Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICS---------------------------------------------------------------------------------Active% | SQL_ID | EVENT | WAIT_CLASS---------------------------------------------------------------------------------393% | b4kvh3sqsttkw | cell smart table scan | User I/O7% | b4kvh3sqsttkw | ON CPU | ON CPU-- End of ASH snap 1, end=2011-04-03 19:57:35, seconds=5, samples_taken=44This output shows that there are apparently four different PX slave sessions (notice the SID column) thathave done physical reads during the 5-second Snapper run. The physical read total bytes tells us thateach slave has read between 2.63 and 3.05GB worth of data blocks from datafiles during the 5-secondSnapper run, using cell smart scan, because the cell scans statistic has increased for each slave.Moreover, the fact that cell physical I/O interconnect bytes returned by smart scan is also non-zero meansthat a cell smart scan was used and cellsrv processes returned some data to the database layer. Notethat only around 381–443KB of data was returned to the database layer, while cells scanned throughgigabytes of blocks for their “master” PX slave sessions. This is the Smart Scan at its best—parallel bruteforce scanning in cells at extreme speeds— and return only a small proportion of matching data to thedatabase layer.The Meaning and Explanation of Exadata Performance CountersNow, it’s time to explore the meaning of performance counters. No matter how pretty the charts orpictures a performance tool draws using these metrics, if you don’t know what they actually mean, theywill be useless for troubleshooting. In this book we will cover mostly Exadata-specific statistics and a fewclosely related ones. You can find more info about some common performance counters in Appendix E:“Statistics Descriptions,” in Oracle Database Reference guide 11g Release 2 (11.2), currently located athttp://download.oracle.com/docs/cd/E14072_01/server.112/e10820/stats.htm.Here’s a script that lists all statistics related to storage cells from v$statname, with the statistic class,which indicates the purposes for which Oracle kernel engineers have expected to use these counters:SQL> SELECT2 name3 , TRIM(4 CASE WHEN BITAND(class, 1) = 1 THEN 'USER ' END ||5 CASE WHEN BITAND(class, 2) = 2 THEN 'REDO ' END ||6 CASE WHEN BITAND(class, 4) = 4 THEN 'ENQ ' END ||7 CASE WHEN BITAND(class, 8) = 8 THEN 'CACHE ' END ||8 CASE WHEN BITAND(class, 16) = 16 THEN 'OSDEP ' END ||9 CASE WHEN BITAND(class, 32) = 32 THEN 'PARX ' END ||10 CASE WHEN BITAND(class, 64) = 64 THEN 'SQLT ' END ||11 CASE WHEN BITAND(class,128) = 128 THEN 'DEBUG ' END12 ) class_name13 FROM14 v$statname15 WHERE16 name LIKE '%cell%'17 ORDER BY18* nameSQL> /354