Sizing for Service Level Objectives

As part of planning configuration changes, you can also consider your service level objectives or quality of service goals.

For example, as shown in Figure 2–5, CPU utilization has one peak at 8 CPUs, with many lower peaks. If you configure your system to always meet the demand of this single 8–CPU peak, you require 100% capacity all the time, you must keep 8 CPUs in your configuration, and you cannot add new processes.

Figure 2-5  CPU Utilization with Selected Percentiles

This utilization graph shows three months of data. As there are 288 data points per day, there are about 26,000 data points in the quarter shown. The 99.9th percentile, the value for which 99.9% of the data points are below that value, is the 26th highest reading; the 99th percentile is the 260th highest reading. Thus, if you set your requirement at the 99% percentile (260th highest peak), five CPUs might be sufficient. With Capacity Advisor's visualization and reporting tools, you can make a considered estimate of server utilization using different scenarios. Such estimates can help you minimize overspending for capacity you use rarely, and maximize utilization of your systems to ensure that your systems have the capacity needed at the level you require.

Additionally, with utilization graphs and associated reports, you can identify how to optimize use of your servers by examining your quality of service requirements. Capacity Advisor reports provide additional detail to connect utilization graphs with specific quality of service metrics. For additional information, see the report examples in “Producing Graphs and Reports” later in this document.