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Donald K. Burleson

Oracle Tips

Tuning Value of Oracle Buffer Cache Hit Ratio?

The buffer cache hit rationdicates how often data is found in memory vs. disk.  Critics of this ratio complain that it is not a good indicator of performance because (a) many analysts use cumulative numbers for the computations, which can artificially inflate the value to a meaningless measure, and (b) it does not negatively reflect excessive logical I/O activity, which, although faster than disk I/O, can certainly suppress performance on any database. 

These complaints have merit.  One must use delta statistics over time to come up with a meaningful value for the ratio, and high logical I/O values can definitely be a leading cause of bad execution times.  However, when properly computed, the buffer cache hit ratio  is an excellent indicator of how often the data requested by users is found in RAM instead of disk, a fact of no small importance.

The global statistic shown above is a good place to start, but you do not have to stop there.  You can penetrate deeper to find cache hit ratios at the buffer pool, session, and SQL statement level. 

The above is an excerpt from Oracle Performance Troubleshooting by Robin Schumacher.

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