||Oracle Tips by Burleson
Internals of the Oracle Data
This section is a little more
advanced and explores the internal mechanisms of the Oracle data
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:
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.
text is an excerpt from "Creating
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