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 Oracle Database and Database Instance
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Database and Database Instance

Oracle Database Server represents a collection of physical files, logical database objects such as tables and indexes, and the host level memory structures and processes. The physical host level files reside on storage arrays, directly attached, network attached, or on a storage area network (SAN). The combination of background processes and memory buffers is called the database instance. An Oracle database instance, which resides on a host, is the actual database processing area that allows access to the physical and logical structures.


A RAC database consists of multiple database instances.  Access to the database (dbf files) is shared by the multiple instances.  In other words, the database consists of a single set of physical data files that can be accessed by multiple database instances.


As shown in Figure 2.1, each of the instances resides on a separate host and forms its own set of background processes and memory buffers. Thus, RAC enables access to a single database via multiple database instances.

instance_name vs. db_name

When the database is not a RAC system, it has one instance and one database. Sometimes, the instance and database are construed to be the same. In that case, it is called a standalone database system.


As an example,


Database Name       : NYDB50


Instance-1 Name     : NYDB51

Instance-2 Name     : NYDB52

Instance-3 Name     : NYDB53


The parameter db_name will have the value of NYDB50; this represents the name of the database.  The parameter instance_name will be one of the names listed above. All of these instances provide access to the same database named NYDB50.


Figure 2.1:  Multi-Instance RAC Database System – At a Glance

Database Instance

A typical RAC instance is similar to a stand-alone instance.  A RAC instance has extra processes, memory structures, and logical structures. Since RAC must maintain concurrency of data across multiple instances, it creates additional structures to manage and coordinate the resources.

System Global Area (SGA)

The SGA components include the database buffer cache, large pool, java pool, streams pool, redo log buffer and the shared pool.  The main memory structures of the shared pool are the dictionary cache, library cache and the result cache.  Unlike the PGA, the SGA memory structures are shared.


The SGA and PGA memory structures are shown in the Figure 2.2. The SGA resources are formed at the time of database instance launch based on the instance initialization parameters. However, many of the parameters can dynamically be altered to suit the database processing needs.


Figure 2.2:  SGA and PGA Components of a Typical Instance


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