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  Oracle Tips by Burleson

Tuning the Buffer Cache

The buffer cache is the area in memory where data is stored from data tables, indexes, rollback segments, clusters, and sequences. By ensuring that enough buffers are available for storage of these data items, you can speed execution by reducing required disk reads. In Oracle7 and early Oracle8 releases, you only had the “normal” buffer area to worry about. To Oracle8i was added the capability to subdivide this buffer area into KEEP and RECYCLE buffer areas. Later in this section, we will examine how these areas interact and how they should be tuned and sized. To add to the complexity you can, in Oracle9i, also have multiple areas inside the buffers that do not have the same blocksize as the rest of the database. We will cover that in the Oracle9i tuning section.

If DB_BLOCK_BUFFERS is set too high, you may exceed shared memory size on UNIX or NT for your instance. Another possible result is that the entire Oracle process could be swapped out due to memory contention with other processes. In either case, it is not a desirable condition. To avoid exceeding  shared memory area size, be sure you set these operating system values (on UNIX) high when the instance is created. To avoid swapping, know how much memory you are able to access; talk with your system administrator to find this out.

Tuning the Multipart Oracle8, Oracle8i, and Oracle9i Buffer Cache

In Oracle8 and Oracle8i, the database block buffer has been split into three possible areas: the default, keep, and recycle buffer pool areas. It is not required that these three pools be used, only one: the default pool, which must be configured with the DB_BLOCK_BUFFERS or DB_CACHE_SIZE (in Oracle9i, this should be used instead of DB_BLOCK_BUFFERS) initialization parameters; the others are “sub” pool to this main pool.

This is an excerpt by Mike Ault’s book “Oracle9i 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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