Abnormal values in monitoring data
Being able to identify abnormal values is key to interpreting system performance monitoring data when troubleshooting performance problems.
A monitor element provides a clue to the nature of a performance problem when its value is worse than normal, that is, the value is abnormal. Generally, a worse value is one that is higher than expected, for example higher lock wait time. However, an abnormal value can also be lower than expected, such as lower buffer pool hit ratio. Depending on the situation, you can use one or more methods to determine if a value is worse than normal.
One approach is to rely on industry rules of thumb or best practices. For example, a rule of thumb is that buffer pool hit ratios of 80-85% or better for data are generally considered good for an OLTP environment. Note that this rule of thumb applies to OLTP environments and would not serve as a useful guide for data warehouses where data hit ratios are often much lower due to the nature of the system.
Another approach is to compare current values to baseline values collected previously. This approach is often most definitive and relies on having an adequate operational monitoring strategy to collect and store key performance metrics during normal conditions. For example, you might notice that your current buffer pool hit ratio is 85%. This would be considered normal according to industry norms but abnormal when compared to the 99% value recorded before the performance problem was reported.
A final approach is to compare current values with current values on a comparable system. For example, a current buffer pool hit ratio of 85% would be considered abnormal if comparable systems have a buffer pool ratio of 99%.