Database Reference
In-Depth Information
Figure 3.12
Oracle Standby/
Fail-over Database
Architecture.
safety can be transferred to a standby as they are created using the Log
Writer (LGWR), filling a precreated archive log file on the standby data-
base. On the other hand, maximum performance can be achieved at the
expense of safety, thus potentially presenting possible data loss using the
Archiver (ARC
n
) to transfer log entries from primary to standby. In this
case, redo log entries are transferred when a primary database log switch
occurs, copying each archive log file to a standby database as it is created.
Using the Archiver, redo log entries are not copied as they are created but
only after primary database archiving.
Physical standby has disadvantages. A physical standby can only be
accessed externally in read-only mode, and it must duplicate the source
(primary) database exactly. A logical standby database is maintained in
read-write mode, a completely open and accessible database. Also, a logical
standby can have a subset of source database objects and can even contain
objects in addition to the primary database. Once again, logical standby is
much more flexible than physical standby.
3.5.5
Clustering and Oracle RAC
Clustering was previously called Oracle Parallel Server and is now called
Oracle Real Application Clusters (RAC). Oracle RAC allows for sharing of
a single large data source's data across more than one Oracle instance, run-
ning on more than a single database server. Thus multiple database servers
share the same data, allowing for high availability, enormous scalability,
and flexibility.
