In this solution, Oracle RAC is deployed on a two-layer bare-metal PowerFlex cluster with three CO nodes and four SO nodes, with Red Hat Enterprise Linux 8.4 as the operating system.

Multitenant Container Database

Beginning with Oracle 21c, the Multitenant Container Database (CDB) is the default and only supported architecture for the Oracle database. In the previous releases Oracle supported non-CDBs. 

A CDB contains one or more user-created pluggable databases (PDBs) and application containers. CDB contains the database system components, such as control files, system datafiles, undo files, temp files, and redo log files. A PDB contains user-specific database data and metadata. This design makes it easy to manage multiple PDBs within a single CDB, move PDBs between CDBs, maintain separation of responsibilities, and add security.

• CDB$ROOT container is the first container that is created when the Oracle CDB is installed. CDB$ROOT contains datafiles, control files, and redo logs, but does not contain any user data. The root container is used to store internal database objects used by the users: SYS and SYSTEM. The root container also contains information about all pluggable databases that are associated with it.
• PDB$SEED is another default pluggable database which is created during the database installation. PDB$SEED is used to create new user-defined pluggable databases. This PDB is read-only because it is used as a template for creating a pluggable database. 

PDBs contain all the user-specific data, including their users that is, users who are confined within that PDB itself. Multiple PDBs can be created within the same CDB.

The following figure shows the multitenant architecture of the CDB:

Figure 6.   Oracle multitenant architecture

For more information about multitenant architecture, see the Multitenant administrator guide.