Expert Oracle Exadata - Parent Directory

CHAPTER 2 OFFLOADING / SMART SCANReducing the volume was the main focus and primary goal. The majority of the optimizationsintroduced by Offloading contribute to this goal. Reducing CPU load is important as well, but is not theprimary benefit provided by Exadata and therefore takes a back seat to reducing the volume of datatransferred. (As you’ll see, however, decompression is a notable exception to that generalization, as it isperformed on the storage servers.) Several optimizations to reduce disk access time were alsointroduced, and while some of the results can be quite stunning, we don’t consider them to be thebread-and-butter optimizations of Exadata.Exadata is an integrated hardware/software product that depends on both components to providesubstantial performance improvement over non-Exadata platforms. However, the performance benefitsof the software component dwarf the benefits provided by the hardware. Here is an example:SYS@SANDBOX> alter session set cell_offload_processing=false;Session altered.Elapsed: 00:00:00.06SYS@SANDBOX> select count(*) from kso.skew3 where col1 < 0;COUNT(*)----------21 row selected.Elapsed: 00:00:51.09SYS@SANDBOX> alter session set cell_offload_processing=true;Session altered.Elapsed: 00:00:00.07SYS@SANDBOX> select count(*) from kso.skew3 where col1 < 0;COUNT(*)----------21 row selected.Elapsed: 00:00:00.15This example shows the performance of a scan against a single table with 384 million rows. We ran itonce with Offloading disabled, effectively using all the hardware benefits of Exadata and none of thesoftware benefits. You’ll notice that even on Exadata hardware, this query took almost a minute. Keep inmind that this was only spread across three storage servers on our V2 quarter rack and did not utilize theflash cache at all. We then re-enabled Offloading, and the query completed in substantially less than asecond. Obviously the hardware in play was the same in both executions. The point is that it’s thesoftware’s ability via Offloading that made the difference.25