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Components of a Cluster

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%.

Cluster configuration is usually comprised of two or more servers or nodes and an external SCSI or Fibre Channel array. The cluster nodes are interconnected by high-speed interconnect and controlled by some cluster software or a cluster manager application. The cluster functions as a single computing resource, but it is comprised of a logical stack of integrated components.

The following section examines characteristic features of the important cluster components.

Cluster Nodes

A node in the cluster can be as simple as a computer with a single processor or it can have a symmetric multiprocessor (SMP) or NUMA architecture. Each cluster node is usually characterized by an independent copy of the Operating Environment. However, some may share a single boot image from the central shared disk storage unit. This could be, for example, an independent physical server such as the Sun Enterprise? 6800, or a single domain within a Sun Enterprise 15000 or low cost Dell workstation running a Linux operating system.

There are two main architectural approaches for multiple processor systems: Symmetric Multi-Processors (SMP) and NUMA (or DSM - Distributed Share Model). These architectures will be reviewed next.

SMP Systems

In the SMP architecture, the computers utilize internal, multiple, processors (CPU/s) and they all share the same system memory and I/O resources. See Figure. 3.3. Sharing is achieved through the use of a high-speed systems bus. One multi-CPU aware copy of the O/S runs on the computer and controls all of the processors.  Thus, the SMP architecture is also called the Shared Memory Architecture. In an SMP environment, each processor executes processes independently. A database application like Oracle, which has the ability to spawn multiple processes, is quite suitable for the SMP architecture and takes advantage of the SMP architecture in performing many parallel operations simultaneously. 

Figure 3.3: SMP Based Node

With current technology advances, present SMP machines are able to house up to 100 CPUs in a single SMP computer system. Advantages of utilizing SMP based servers include:

* Provides incremental paths to improve performance by adding extra processors as needed.

* Many Applications designed for a single processor do work seamlessly for SMP and take advantage of the existence of additional processors.

* SMP technology is mature and widely used.

* Administrative overhead from the operating system point-of-view is minimal.

Traditionally, scalability has been a problem in the case of SMP, although many SMP based systems from Sun and IBM support 64 to 128 processors. To scale an SMP it is not just enough to add a few sockets to plug in processors, the entire machine needs to be upgraded to achieve higher computing power. A high-speed bus and additional memory are needed. Another issue with SMP is the lack of high-availability. If one of the components of an SMP machine goes down, the entire machine can become unusable, whatever the number or processors present. In addition, the SMP servers with large numbers of processors are prohibitively expensive. For example, it may cost over one-hundred thousand dollars to simply add two processors and their companion memory boards to an existing system. 

However, when several individual SMP nodes are aggregated to form a cluster, they provide a relatively inexpensive way to achieve scalability. Smaller SMP nodes, such as those provided by DELL and HP-Compaq, are relatively cheap and the aggregation of such SMP systems would result in a powerful computing cluster.


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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