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Donald K. Burleson

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System Considerations

Within each Oracle installation there are several operating system considerations that must be taken into account. These affect how Oracle uses global memory and processes memory areas. The DBAs will be responsible for tuning and maintaining these areas.

Oracle SGA

SGA is an abbreviation for Shared Global Area. As the term global implies, this area is accessible to all Oracle processes and users. Each instance will have its own SGA. Oracle processes and users must share large amounts of data. If all of the processes had to get the data from the disk, the I/O load would soon render totally unacceptable response times. To prevent this, Oracle uses global memory areas, that is, CPU memory. This memory is dedicated to use for Oracle alone. In Oracle8i, the SGA contains data buffer areas, redo log buffers, and the shared pool (context areas). Each area is important to the database’s overall performance. An additional area, the LARGE POOL, is also configured. Under Oracle8i, another area has been added, the Java shared pool area. Oracle has the capability to divide the database buffer regions into multiple varying block sized areas (2K, 4K, 8K, 16K and 32K block sizes are supported).

The shared pool context areas and database buffers provide immediate access to data that has been preread from either the data dictionary tables or the data tables. The Oracle kernel process uses an LRU (least recently used) algorithm to write data back to the disks. Data is never altered on the disks directly; it is altered in memory first. In Oracle8 the ability to have areas where LRU aging was turned off, know as the KEEP area and where LRU aging was accelerated, known as the RECYCLE area were added.

This is an excerpt by Mike Ault’s book “Oracle Administration & Management”.  If you want more current Oracle tips by Mike Ault, check out his new book “Mike Ault’s Oracle Internals Monitoring & Tuning Scripts” or Ault’s Oracle Scripts Download.




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