Expert Oracle Exadata - Parent Directory

CHAPTER 10 EXADATA WAIT EVENTSWAIT #4: nam='cell smart flash unkeep' ela= 216 cellhash#=3176594409 p2=0 p3=0 obj#=-1WAIT #4: nam='cell smart flash unkeep' ela= 222 cellhash#=379339958 p2=0 p3=0 obj#=-1WAIT #4: nam='cell smart flash unkeep' ela= 196 cellhash#=2133459483 p2=0 p3=0 obj#=-1WAIT #4: nam='cell smart flash unkeep' ela= 216 cellhash#=3176594409 p2=0 p3=0 obj#=-1Note that the cell smart flash unkeep event is preceded by a handful of enq: RO – fast objectreuse events, which are used to mark time associated with cleaning up the buffer cache after a drop ortruncate. The cell smart flash unkeep is basically an extension of that event to clean up the ExadataSmart flash cache on the storage server as well.ParametersThe only parameter used for this event is P1, which shows which cell was responsible for generating theevent.Old EventsP1 - Cell hash numberP2 - Not usedP3 - Not usedIn addition to the new cell events, there are also a few non-Exadata-specific wait events that you shouldbe aware of. These are events that you may already be familiar with from managing Oracle on otherplatforms. They happen to also be important in an Exadata environment, so they represent cases inwhich your existing knowledge and skill can carry over and stand you in good stead as you move in tomanaging Exadata.direct path readDirect path reads are used by Oracle to read data directly into PGA memory (instead of into the buffercache). They are an integral part of Exadata Offloading because SQL processing can only be offloaded tothe storage cells when the direct path read mechanism is used. The direct path read wait event isactually replaced by the cell smart table scan and cell smart index scan wait events when a query isoffloaded. However, the direct path read mechanism is still used by the code covered by those new waitevents. That is, either the plan must include a parallel scan or Oracle must decide to use the serial directpath read mechanism.Event MeaningThis event records time that Oracle spends waiting on a direct path read to complete. You should knowthat the direct path read wait event can be very misleading. Both the number of events recorded and thetimings associated with them can appear to be inaccurate. This is due to the fact that direct path readsare done in an asynchronous and overlapping fashion. See MOS note 50415.1 for further details aboutwhy time recorded by this event can be misleading. It also bears mentioning that 11gR2 contains anenhancement that causes serial direct path reads to occur more frequently than in previous releases. SeeMOS Note 793845.1, which briefly mentions this change.338