||Oracle Tips by Burleson
Oracle’s Seven Data Buffer Hit
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
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.
text is an excerpt from "Creating
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