Expert Oracle Exadata - Parent Directory

CHAPTER 16 UNLEARNING SOME THINGS WE THOUGHT WE KNEWSession altered.Elapsed: 00:00:00.00SYS@SANDBOX1> alter session set cell_offload_processing=true;Session altered.Elapsed: 00:00:00.00SYS@SANDBOX1> alter system flush buffer_cache;System altered.Elapsed: 00:00:00.08SYS@SANDBOX1> select avg(length(c)) from tanel.t_chained2;AVG(LENGTH(C))--------------4000Elapsed: 00:01:59.24 @fsx4Enter value for sql_text: select avg(length(c)) from tanel.t_chained2Enter value for sql_id:SQL_ID CHILD OFFLOAD IO_SAVED_% AVG_ETIME SQL_TEXT--------------------------------------------------------------------------------------3upcma5nsnbcd 0 Yes -27.41 159.24 select avg(length(c)) from tanel.t_chainElapsed: 00:00:00.06Notice that the statement was offloaded and that the IO_SAVED_% calculation produced a negativenumber. This shows that more bytes were returned by the storage cells than the total number of bytesthat the table occupies. It’s clear from this example that chained rows can be much more detrimental toperformance when attempting to make use of Exadata’s Smart Scans than to queries that are handled innormal block-shipping mode. It’s worth noting that this is an extreme example, as every row waschained. In fact, many of the records spanned three blocks. In this extreme case, it is clear that avoidingoffloading actually improves the performance significantly. Of course, eliminating the chained rowswould be the best solution.Very Busy CellsWhen a storage cell becomes very busy with CPU operations, it is possible for the storage software tobegin refusing to perform offload activities on some portion of the requests. That is, if a particular cellbecomes extremely busy and is getting offloadable requests from a database server that is notparticularly busy, it may decide to send some data back to the database server that has not beenprocessed or has only partially been processed. In some cases column projection may be done but notfiltering, in other cases, cellsrv may revert to shipping entire blocks back to the database layer. So whilethis issue does not result in a complete shutdown of offloading, it can reduce the amount of work that isdone on the storage tier.522