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Spotlight on Oracle 11.0 - Getting Started Guide

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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

  1. Select the Spotlight on Oracle RAC connection in the Spotlight Browser.

  2. 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:

  • Grants - These misses occur when no remote instance has the block requested: the requesting instance is "granted" permission to retrieve the block from disk.
  • 2-Way - A 2-way get occurs when an instance requests a block from the master instance for that segment and the block is found on the master instance. Only two instances are involved in the transaction.
  • 3-Way - A 3-way get occurs when an instance requires a block from the master instance and that block is not available on the master but is available on a third instance.

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.

 

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