9.7.4.1 Creating a Snapshot of a Pluggable Database

Creating an Exadata snapshot of a pluggable database (PDB) is the simplest method for creating a snapshot because it requires no additional manual steps. Two new clauses in the CREATE PLUGGABLE DATABASE statement identify the PDB as an Exadata snapshot.

Creating individual Exadata snapshot PDBs is best used when creating snapshots for a smaller number of PDBs within a given CDB. The following figure shows a high-level example of a typical lifecycle for a PDB with two PDB snapshots.

Figure 9-12 Lifecycle of Exadata Snapshots Using PDBs

Description of "Figure 9-12 Lifecycle of Exadata Snapshots Using PDBs"

One of the benefits of Oracle Multitenant and PDBs is the ability to easily clone an existing PDB to create a test master and move it from one CDB to another. Oracle recommends that you clone your source PDB to create the test master and then migrate it to your test environment where you can perform any data scrubbing that may be needed. Once complete, the test master PDB can be used to create numerous Exadata snapshot PDBs.

An Exadata snapshot PDB is created as a PDB in the CDB. Like all other PDBs, an Exadata snapshot PDB is subject to the general limit for the maximum number of PDBs in a single CDB. For Oracle Database 12c release 1 (12.1), the limit is 252 PDBs in a single CDB. For Oracle Database 12c release 2 (12.2) and later, the limit is 4096 PDBs in a single CDB.

When creating an Exadata snapshot PDB, the command changes file permissions on the test master PDB, marking the files as READONLY in Oracle ASM. Consequently, if you want to delete the test master PDB, you must first drop all the snapshot copies and change the test master PDB file permissions back to READ WRITE.

Note:

If you drop the test master PDB before changing the file permissions to READ WRITE, the command completes without error. However, the underlying database files remain, and an entry is written to the database alert log file. In this case, you must manually delete the files in Oracle ASM to free up the space occupied by the files.

After creating the test master PDB, perform the following steps to create an Exadata snapshot PDB:

1. In SQL*Plus, connect to the CDB root container (cdb$root).
2. Close the test master PDB in all instances.

   SQL> alter pluggable database PDB1TM1 close instances=all;

3. Open the test master PDB in the local instance in read only mode.

   SQL> alter pluggable database PDB1TM1 open read only;

4. Create an Exadata snapshot PDB of the test master.

   SQL> create pluggable database PDB1S1 from PDB1TM1 tempfile reuse create_file_dest='+SPARSE' snapshot copy;

   The create_file_dest argument must specify the name of a sparse disk group. The snapshot copy clause creates the PDB as a snapshot rather than a full PDB clone.

   If you need to create more PDBs than can fit in a single CDB, you can create the Exadata snapshot PDB in another CDB by creating a remote clone using a database link. For example:

   SQL> create pluggable database PDB1S1 from PDB1TM1@cdb1_dblink tempfile reuse create_file_dest='+SPARSE' snapshot copy;