Miss Ratio Page
A cache miss occurs in an Oracle RAC instance when a data block requested by the user session is not found in the local cache for the instance, and the block has to be obtained from a remote instance. The cache miss ratio is the ratio of total cache misses to total data requests across the cluster.
The charts on the Miss Ratio page displays four different views of the cache miss ratio across ALL instances in the cluster.
To open the Miss Ratio page
- Select the Spotlight on Oracle RAC connection in the Spotlight Browser.
-
Click Cluster| Miss Ratio.
Charts on the Miss Ratio page
Notes:
- Every chart has a legend (list of symbols) to its right that describes the various series (line graphs) on the chart.
- Click an item in the legend to highlight its series (line) in the chart. Click a second time to return the series to its normal appearance.
- Move the mouse pointer over an item in the legend to view the current value for that series within the chart.
| Chart | Description |
|---|---|
| Miss Type |
A cache miss occurs in an Oracle RAC instance when a data block requested by the user session is not found in the local cache for the instance, and the block has to be obtained from a remote instance. Miss Type shows the rate of misses and categorizes them as follows:
Oracle RAC architecture always ensures that misses involve at most three instances: there are no "4-way" misses in Oracle RAC. |
| Miss Ratio by Instance | The multiple data series in Miss Ratio by Instance display the cache miss ratio for every instance in the cluster. |
| Consistent Blocks |
Consistent Blocks displays the cache miss ratio for consistent block requests. A consistent block request is a request for a block for a read operation from global cache. The multiple data series in this chart display the consistent block requests for the specified instances in the cluster. |
| Current Blocks |
Current Blocks displays the cache miss ratio for current block requests across ALL instances in the cluster. A current block request is a request for a block for update operation from a cluster instance. Oracle cannot not find a recent version of the block in its local buffer and has to request the block from another instance in order to update it. The multiple data series in this chart display the cache miss ratio for current block requests for every instance in the cluster. |
Tip: For complete descriptions of Oracle RAC terms and processes, see the Glossary on the Oracle Real Application Clusters Concepts Web site.