This metric represents dictionary cache efficiency as measured by the percentage of requests against the dictionary data that were already in memory. It is important to determine whether the misses on the data dictionary are actually affecting the performance of the Oracle Server. The shared pool is an area in the SGA that contains the library cache of shared SQL requests, the dictionary cache, and the other cache structures that are specific to a particular instance configuration.
Misses on the data dictionary cache are to be expected in some cases. Upon instance startup, the data dictionary cache contains no data, so any SQL statement issued is likely to result in cache misses. As more data is read into the cache, the likelihood of cache misses should decrease. Eventually the database should reach a steady state in which the most frequently used dictionary data is in the cache. At this point, very few cache misses should occur. To tune the cache, examine its activity only after your application has been running.
This test checks the percentage of requests against the data dictionary that were found in the Shared Pool. If the value is less than or equal to the threshold values specified by the threshold arguments, and the number of occurrences exceeds the value specified in the "Number of Occurrences" parameter, then a warning or critical alert is generated.
The rest of the information in this section is only valid for this metric when it appears in either the Enterprise Manager Grid Control or the Enterprise Manager Database Control (if applicable).
The following table shows how often the metric's value is collected and compared against the default thresholds. The 'Consecutive Number of Occurrences Preceding Notification' column indicates the consecutive number of times the comparison against thresholds should hold TRUE before an alert is generated.
|
Target Version |
Evaluation and Collection Frequency |
Upload Frequency |
Operator |
Default Warning Threshold |
Default Critical Threshold |
Consecutive Number of Occurrences Preceding Notification |
Alert Text |
|
pre-10g |
Every 5 Minutes |
After Every Sample |
< |
Not Defined |
Not Defined |
2 |
Data dictionary hit ratio is %value%%%. |
|
Target Version |
Server Evaluation Frequency |
Collection Frequency |
Upload Frequency |
Operator |
Default Warning Threshold |
Default Critical Threshold |
Consecutive Number of Occurrences Preceding Notification |
Alert Text |
|
10.1.0.x; 10.2.0.x; 11.1.0.x; 11.2.0.x |
Every Minute |
Not Defined |
After Every Sample |
< |
Not Defined |
Not Defined |
2 |
Generated By Database Server |
(Gets/Misses) * 100 where:
Misses: select sum(getmisses) from v$rowcache
Gets: select sum(gets) from v$rowcache
If the percentage of gets is below %90 to %85, consider increasing SHARED_POOL_SIZE to decrease the frequency in which dictionary data is being flushed from the shared pool to make room for new data. To increase the memory available to the cache, increase the value of the initialization parameter SHARED_POOL_SIZE.
Related Topics
About Alerts
About the Metric Detail Page
Editing Thresholds
Understanding Line Charts
Copyright © 1996, 2009, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its affiliates.
Other names may be trademarks of their respective owners.