Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICSTable 11.1. V$SQL Columns and Their MeaningsColumn NameIO_CELL_OFFLOAD_ELIGIBLE_BYTESIO_INTERCONNECT_BYTESOPTIMIZED_PHY_READ_REQUESTSIO_CELL_UNCOMPRESSED_BYTESIO_CELL_OFFLOAD_RETURNED_BYTESMetric MeaningHow many bytes worth of segment reads were offloaded tothe cells. The cells either did read this data or skipped it ifstorage indexes helped to skip block ranges. This metriccorresponds to the cell physical IO bytes eligible forpredicate offload statistic in V$SESSTAT.The total traffic bytes (read and write) sent between thedatabase node and cells. Note that thanks to the bug10036425 mentioned earlier, the IO_INTERCONNECT_BYTEScolumn is populated also on non-Exadata platforms, butyou can ignore it on these systems.The number of disk IO requests that were eithercompletely avoided thanks to storage indexes or doneagainst cell flash cache cards.The size of uncompressed data the cells have scannedthrough during a Smart Scan. Note that the cells do nothave to actually uncompress all the data to know theuncompressed length. The EHCC compression unitheaders store both the compressed and uncompressed CUlength info in them. This metric is useful for estimating theI/O reduction from EHCC compression. Note that thismetric works for EHCC segments only. For regular blocklevelcompression, this metric just shows the compressedsize of data.This metric shows how much data was returned as a resultfrom an offloaded Smart Scan access path. This is a mainindicator of smart scan offloading efficiency whencompared with IO_CELL_OFFLOAD_ELIGIBLE_BYTES (tomeasure the IO reduction between cells and database) orIO_CELL_UNCOMPRESSED_BYTES when measuring the total IOreduction thanks to offloading and compression.Here is example output from a query on an EHCC-compressed table where the table scan was offloadedto the cell. The V$SQL table output is pivoted for better readability:375