Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICSIf this new feature stands the test of time, it will add many variables (in multiple dimensions) to performanceanalysis. In the end, however, the only thing that really matters is whether the users’ responsiveness from thesystem is acceptable. I choose to remain dubious, however, because of the added complexity. It has taken manychapters of this well-written book to explain when Smart Scan, and thus offload processing, will occur. Now wehave a Smart Scan situation that doesn’t perform offload processing for varying percentages of the data streamingoff of storage. It has been said many times that offload processing is the “secret sauce” of Exadata and, of course,Smart Scan is the main agent for delivery. It is yet to be seen how complex workloads behave when Smart Scansometimes chooses not to do its job.cell physical IO bytes saved by storage indexThis is another important statistic, which shows how many bytes the Smart Scan sessions could simplyskip reading from disk thanks to the in-memory storage index in cellsrv. If this statistic, cell physical IObytes saved by storage index is close to cell physical IO bytes eligible for predicate offload, it indicates thatSmart Scans are greatly benefiting from storage indexes and have avoided disk reads thanks to that.cell physical IO bytes saved during optimized file creationThis statistic shows how many bytes worth of data file creation and file extension work was done in thecell. Without this optimization, Oracle would format the new data blocks in the database session’s PGAand send physical block writes to the cell over interconnect. In Exadata, the file block formatting andwrites can be offloaded to the cell, and the database does not have do physical writes itself. In Exadata,you would see a cell smart file creation wait event when cells are formatting blocks for new files or whenextending files. Without this optimization, you would see the Data file init write wait events.cell physical IO bytes saved during optimized RMAN file restoreThis statistic shows how many bytes worth of datafiles were restored within cells, without having toinvolve the database layer, thanks to offloading the RMAN restore to cells. You should see your recoverysessions waiting for cell smart restore from backup wait event when restoring on Exadata RMAN backup& recovery I/O wait event.cell physical IO bytes sent directly to DB node to balance CPU usageThis statistic is the same as the “cell physical IO bytes pushed back due to excessive CPU on cell” statisticearlier, which is used in Oracle 11.2.0.2 with Bundle Patch 6, but in newer Oracle versions and patchlevels the current statistic name is used instead.367