Expert Oracle Exadata - Parent Directory

CHAPTER 16 UNLEARNING SOME THINGS WE THOUGHT WE KNEWid:7f60871e-d5f2-4990-b7e5-30c4a238de12size: 365.25Gstatus:normalNotice the new attribute degradedCelldisks . Also notice that the flash cache on this cell shows astatus of normal. Monitoring storage software behavior is covered in more detail in Chapter 12.ScalabilityAnother thing to keep in mind when dealing with OLTP workloads is that the Exadata platform providesexceptional scalability. Upgrading from a half rack to full rack doubles the number of CPUs at both thedatabase layer and the storage layer. The amount of ESFC is also doubled, as is the available memory.This allows Exadata to scale in a nearly linear fashion for many systems. Kevin Says: The authors are correct that doubling from a half rack to a full rack is perfect hardware scale-up. Iurge caution, however, in presuming that the software—specifically Real Application Clusters (RAC)—will scalelinearly with all applications. RAC has been around for over ten years, so let’s not throw away our understanding ofits scalability characteristics when considering Exadata. There is nothing about Exadata architecture that changesthe intrinsic RAC scalability characteristics.Write-Intensive OLTP WorkloadsWrite-intensive workloads are a subset of OLTP-oriented systems. There are some systems that just bangaway at single-row inserts. These systems are often limited by the speed at which commits can be done,which often depends on the speed with which writes to the log files can be accomplished. This is onearea where Exadata competes with other platforms on a fairly even playing field. There are no majorenhancements that make Exadata run orders of magnitudes faster for systems that are bottlenecked onwrite operations. This means that there is no magic bullet, and traditional methods of tuning, such asminimizing commits, are appropriate for these types of systems. Kevin Says: Write-intensive workloads warrant deep thought. The authors point out that there are no majorenhancements that make Exadata perform orders of magnitude better than Oracle systems connected toconventional storage. There are, in fact, no attributes of Exadata that favor write-intensive workloads as of thepublication date of this book. The V2 and X2 models are capable of servicing over 1 million random single-blockreads per second—from Exadata Smart Flash Cache on the full rack configuration. Random writes, on the otherhand, have to go to spinning disks, of which there are 168 in the full rack configurations. The high-performanceSAS drives can service roughly 300 random write IOPS—as long as the seeks are short. However, writes must beredundant, and thus the net random write bandwidth available to databases is roughly 25,000 IOPS. To scale anapplication to 1 million read IOPS on Exadata, the imposed read:write ratio is 40 to 1. That is, Exadata is 40-fold513