The Buffer Pool Overall Hit Rate alarm becomes active when the buffer pool hit ratio, expressed as a percentage, for a database is low.
When the buffer pool hit rate is high, many of the index and data pages required to satisfy a query are found in a buffer pool. When the buffer pool hit rate is low, I/O is required to load the required pages from external storage.
Certain applications have low buffer hit rates for various reasons. Applications (such as data warehousing applications) that perform frequent table scans of large tables see little buffer pool improvement even with large buffer pools.
The default thresholds are predefined as follows:
Threshold 1—The zero to 69 percent range. This raises the High severity alarm stating, The buffer pool hit ratio is extremely low.
Threshold 2—The 70 to 79 percent range. This raises the Medium severity alarm stating, The buffer pool hit ratio is low.
Threshold 3—The 80 to 89 percent range. This raises the Low severity alarm stating, The buffer pool hit ratio is moderate.
Threshold 4—The 90 to 100 percent range. The buffer pool hit ratio is high. No alarms are raised.
Note: These hit rate thresholds should take into account the normal buffer pool activity incurred by the types of applications your database supports.
The underlying metricsfor this alarm can be adjusted to suit your environment by using the Metric Editor.