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Donald K. Burleson

Oracle Tips

Oracle Data Buffer Caches

Oracle allows the SGA to include up to seven RAM regions for caching incoming disk data blocks. They are the KEEP pool, RECYCLE pool, and DEFAULT pool, plus instantiated pools for 2K, 4K, 8K, 16K, and 32K data blocks (Figure 1.2).

Please note that the KEEP and RECYCLE regions were subsets of the DEFAULT pool prior to Oracle, but are separate RAM regions in Oracle and beyond. Remember, once one of the sized pools (2K-32K) is assigned to the DEFAULT pool, its size cannot be assigned to another buffer. The sized data buffers correspond to tablespaces, and a 32K block tablespace will load into the 32K data buffer.

Prior to Oracle, the KEEP and RECYCLE pools were a sub-set of the DEFAULT pool. The DEFAULT pool is created by the db_cache_size parameter in Oracle, while former versions use the db_block_buffers parameter. Starting with Oracle, the KEEP and RECYCLE pools are allocated, in addition to the db_cache_size. Also, note that Oracle allows the use of multiple block regions, ranging in size from 2K-32K, each with its own distinct buffer cache.

The buffer caches are in memory areas of the SGA where incoming Oracle data blocks are kept. These data buffer sizes can have a tremendous impact on overall system performance. There is a greater probability that data from past transactions will still be in a large buffer, minimizing expensive physical disk I/O in Oracle. RAM access is at least two thousand times faster than disk access.

The above text is an excerpt from "Creating a Self Tuning Oracle Database", by Rampant TechPress.  It is only $9.95 and all scripts in this tips can be immediately downloaded.


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