The Read Hit Ratios graph plots hit ratios over time for a buffer pool. It contains series for three types of hit ratios:
Data—Shown in light blue, this series plots the data page hit ratio.
Temp Data—Shown in orange, this series plots the data page hit ratio for buffer pools located in temporary tablespaces.
XML Data—Shown in dark blue, this series plots the data page hit ratio for XML storage objects (XDAs).
The hit ratios for all series are taken from statistics accumulated since a database was started or statistics were reset. They are shown as percentages on the vertical axis. Check these values along the horizontal axis to see how percentages are changing over consecutive monitoring intervals of ten minutes.
Buffer pool hit ratio is the ratio of physical reads to logical reads, expressed as a percentage. It is a measure of how often reads are made from pages that are already in a buffer pool compared to how often they are made from pages that must be loaded from disk. A higher percentage indicates more data is being read from the buffer pool. This means a lower frequency of disk I/O. A lower percentage means more data is being read from disk. This means a higher frequency of disk I/O and slower performance.
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