Expert Oracle Exadata - Parent Directory

CHAPTER 5 EXADATA SMART FLASH CACHESYS@EXDB1>SYS@EXDB1> select2 'cell single + multiblock reads' c1,3 c2, c3, c5, c6,4 c6/decode(nvl(c2,0),0,1,c2) hit_ratio5 from (6 select7 sum(total_waits) c2,8 avg(value) c6,9 sum(time_waited / 100) c3,10 avg((average_wait /100)*1000) c511 from12 sys.v_$system_event, v$sysstat ss13 where14 event in (15 'cell single block physical read',16 'cell multiblock physical read')17 and18 name like 'cell flash cache read hits'19 and20 event not like '%Idle%')21 order by22 c323 ;AvgEvent Total Seconds Wait HitNameWaits Waiting (ms) Flash Cache Hits Ratio------------------------------ ------- ------- ----- ---------------- -----cell single + multiblock reads 106,642 374 3.5 247,951 2.325As you can see, the script vastly overestimates the hit ratio on this particular system, coming up witha hit ratio of over 200%. This happens because this system has several objects that are designated foraggressive caching, resulting in many cache hits for Smart Scan events which are not included in thesingle block and multi-block read statistics . For systems that don’t use the KEEP attribute, the numbersshould be closer to reality. However, calculated hit ratios are probably not the best approach. It isgenerally more enlightening to look at the actual single block access times than to try to calculate a hitratio.SummaryExadata Smart Flash Cache provides yet another way to pare down the I/O cost associated with Oracledatabases. Most of the optimizations that the Exadata platform provides require the use of Smart Scans(full table or index scans). ESFC does not depend on Smart Scans and in fact is most useful for speedingup access on random reads of single blocks. These operations are generally associated with OLTPworkloads, and so ESFC is the key component of Exadata for OLTP or mixed workloads. The fact thatESFC does not provide a write-back cache, though, severely limits its effectiveness with systems that arebottlenecked on writes. Nevertheless, the large cache and the intelligent caching algorithms used by theOracle storage software allow ESFC to provide read performance similar to solid-state based disk141