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  Oracle Database Tips by Donald Burleson

Single-task versus two-task or multitask structure.      

If the tools are linked single-task, they address a specific node's Oracle kernel by default. To access another node or another system, a connect string must be used (connect strings will be covered in Chapter 14, Managing in a Distributed Environment). This mode is useful for a single-node database situation and saves on memory and task usage. This is generally used where a client/server architecture is not used. It has been demonstrated that relinking some tools single-task, such as the import and export utilities, will increase their performance by up to 30 percent.


Single-task linking will be allowed only in preOracle9i releases, so plan now to remove it from use.     

If the tools are linked independent, or two-task, a connect string must always be used. It is called "two-task" because the tools must run as one task while the Oracle executable runs as another. Two-task is generally used in a client/server situation. This allows the following benefits:       

Client machines to perform CPU-intensive tasks, offloading these tasks from the server.

* Movement of tools from one environment to another (such as from a development area to a production one) without relinking.

* The Oracle8i server to be relinked without relinking all of the tools.

* Two-task tools can reduce throughput, depending on the machine they are installed upon. The DBA needs to consider the costs versus the benefits when deciding whether to use single- or two-task-linked tools. 

Will This Database Be 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 Oracle9i, it's known as Real Application Clusters, or RAC.

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 following characteristics:

* An Oracle instance can be started on each node in the loosely coupled system.

* 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 transactions concurrently.

* Row locking is preserved.

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