Oracle8i Parallel Server Concepts
Release 2 (8.1.6)

Part Number A76968-01





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Introduction to Oracle Parallel Server

This chapter introduces parallel processing and the parallel database technology features of Oracle Parallel Server. Oracle Parallel Server offers significant advantages for Online Transaction Processing (OLTP), Electronic Commerce, Decision Support Systems (DSS), and hybrid system types. With the functionality of Oracle Parallel Server, such systems can effectively exploit the redundancy of parallel environments.

You can also use Oracle Parallel Server to deliver high performance, throughput, and high availability. Whatever your goal, your challenge is to successfully deploy these technologies to take full advantage of their multiprocessing powers. To do this, you must understand how Oracle Parallel Server works, what resources it requires, and how to effectively use it.

This chapter includes the following topics:

What Is Oracle Parallel Server?

Oracle Parallel Server is a robust computing environment that harnesses the processing power of multiple, interconnected computers. Oracle Parallel Server software and a collection of hardware known as a "cluster", unites the processing power of each component to become a single, robust computing environment. A cluster generally comprises two or more computers, or "nodes".

In Oracle Parallel Server environments, all nodes concurrently execute transactions against the same database. Oracle Parallel Server coordinates each node's access to the shared data to provide consistency and integrity.

Harnessing the power of multiple nodes offers obvious advantages. If you divide a large task into sub-tasks and distribute the sub-tasks among multiple nodes, you can complete the task faster than if only one node did the work. This type of parallel processing is clearly more efficient than sequential processing. It also provides increased performance to process larger workloads and accommodate growing user populations.

If you establish high node-to-data affinity with accurate partitioning, you can effectively scale your applications to meet increasing data processing demands. As you add resources, Oracle Parallel Server can exploit them and extend their processing powers beyond the limits of the individual components.

You can use Oracle Parallel Server for many system types. For example, data warehousing applications accessing read-only data are prime candidates for Oracle Parallel Server. In addition, Oracle Parallel Server successfully manages increasing numbers of online transaction processing systems as well as hybrid systems that combine the characteristics of both read-only and read/write applications.

Oracle Parallel Server also serves as an important component of robust High Availability solutions. A properly configured Oracle Parallel Server environment can tolerate failures with minimal or no downtime.

Benefits of Oracle Parallel Server

Some of the most important benefits beyond the obvious advantages of parallel processing are described in the following sections. These benefits include improved throughput and scalability over single-instance systems and improved response time. An Oracle Parallel Server also provides an ideal High Availability solution by resolving node failure in a clustered environment.

Oracle Parallel Server environments are functionally transparent when compared to single-instance environments because they are functionally identical to single-instance environments.


Scalability is the ability to add additional nodes to properly deployed Oracle Parallel Server applications and achieve markedly improved performance. Oracle Parallel Server can take advantage of additional equipment and harness the processing power of multiple systems.

High Availability

High availability refers to systems with redundant components that provide consistent, uninterrupted service, even in the event of hardware or software failures. In most high availability configurations, nodes are isolated from each other so a failure at one node does not affect the entire system. In such a case, surviving nodes recover the failed node and the system continues to provide data access to users. This means data is consistently available, more so than it would be with a single node upon node failure. High availability also implies increased database availability.


Transparency is the functional equivalent of single-instance exclusive Oracle and shared configurations that use Oracle Parallel Server. Applications that run on single instance Oracle execute with the same results using Oracle Parallel Server. An Oracle database can be configured to execute in three different modes:

Installation of the Oracle Parallel Server option is required if you want to execute transactions from multiple nodes in shared mode. Oracle Parallel Server offers many performance features beyond those available in a single instance environment.

High Performance Features of Oracle Parallel Server

Oracle Parallel Server takes advantage of the parallel processing in a computer cluster without sacrificing Oracle's inherent transaction processing features. The following sections discuss certain features in Oracle, both in exclusive and shared modes, that result in improved application performance when these applications run using Oracle Parallel Server.

Buffer Cache Management

Within a single instance, Oracle stores resources, such as data block and lock information, in a buffer cache that resides in memory. Storing this information locally reduces the amount of disk I/O necessary for database operations. Since each node in the Parallel Server has its own memory that is not shared with other nodes, Oracle Parallel Server must coordinate the buffer caches of different nodes while minimizing additional disk I/O that could reduce performance. The Oracle parallel cache management technology maintains the high-performance features of Oracle while coordinating multiple buffer caches.

See Also:

Oracle8i Concepts for detailed information about the buffer cache.  

Fast Commits, Group Commits, and Deferred Writes

Fast commits, group commits, and deferred writes operate on a per-instance basis in Oracle and work the same whether in exclusive or shared mode.

Oracle only reads data blocks from disk if they are not already in the buffer cache of the instance requesting the data. Because data block writes are deferred, they often contain modifications from multiple transactions.

Optimally, Oracle writes modified data blocks to disk only when necessary:

Row Locking and Multiversion Read Consistency

Oracle's row locking feature allows multiple transactions from separate nodes to lock and update different rows of the same data block. This is done without any of the transactions waiting for the others to commit. If a row has been modified but not yet committed, the original row values are available to all instances for read access. This is called multiversion read consistency.

Online Backup and Archiving

Oracle Parallel Server supports all Oracle backup features that are available in exclusive mode, including both online and offline backups of either an entire database or individual tablespaces.

If you operate Oracle in ARCHIVELOG mode, online redo log files are archived before they are overwritten. In Oracle Parallel Server, each instance can automatically archive its own redo log files or one or more instances can manually archive the redo log files for all instances.

In ARCHIVELOG mode, you can make both online and offline backups. If you operate Oracle in NOARCHIVELOG mode, you can only make offline backups. If you cannot afford any data loss, Oracle strongly recommends that you operate your production databases in ARCHIVELOG mode.

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