Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 4 PLANNING
resources and all of them suffer. The easiest solution is to increase the number of CPUs
and the amount of RAM, but large and powerful computers are also very expensive.
Real Application Clusters is a technology that combines the resources of more than
one computer. Two or more Oracle Database instances share access to the same set of
disks and coordinate with each other over a fast network. Additional instances can be
added to the cluster as the workload increases. This allows us to start with cheap commodity
hardware and “scale out” as the workload increases instead of “scaling up” to
a more powerful (and more expensive) computer.
RAC also has implications for database availability. For example, any single instance
can be shut down for hardware maintenance or OS patching without affecting the availability
of the database. In some cases, it is even possible to apply Oracle patches in rolling
upgrade fashion.
Standby Database
A standby database can improve application availability. In this scenario, redo information
from the main database is shipped and applied to another database, called the standby.
In the event of a primary database outage, applications can use the standby database.
Applications can also be switched to the standby database when hardware maintenance
or operating system maintenance needs to be performed on the primary database; even
the outages associated with Oracle patch sets and upgrades can be avoided. Note that the
maintenance of the standby database can be automated and simplified using an Oracle
product called Data Guard, which is part of Enterprise Edition.
To save money, it is typical for the standby database to have fewer hardware resources
(CPU and memory) than the primary database; it is also typical to use a non-RAC standby
database for an RAC primary database. Another interesting option is to use an active-active
configuration of two or more computers. A typical active-active configuration has two
databases, each on a separate server; the standby database of the first database is placed
on the second server and the standby database of the second database is placed on the
first server. Each server thus hosts a primary database as well as a standby database. If one
server suffers an outage, the standby database on the other node can be activated; this is a
very cost-effective way of using hardware resources.
Maximum Available Architecture (MAA)
Maximum Available Architecture (MAA) can be used when both availability and performance
are important and budgets are generous. The typical standby configuration uses
fewer hardware resources for the standby database since the probability of a switchover is
low; that is, there will be some amount of degradation in performance if the applications
are pointed to the standby database. The MAA configuration combines RAC technology