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  Oracle Tips by Burleson

Oracle’s Seven Data Buffer Hit Ratios

We have seen that the data buffer hit ratio is a common metric used by Oracle tuning experts to measure the propensity of a row to be in the data buffer. For example, a hit ratio of 95 percent means that 95 percent of row requests were already present in the data buffer, thereby avoiding an expensive disk I/O. In general, as the size of the data buffers is increased, the DBHR will also increase and approach 100 percent.

Oracle has a separate data buffer hit ratio for all seven data buffer caches. For optimum performance, the Oracle DBA should constantly monitor all seven data buffers and adjust their size, based on each data buffer hit ratio. Oracle provides the exciting feature of allowing the number of RAM buffers within any of the data buffer caches to be changed dynamically.

This is done through alter system commands that allow the size of the buffers to be changed while Oracle remains available. This means that the DBA can maximize performance in response to current statistics by manually de-allocating RAM from one data buffer and shifting it to another buffer cache.

The general rule is that the more data that can be retrieved from a single I/O, the better the overall hit ratio. However, we need to delve a little deeper to get a more complete understanding of how multiple data buffers operate.


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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