Oracle Data Guard Licensing Tips
Oracle Data Guard Failover and Oracle
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
The least expensive option is the Oracle manual
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
where it is automatically accepted into the Oracle instance
it remains in RECOVER DATABASE mode.
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
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:
Data Guard licensing guidelines