Overview

Oracle Data Guard is a feature of 10g Enterprise Edition that allows for the creation of standby databases that can be kept transactionally consistent with a primary database. To achieve this, Oracle ships log buffers (or log files in some configurations) from the primary server to the standby server where the log records are replayed on the standby database. A Data Guard standby comes in two flavors : logical standby and physical standby. In logical standby mode, log records are converted to SQL statements and replayed on the standby database. This more closely resembles DB2 s SQL Replication and Q Replication capabilities and as such will not be discussed here. In physical standby mode, log records are applied using redo logic, which applies the records much in the same fashion as would occur when rolling forward a database through log files.

In this mode, both the primary and standby databases are exact physical copies of each other and the application of log buffers is similar to that of IBM s HADR. However, there are many differences that appear when you look just below the surface that make HADR a superior solution to Data Guard for high-availability scenarios. A summary of the differences, each of which will be described in detail, follows.

Both IBM s HADR and Oracle s Data Guard protect from failures, such as software failure, primary server failure, storage failure, or even primary site failure. In both cases the configuration can include a second complete copy of the database in a remote...