Donald K. Burleson
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.
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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