Expert Oracle Exadata - Parent Directory

CHAPTER 2 OFFLOADING / SMART SCANXMLTRANSFORM V92 Oracle NOXMLTRANSFORMBLOB V10 Oracle NOXS_SYS_CONTEXTV11R1 Oracle NO921 rows selected.Offloading functions does allow the storage cells to do some of the work that would normally bedone by the CPUs on the database servers. However, the saving in CPU usage is generally a relativelyminor enhancement. The big gain usually comes from limiting the amount of data transferred back tothe database servers. Being able to evaluate functions contained in WHERE clauses allows storage cells tosend only rows of interest back to the database tier. So as with most Offloading, the primary goal of thisoptimization is to reduce the amount of traffic between the storage and database tiers.Compression/DecompressionOne Exadata feature that has received quite a bit of attention is Hybrid Columnar Compression (HCC).Exadata offloads the decompression of data stored in HCC format during Smart Scan operations. That is,columns of interest are decompressed on the storage cells when the compressed data is accessed viaSmart Scans. This decompression is not necessary for filtering, so only the data that will be returned tothe database tier will be decompressed. Note that all compression is currently done at the database tier,however. Decompression may also be done at the database tier when data is not accessed via a SmartScan. So to make it simple, Table 2-1 shows where the work is done.Table 2-1. HCC Compression/Decompression OffloadingOperation Database Servers Storage ServersCompression Alwa ys NeverDecompression Non-Smart Scan Smart ScanDecompressing data at the storage tier runs counter to the theme of most of the other Smart Scanoptimizations. Most of them are geared to reducing the volume of data to be transported back to thedatabase servers. Because decompression is such a CPU-intensive task, particularly with the higherlevels of compression, the decision was made to do the decompression on the storage servers. Thisdecision is not be locked in stone, however, as in some situations there may be ample CPU resourcesavailable to make decompressing data on the database servers an attractive option (that is, in somesituations the reduction in data to be shipped may outweigh the reduction in database server CPUconsumption). In fact, as of cellsrv version 11.2.2.3.1, Exadata does have the ability to return compresseddata to the database servers when the storage cells are busy.39