Oracle Data Guard Licensing Tips

Oracle Data Guard Failover and Oracle licensing

• Manual failover is allowed in Oracle SE, but there is the risk of data loss.  Also, it may take up to 30 minutes to recover the server
   

• Data Guard requires Oracle EE, but it automates the failover process to reduce downtime
   

• Oracle Streams requires Oracle10g Enterprise Edition.

The least expensive option is the Oracle manual standby database.

Manual standby database

If the goal is to save money by using Oracle Standard Edition, which does not have Oracle Data Guard included, multimaster replication and Oracle Streams, a manual approach might be the solution.

In this approach, a daemon process is written on the OS that will detect when the Oracle ARCH process has completed writing an archived redo log.  Then, the daemon uses FTP or rsh to move the archived redo to the Oracle instance server where it is automatically accepted into the Oracle instance because it remains in RECOVER DATABASE mode.

Oracle Data Guard manual standby database

The trick to this approach is to set the size of the online redo logs to minimize data loss.  This is done by examining the log switch rate for the Oracle database at its PEAK DML load.  For example, if the size of the online redo logs is set such that it has log switching every 5 minutes at peak load with an average of 15 minutes, the maximum data loss is 15 minutes.

Many shops improve this mechanism with solid-state disk (SSD) for the online redo logs and archived redo log filesystem.  SSD can increase write speeds by 100 times, reducing recovery time by two orders of magnitude and reducing the maximum data loss from 15 minutes to less than one minute.  In summary, the Oracle instance approach has these issues:

• Allows the use of Oracle Standard Edition, which is far less expensive than the Enterprise Edition.
   

• Requires the purchase of a license to use Oracle Data Guard for the standby database.
   

• If Oracle instance is geographically removed from the primary system, it also provides a disaster recovery option.
   

• Recovery can take up to 15 minutes.
   

• If the instance fails, the last redo log remains in the log buffer and online redo log files.  This means that there might be a small data loss.
   

• Requires manual intervention to open the surviving Oracle instance and re-direct incoming requests via tnsnames.ora or Apache to the new server.

NOTE: Oracle standby servers need to be fully licensed if they are "hot standby" servers that can be used for queries.  At one time, "failover" configurations that were used less than 10 days per year did not need to be licensed, but current 2006 licensing policies suggest that if Data Guard is installed for "standby" purposes, it requires full licensing.  Check with your Oracle CSR for details on current licensing.  See also:

Oracle Data Guard licensing guidelines