Capacity Planning and Architecture - Expert Oracle RAC Performance Diagnostics and Tuning - page 28

Oracle Single-Instance vs. Clustered Configuration
Every Oracle configuration starts with a single instance, in the sense that even in the clustered Oracle configuration
such as RAC, it starts with a single instance. From the basic level of database creation, database management,
database performance tuning, and so forth, all operations start with a basic single-instance configuration and
move to a clustered configuration. It is very important in every aspect of database administration and maintenance
that each instance is considered as an individual unit before considering it as a combined cluster. Stand-alone, or
single-instance, configuration in an enterprise system does not provide all the functionalities, such as availability
and scalability. One way of providing for availability is by using some of the high-availability options accessible
from Oracle, for example, the ODG. With this feature, data is migrated to a remote location by pushing data from the
redo logs or archive logs to the remote location. The difficulty with such a configuration is that there could be loss of
data when the node that contains the primary database fails, and the last set of redo logs are not copied over to the
destination database. This creates an inconsistent environment.
Another high-availability option would be to use the Oracle Advanced Replication (OAR) or the Oracle Streams
feature. This option is very similar to the ODG option; however, instead of copying the redo logs from the primary
instance to the secondary, or target-replicated environment, data could be transferred more frequently like a
record, or a group of records. This feature, when compared to the ODG option, provides a much closer level of data
consistency. This is due to the fact that in the case of failure of the node that contains the primary database, only the
last few rows, or sets, of data are not transferred.
From a disaster recovery or reporting solution, the ODG and OAR feature are high-availability options. Where
data consistency is not an immediate concern, such as in the case of disasters basically due to an “act of God,” where
the primary database is not available, a remote database created by either of these options could help provide a
backup opportunity to the enterprise system.
Oracle's clustered, or multi-instance configurations, comprise multiple nodes working as a cohesive unit, with
each node in the cluster consisting of two or more instances talking to a common shared database. As has been
discussed, this feature is the RAC configuration.
RAC Architecture
RAC is a clustered database solution that requires a two or more node hardware configuration capable of working
together under a clustered software layer. A clustered hardware solution is managed by cluster management software
that maintains cluster coherence between the various nodes in the cluster and manages common components such
as the shared disk subsystem. Several vendors have provided cluster management software to manage their respective
hardware platforms. For example, HP Tru64 manages HP platforms; Sun Cluster manages Sun platforms; and so
forth; and there are others such as Veritas cluster manager that has cluster management software that supports more
than one hardware vendor. In Oracle Database version 11g and above, the cluster management is provided using
Oracle's ClusterWare. 1
1 Oracle Clusterware is part of Oracle Grid Infrastructure starting with Oracle Database 11g Release 2.