appliance release 19.23, the new DB system created has multi-database support by default. For existing DB systems, after the DB systems are updated to Oracle Database Appliance release 19.23, either by patching to 19.23 with the `odacli update-server` command or upgrading to 19.23 with the `odacli upgrade-dbsystem` command, multi-database support is enabled. If an existing DB system is not updated to 19.23, the DB system continues to support only one database. To check if the DB system has multi-database enabled, you can view the shape with the `odacli list-dbsystems` command. If the shape is dbsX, then multi-database feature is enabled. If the shape is odbX, then the DB system supports only one database.

How do I choose the DB system shapes to support multiple databases? How many database can be created in the multi-database enabled DB system?

Prior to Oracle Database Appliance release 19.23, one DB system supported one database. Starting with Oracle Database Appliance release 19.23, DB system supports running multiple databases. Selecting the shape of the DB system depends on the sizing requirements of the databases running in the DB system. Similarly, the number of databases supported in the DB system depends on the resources such as CPU, memory of DB system. DB system must be sized properly based on the sizing requirement of databases running in the DB system. Generally, the shapes of databases together running in a DB system must be no greater than the shape of the DB system. For example, if databases odbA, odbB, ..., odbN run in the DB system with shape dbsX, then A+B+...N <= X. You can change the DB system shapes to meet the new sizing requirement of the databases in the DB system.

Why must I manually change the database shapes when modifying DB system shapes in 19.23 and later?

Prior to Oracle Database Appliance release 19.23, one DB system supported one database and the database shape was the same as the DB system shape. Hence, modifying the DB system shape would automatically change the shape of the database. Starting with Oracle Database Appliance release 19.23, multi-database feature is enabled by default on the DB system. When you modify the DB system shape with the `odacli modify-DB system --shape` command, the shape of the DB system is modifed; the command does not change the shapes of the databases inside the DB system any more. You must modify database shapes accordingly based on the new shape of the DB system. For DB system shape scale up, increase the database shapes after DB system shape is scaled up. For DB system shape scale down, scale down database shapes before DB system shape is scaled down; otherwise databases could fail to start because of reduced DB system memory.

After the DB system shape is scaled down, the databases in the DB system no longer starts, why? What is the solution?

After DB system shape is scaled down, the CPU and memory are also reduced, so are the huge pages of the system. When the memory of the system cannot meet the memory requirements of the database, especially if the huge pages of the system cannot meet the SGA of the database, then databases fail to start. To fix the issue, scale up the DB system shape to the original shape, scale down the databases shape, and then scale down the shape of the DB system.

Parent topic: Oracle Database Appliance Frequently Asked Questions