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Donald K. Burleson

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Data Buffer Cache Hidden Parameters

For the brave DBA, the caching and aging rules within the Oracle db_cache_size can be changed.  This will modify the way that Oracle keeps data blocks in RAM memory. While modifying these parameters is somewhat dangerous, some savvy DBAs have been able to get more efficient data caching by adjusting these values:


These parameters determine the dynamics of the buffer aging and are dynamically generated. It influences Oracle’s LRU approximation and should never be touched without the direction from Oracle Support.


§       _db_aging_cool_count:  Touch count set when buffer cooled.

§       _db_aging_freeze_cr: Make consistent read buffers always be FALSE; too cold to keep in cache.

§       _db_aging_hot_criteria: Adjust aging for the touch count which sends a buffer to the head of the replacement list.

§       _db_aging_stay_count: Adjust aging stay count for touch count.

§       _db_aging_touch_time: Touch time that sends a buffer to the head of LRU.

§       _db_block_cache_clone: Always clone data blocks on get, for debugging.

§       _db_block_cache_map: Map/unmap and track reference counts on blocks, for debugging.

§       _db_block_cache_protect: Protect database blocks.  This is true only when debugging.

§       _db_block_hash_buckets: Number of database block hash buckets.

§       _db_block_hi_priority_batch_size: Fraction of writes for high priority reasons.

§       _db_block_max_cr_dba: Maximum Allowed Number of CR buffers per DBA.

§       _db_block_max_scan_cnt: Maximum number of buffers to inspect when looking for free space.

§       _db_block_med_priority_batch_size: Fraction of writes for medium priority reasons.

Oracle does not support changing the hidden parameters, and any change should be carefully tested prior to employing them in any production database.


After this introduction to the importance of setting instance wide parameters, it is time to look at a general approach to Oracle instance tuning.



This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:




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