Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICSSummaryThe emphasis of this chapter was on understanding Exadata performance and the various relatedmetrics Oracle offers for us. It is important to remember that the Exadata Smart Scan potentially makesyour data retrieval scanning faster, but Smart Scans happen only when direct path reads and fullsegment scans are used. Also, remember that it is not possible to determine whether a Smart Scanactually kicked in just by looking at the execution plan alone. You should always check additionalmetrics, like whether you see the cell smart table/index scan wait events in your session and whether theIO_CELL_OFFLOAD_ELIGIBLE_BYTES (in V$SQL) or cell physical I/O bytes eligible for predicate offloadstatistic (in V$SESSTAT) increases while you run your SQL. Many of the other metrics explained here willhopefully be helpful for understanding and troubleshooting advanced performance issues, such as whena Smart Scan kicks in but is throttled by a multitude of special conditions like chained rows, consistentread rollbacks, or just running out of cell server resources. In the next chapter we will see how to use thisknowledge in monitoring and troubleshooting Exadata performance, and we will look deeper into thecell-level performance metrics from cellsrv and the operating system, too.377