Donald K. Burleson
Oracle Database Shared
between Multiple Instances
A shared database (RAC) allows a
number of instances to access the same database. This allows the 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 Oracle, it’s known as Real Application Clusters, or
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
* An Oracle instance can be started on each node in the loosely
* 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
* 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 Oracle 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 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 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.
This is an
excerpt by Mike Ault’s book “Oracle
Administration & Management”. If you want more current Oracle tips
by Mike Ault, check out his new book “Mike
Ault’s Oracle Internals Monitoring & Tuning Scripts” or
Ault’s Oracle Scripts Download.