Refreshable PDB (Pluggable Databases) Switchover as Disaster Recovery
As noted in our blog; “Having a data recovery strategy in place is critical to ensuring business continuity in the event of an outage or disaster. When developing a data recovery strategy, it is essential to take into consideration the different licensing rules when deploying backups, failover (active/passive), and high-availability (active/active) scenarios.”
Even though a bullet-proof disaster recovery (DR) solution offers the ultimate protection for enterprise data, businesses often don’t place a high priority on true DR since the value they get is not realized, unless they have a disaster. Disaster recovery infrastructure can be nearly as costly as the production environment, but it is rarely used, so many businesses do not see the value.
Evaluating disaster recovery solutions is very important for organizations who are serious about the recovery of their data. An effective disaster recovery solution needs to be able to satisfy the service level agreements for Recovery Time Objectives (RTO), Recovery Point Objectives (RPO) and Oracle data corruption protection. RTO and RPO for mission critical business data need to be established while evaluating disaster recovery solutions.
Here we summarize the various types of Oracle Disaster Recovery methods and their licensing impacts
Refreshable PDB (Pluggable Databases) Switchover as Disaster Recovery:
As noted above, there are many reasons for businesses not to invest in a true Disaster Recovery method and try to find alternatives. Many believe that the Refreshable PDB Switchover function would make an effective disaster recovery solution.
Refreshable PDB is a feature that was introduced in Oracle database from 12.2 onwards. It builds upon hot clone capability in Oracle Database 12.2. It basically means the client can refresh the remote PDB from source PDB on regular intervals by just applying the incremental redo. A refreshable clone PDB is a read-only clone that can periodically synchronize itself with its source PDB. The intended use case of Refreshable PDB is as a “golden master” database.
The read-only PDB can be used for reporting purposes, or as the source for other clones, to minimize the impact on a production system when multiple up-to-date clones are required on a regular basis. Refreshable PDB is a feature included with the Multitenant option and is available in Standard Edition, Standard Edition One, Standard Edition Two, and in Enterprise Edition limited to a maximum of one PDB. You can buy the Enterprise Edition Database Multitenant option, which will expand the PDBs from one to 252.
Oracle 18c allows you to switch the roles of a source PDB and a refreshable PDB at the individual PDB level, enabling availability at the PDB level within the CDB. This functionality is known as Refreshable PDB Switchover. The refreshable PDB switchover capability, allows you to create and maintain replicas on a per-PDB basis with only two CDB’s to manage.
This feature eliminates the need to re-create a cloned environment from scratch and also ensures that a cloned PDB can be periodically synchronized with source changes. Once the switchover completes, the original source PDB becomes the refreshable clone PDB (read only mode), and the original refreshable clone PDB is now open in read/write mode functioning as a source PDB. In situations where the primary PDB fails for some reason, you can switch over to the replica and resume operations there, known as an unplanned switchover.
Restrictions and Limitations:
- There are significant prerequisites;
- In addition to the prerequisites of a typical PDB Refresh, the Refreshable PDB Switchover is currently restricted to Enterprise Edition on Engineered Systems, (Exadata or ODA), but is available on all editions on Oracle Database Cloud Services.
- A Refreshable PDB “is not Data Guard”;
- There is a lag between the initiation and completion of the switchover, where transactions against the original primary database could be applied, and not synced with the read-only database before the roles are switched. As a result, transactions may be lost.
- A Data Guard solution contains benefits like Automatic Failover, Reinstate on Failover, Backup from Standby, Flashback Database and more.
- The switchover time for large databases is significant and can take up substantial network bandwidth.
- If you are not licensed for Oracle Multitenant, depending upon your DB version, you may be limited in the number of PDB’s available.
Conclusion:
Although useful for planned switchovers for load balancing purposes, the Refreshable PDB Switchover feature is not a true disaster recovery method, is not a replacement for Data Guard, and is not recommended for mission critical applications.
Please contact your trusted Miro Analyst or Miro Account Manager if you are considering or have a data recovery strategy in place. Miro can assist your organization with assessing all of the risks and opportunities to maximize your software investment while ensuring a fully compliant environment.