Oracle Tips by Burleson

 Oracle Database Shared between Multiple Instances

A shared database (RAC) allows a number of instances to access the same database. This allows the Remote DBA to spread the SGA usage for a large database system across the CPUs of several machines. The CPUs must be part of the same CLUSTER. In previous releases this was also known as a parallel or shared database; in Oracle9i, it’s known as Real Application Clusters, or RAC.

In order to use this option on UNIX, the disks that are shared must be configured as raw devices. This requires what is known as a loosely coupled system; a set of clustered Sun, HP, or Windows machines is an excellent example. This parallel server mode has the following characteristics:

• An Oracle instance can be started on each node in the loosely coupled system.
• Each instance has its own SGA and set of detached processes.
• All instances share the same database files and control files.
• Each instance has its own set of redo log groups.
• The database files, redo log files, and control files reside on one or more disks of the loosely coupled system.
• All instances can execute transactions concurrently against the same database, and each instance can have multiple users executing transactions concurrently.
• Row locking is preserved.

Since the instances must share locks, a lock process is started, called LCKn. In addition, the GC_ parameters must be configured in the INIT.ORA files. In Oracle8, Oracle Corporation supplies the required DLM. Under Oracle9i RAC, there are many changes, which we will discuss them in Chapter 14, Distributed Database Management. If the answer to the question, Will this database be shared between multiple instances?, is yes, the Remote DBA needs to know how many instances will be sharing this database. This parameter will be used to determine INIT.ORA parameters. This answer is also important when determining the number and type of rollback segments. Rollback segments can either be private and only used by a single instance, or public and shared between all instances that access the database.

The Remote DBA will need to know the names for all instances sharing a database. He or she should also know the number of users per instance. Figure 1.2 illustrates the concepts of shared and exclusive mode; Oracle is usually run in exclusive mode. Essentially, exclusive mode is the “normal” mode for Oracle nonshared databases.

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