星期三, 十一月 13, 2019

BothdbfilesequentialreadanddbfilescatteredreadeventssignifytimewaitedforI/Oreadrequeststocomplete。

Timeisreportedin100sofasecondforOracle8ireleasesandbelow,and1000sofasecondforOracle9iandabove。

Mostpeopleconfusetheseeventswitheachotherastheythinkofhowdataisreadfromdisk。

InsteadtheyshouldthinkofhowdataisreadintotheSGAbuffercache。

dbfilesequentialread:

Asequentialreadoperationreadsdataintocontiguousmemory(usuallyasingle-blockreadwithp3=1,butcanbemultipleblocks)。

SingleblockI/Osareusuallytheresultofusingindexes。

Thiseventisalsousedforrebuildingthecontrolfileandreadingdatafileheaders(P2=1)。

Ingeneral,thiseventisindicativeofdiskcontentiononindexreads。

dbfilescatteredread:

Similartodbfilesequentialreads,exceptthatthesessionisreadingmultipledatablocksandscattersthemintodifferentdiscontinuousbuffersintheSGA。

ThisstatisticisNORMALLYindicatingdiskcontentiononfulltablescans。

Rarely,datafromfulltablescanscouldbefittedintoacontiguousbufferarea,thesewaitswouldthenshowupassequentialreadsinsteadofscatteredreads。

Theindexfullscantypicallyusesdbfilesequentialreadstogetdatafromdisk,althoughnewerversionsofOraclecandodbfileparallelreadswhicharereadrequestsformultipleOracleblocksthatarenot?

adjacentblocksintheOracledatafiles。

Theindexfastfullscantypicallyusesdbfilescatteredreadstogetdatafromdiskandtheindexesarethensubjecttoallthedetailsofthespecialtreatmentfortablescans。

ThenitwillusuallybeareadfromaSORT(TEMPORARY)Segmentasmultiblockreadsfortablescansusuallyshowupaswaitingondbfilescatteredread。

Sometimesdelayedblockcleanoutalsocausedbfilescatteredreadevent。

Relatedwaitevents:

directpathread

bufferbusywaits

readbyanothersession

Justfortherecord

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