Expert Oracle Exadata - Parent Directory

C H A P T E R 10Exadata Wait EventsThe Oracle database is a very well instrumented piece of code, and it has been so for quite a while. Itkeeps track of the amount of time spent in discrete operations via the use of wait events. While thedatabase software is quite complex, wait event analysis allows performance analysts to determineprecisely where the database is spending its time. Many difficult performance problems can be resolvedby analyzing these wait events. The introduction of Exadata has resulted in the creation of several newwait events to support the unique operations that are performed on the platform. This chapter will focuson describing these new events and how they relate to the activities actually being performed, whilecontrasting them with the wait events used by the database on non-Exadata platforms. It will alsodescribe a few wait events that aren’t specific to Exadata but play an important role on Exadataplatforms.A wait event is actually a section of code that is timed and assigned a name. The pieces of codecovered by these events are quite often discrete operating system calls such as I/O requests, but somewait events cover considerably larger portions of code. The events may even include sections of codethat contain other wait events. The naming of wait events has been somewhat inconsistent over theyears, and many events have somewhat misleading names. Even though some of the event names areacknowledged to be potentially misleading, Oracle has been understandably reluctant to change them.The Exadata platform has provided an excuse to rename some of the I/O-related events, and as you’ll seeshortly, the developers took the opportunity to do just that.Events Specific to ExadataThere are actually no events that exist exclusively on the Exadata platform. Wait events are built into thedatabase code, and since the compute nodes on Exadata run standard Oracle Database software, all thewait events that are used when invoking Exadata-specific features are available in databases that arerunning on non-Exadata platforms as well. But because the Exadata features are only available on theExadata platform, no time is ever allocated to those events on other platforms. By way of proof, considerthe following example, which compares the events from V$EVENT_NAME (which exposes the valid waitevents) first on an Exadata Database Machine and then on a standard 11gR2 database on a non-Exadataplatform:SYS@EXDB> select count(*) from v$event_name;COUNT(*)----------1118319