Expert Oracle Exadata - Parent Directory

C H A P T E R 12Monitoring Exadata PerformanceBy now you have learned about the key Exadata performance features and the related performancemetrics. Let’s see how we can use these for everyday tasks. We will look at standard tools available fordatabase-layer and cell performance monitoring, and how to interpret their output.Oracle Database and the Exadata cells provide a huge variety of different metrics, but beforemonitoring any metrics, you should ask yourself why you are monitoring them. Additionally, you shouldknow what your action would be if a metric crosses some threshold. This leads to the follow-upquestion: which exact threshold should prompt some action from you—and why? In other words, youshould know what are you trying to achieve (good response times for users) and how performancemetrics relate to that.The monitoring tasks covered here can be divided into the following categories:• SQL statement response time monitoring• Database layer utilization and efficiency monitoring• Storage cell layer utilization and efficiency monitoring• Advanced metrics and monitoring for Exadata performance troubleshootingNote that we will focus on the Exadata-specific performance topics here and not the whole widerange of other Oracle performance topics, like lock contention or general SQL performance issues.A Systematic ApproachWhatever metrics you monitor, you should have a purpose in mind. In other words, don’t collect anddisplay metrics just because they are available; this will lead you nowhere or potentially even misleadyou to fixing the wrong problem. Note that the term “performance” is vague—different people maymean different things when they use it. From an IT system user’s perspective, performance is ultimatelyonly about one thing— response time. And not some response times of individual wait events measuredat a low level; the end users don’t care about that. They do care about how much they have to wait fortheir business task to complete, like the time it takes from the report submission to actually seeing itsoutput. This time is measured in regular wall-clock time; it’s as simple as that.If your purpose in monitoring is to ensure good response times for your application users, then youshould measure what matters—response time as your end-user experiences it. This would be the idealentry point to performance monitoring. In addition to this entry point, you should measure moredetailed lower-level metrics to break the end user response time down into individual components, liketime spent in an application server and database time. Your application instrumentation andmonitoring tools should keep track of which database sessions were used for which end-user task, so379