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Oracle Database and Database Instance

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

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 are actually residing on the storage arrays, directly attached, network attached, and storage area networks (SAN). The combination of background processes and memory buffers is called the database instance. Oracle Database Instance, which resides on a host, is the actual database processing area that allows access to the physical and logical structures.

RAC Database has multiple database instances to access and manage a single database system. With the shared disk architecture, the Database consists of a single set of physical data files for data that can be accessed by multiple database instances. As shown in the Figure 4.2, 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.

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 stand-alone 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 and this represents the name of the database. And the parameter instance_name will be one of the names listed above. All these instances provide access to the same database named NYDB50.

Figure 4.2:  Multi-Instance RAC Database System ? At a Glance

The examination of the Oracle Architecture will start by reviewing the memory pools and the back ground processes in a single-instance database. Then the study will extend to review the extra processes and structures that are formed in case of a RAC configuration.

Database Instance

A typical RAC database instance is very much like a stand alone database system. It has all frills and bells of a typical database instance. But it has some extra processes, memory structures, and logical structures. Since the RAC database system has to maintain concurrency of data across multiple instances, it creates additional structures to manage and coordinate the resources.

The Oracle Instance has various components to support the database processing. The memory components are broadly categorized as System Global Area (SGA) and Program Global Area (PGA).

System Global Area (SGA)

The System Global Area (SGA) consists of various memory components. A component represents a pool of memory used to satisfy a particular class of memory allocation requests. The most commonly configured memory components include the database buffer cache, shared pool, java pool, large pool, streams pool, data dictionary cache, and redo log buffer. PGA consists of session specific information that contains data and control structures.

The SGA and PGA memory structures are shown in the Figure 4.3. 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 4.3:  SGA and PGA Components of a typical instance

The size of the SGA is determined by several initialization parameters. The following Table 4.1 shows the parameters that influence the SGA size. However, when the initialization parameter sga_target is set to a value greater than zero, the automatic SGA configuration kicks in. This will be covered in more detail in a later part of this chapter.




Shows the size of the cache of standard blocks


The number of bytes allocated for the redo log buffer


Shows the size of the area allocated to shared SQL and PL/SQL statements.


Size of the large pool; the default is 0.


The size of the Java pool

Table 4.1: Initialization Parameters that affect SGA size


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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