Ratios – Historical Introduction

Tuning has always been an important part of a Remote DBA’s job, ranking right next to backup and recovery. It has often been viewed as “black magic.” That perception originated from the tuning process that was taught by many experts and touted in the many books on tuning. That process centered on using ratios to determine the health of a database or component of the database.

According to the ratio school of thought, if the Buffer Cache Hit Ratio (BCHR) is too low, then the size of the buffer cache should be increased to improve performance. If this fixed the problem and the database ran better, then the Remote DBA was awarded guru status and managers would likely follow almost any recommendations made. If these changes showed no improvement or made things worse, another Remote DBA would be brought in. The new Remote DBA might determine that another ratio was out of line and make a different change, or simply increase the magnitude of the initial changes. If this change made performance better, then the new Remote DBA became the guru.

This school of thought is based, at least in part, on the rationale that it is faster to access data blocks from memory (RAM) than from disk. Therefore, if too many data blocks were being read from disk, then a possible cause of performance degradation is the buffer cache being too small to keep enough data blocks available for the users. Usually, the solution to that problem was to increase the buffer cache.

To better understand ratios, it may help to illustrate using a non-database example. Theoretically it is faster to get to work if there are no stoplights to wait for. So to reduce commute time, try and reduce the time spent at stoplights. One way would be to simply keep going even when the lights are red, but that could get dangerous and cause increased insurance premiums, so another way would be to tune or change the commute.

The above book excerpt is from:

Oracle Wait Event Tuning

High Performance with Wait Event Interface Analysis

ISBN 0-9745993-7-9

Stephen Andert

http://www.rampant-books.com/book_2004_2_wait_tuning.htm