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Spotlight on DB2 6.10 - Getting Started Guide

Buffer Pool graph

If the overall buffer pool hit rate for the database is low, this graph tells you whether the selected application is contributing to the problem.

About the buffer pool

Buffer pools are memory areas configured for a database. These areas temporarily store data and index pages from the database. If an application needs to read a data or index page and that page is already in the buffer pool, the application reads the page from the buffer pool¾a process that is faster than retrieving the page from disk. Avoiding disk I/O is the main issue when you are trying to improve database performance. Consequently, proper configuration of buffer pools is probably the most important consideration for performance tuning.

The buffer pool hit rate ratio indicates the percentage of page requests that were satisfied by a page already in the buffer pool. In other words, the hit rate represents the percentage of time that the database manager did not need to load a page from disk to fulfill a page request. If the high hit rate is generally high, the frequency of disk I/O is low. Therefore, the buffer pool areas are performing effectively.

The buffer pool graph

The Buffer Pool graph contains a single series that plots the buffer pool hit rate for the application, expressed as a percentage. This hit rate is the ratio of the physical reads to the logical reads requested by the application for both data and index pages. The rate takes into account hits across all buffer pools configured for the database. If the hit rate is generally high, the buffer pool areas are performing effectively for the application. Lower hit rates indicate that the application is performing more reads from disk than from buffer pools and might therefore be contributing to the overall low hit rate for the database.

The graph title area shows the number of index and data pages inserted into and retrieved from buffer pools for the application during the last monitoring interval.

For related statistical information

On this drilldown the following statistical information is presented:

  • The I/O Activity tab contains additional buffer pool details.

  • The Statistics tab contains entries for all the buffer pool statistical counts used to generate this graph. The names for these statistics begin with Buffer Pool. These statistics are derived from the POOL data elements that the DB2 snapshot monitor captures.

 

 

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