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  Oracle Tips by Burleson

Internals of the Oracle Data Buffers

This section is a little more advanced and explores the internal mechanisms of the Oracle data buffers.

Oracle has long provided RAM buffers to prevent expensive data block re-reads from disk. But the way the buffers internally handle the incoming data has evolved radically. Prior to Oracle8i, an incoming data block was placed at the front of the list in the buffer. Oracle8i and beyond place the incoming block in the middle of the buffer chain.

Oracle keeps track of the touch count of the block after it is loaded. If a block receives multiple touches, it is moved closer to the head of the current list, guaranteeing that it is resident in RAM for a longer time period. New blocks are inserted into the middle of the buffer and their positions are adjusted according to access activity. This scheme effectively partitions each data buffer into two sections, a “hot” section that contains the data used most recently, and a “cold” section containing the data used least recently.

This is a tremendous advance over the earlier buffers. The midpoint insertion method essentially creates two sub-regions within the KEEP, RECYCLE, and DEFAULT pools. Each buffer pool has a hot and cold area, and only the data blocks that are requested repeatedly will migrate into the hot area of each pool. This method greatly improves the efficiency of the data buffers.

The size of the hot regions is internally configured by three hidden parameters:

  • _db_percent_hot_default

  • _db_percent_hot_keep

  • _db_percent_hot_recycle

Oracle Corporation does not recommend changing these parameters. They should only be altered by advanced DBAs who thoroughly understand the internal mechanisms of data buffers and wish to alter the performance of the buffers.

 

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