Expert Oracle Exadata - Parent Directory

CHAPTER 10 EXADATA WAIT EVENTSEvent MeaningHere is the output of a query that shows the operations that resulted in the cell single block physicalread wait event on an active production system:SYS@EXDB1> select event, operation, count(*) from (2 select sql_id, event, sql_plan_operation||' '||sql_plan_options operation3 from DBA_HIST_ACTIVE_SESS_HISTORY4 where event like 'cell single%')5 group by operation, event6 order by 1,2,37 /EVENT OPERATION COUNT(*)--------------------------------- ---------------------------------------- ----------cell single block physical read 13321CREATE TABLE STATEMENT 35DDL STATEMENT 118DELETE 269FIXED TABLE FIXED INDEX 3FOR UPDATE 2HASH JOIN 4HASH JOIN RIGHT OUTER 8INDEX FULL SCAN 9283INDEX FULL SCAN (MIN/MAX) 1INDEX RANGE SCAN 2763INDEX STORAGE FAST FULL SCAN 6INDEX STORAGE SAMPLE FAST FULL SCAN 13INDEX UNIQUE SCAN 1676INSERT STATEMENT 1181LOAD AS SELECT 6LOAD TABLE CONVENTIONAL 92MERGE 106SELECT STATEMENT 41SORT ORDER BY 6TABLE ACCESS BY GLOBAL INDEX ROWID 10638TABLE ACCESS BY INDEX ROWID 8714TABLE ACCESS BY LOCAL INDEX ROWID 10446TABLE ACCESS CLUSTER 12TABLE ACCESS STORAGE FULL 776TABLE ACCESS STORAGE SAMPLE 40UPDATE 8116As you can see, row access via an index is the most common operation that generates this event.You should also be aware that Exadata provides a large amount of flash cache (384G) on each storagecell. For that reason, physical reads (both multi-block and single-block) are considerably faster than onmost disk-based storage systems. Here is an excerpt from an AWR report showing a histogram of singleblock reads for the instance:% of Waits-----------------------------------------Total329