||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)
* 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
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
* All instances share the same database
files and control files.
* Each instance has its own set of redo log
* The database files, redo log files, and
control files reside on one or more disks of the loosely coupled
* 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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