System performance tools examine the database server, its configuration, and usage. The most commonly used system performance tool is the performance monitor. Database performance monitoring and analysis tools support many types of performance-oriented requests in many ways. For example, system performance tools can operate:
- In the background mode as a batch job that reports on performance statistics written by the DBMS trace facility
- In the foreground mode as an online monitor that either traps trace information or captures information from the DBMS control blocks as applications execute
- By sampling the database kernel and user address spaces as the program runs and by capturing information about the performance of the job, independent of database traces
- By capturing database trace information and maintaining it in a history file (or table) for producing historical performance reports and for predicting performance trends
- As a capacity planning device that gives statistical information about an application and the environment in which it will operate
- As an after-the-fact analysis tool on a workstation, that analyzes and graphs all aspects of application performance and system-wide performance
Each database performance monitor supports one or more of these features. The evaluation of database performance monitors is a complex task. Sometimes more than one performance monitor is used at a single site—perhaps one for batch reporting and another for online event monitoring. Maybe an enterprise-wide monitoring solution has been implemented and one component of that solution is a database module that monitors your DBMS, but it lacks the details of a more sophisticated DBMS monitor. So, another performance monitor is purchased for daily DBA usage, while the module of the enterprise-wide monitoring solution is used for integrated monitoring by system administrators.
Modern database performance tools can set performance thresholds that, once reached, will alert the DBA, perform another task to report on, or actually fix the problem. These tools are typically agent-based. An agent is a piece of independent code that runs on the database server looking for problems. It interacts with, but does not rely on, a console running on another machine that is viewed by the DBA. This agent architecture enables efficient database monitoring because the agent is not tied to a workstation and can act independently. The agent sends information to the DBA only when required.
Additionally, some system performance tools are available that focus on a specific component of the DBMS such as the buffer pools (data cache). Such a tool can be used to model the memory requirements for database caching, to capture data cache utilization statistics, and perhaps even to make recommendations for improving the performance of the buffers.
Another type of performance optimization tool enables database configuration parameters to be changed without recycling the DBMS instance, subsystem, or server. These tools are useful when the changes require the DBMS to be stopped and restarted. Such tools can dramatically improve availability, especially if configuration parameters need to be changed frequently and the DBMS does not support dynamic parameter modification.
A few ISVs provide invasive system performance tools that enhance the performance of databases by adding functionality directly to the DBMS and interacting with the database kernel. Typically, these products take advantage of known DBMS shortcomings.
For example, products are available that enhance the performance of reading a database page or block or that optimize data caching by providing additional storage and control over buffers and their processing. Care must be taken when evaluating invasive performance tools. New releases of the DBMS may negate the need for these tools because functionality has been added or known shortcomings have been corrected. However, this does not mean that you should not consider invasive database performance tools. They can pay for themselves after only a short period of time. Discarding the tool when the DBMS supports its functionality is not a problem if the tool has already paid for itself in terms of better performance.
One final caution: Because invasive performance tools can interact very closely with the database kernel, be careful when migrating to a new DBMS release or a new release of the tool. Extra testing should be performed with these tools because of their intrusive nature.