数据库系统概念全套配套课件PPT ch10.ppt

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1、Chapter10:StorageandFileStructureChapter10:StorageandFileStructureOverviewofPhysicalStorageMediaMagneticDisksRAIDTertiaryStorageStorageAccessFileOrganizationOrganizationofRecordsinFilesData-DictionaryStorageClassificationofPhysicalStorageMediaSpeedwithwhichdatacanbeaccessed

2、CostperunitofdataReliabilitydatalossonpowerfailureorsystemcrashphysicalfailureofthestoragedeviceCandifferentiatestorageinto:volatilestorage:losescontentswhenpowerisswitchedoffnon-volatilestorage:Contentspersistevenwhenpowerisswitchedoff.Includessecondaryandtertiarystorage,a

3、swellasbatter-backedupmain-memory.PhysicalStorageMediaCache–fastestandmostcostlyformofstorage;volatile;managedbythecomputersystemhardware.Mainmemory:fastaccess(10sto100sofnanoseconds;1nanosecond=10–9seconds)generallytoosmall(ortooexpensive)tostoretheentiredatabasecapacities

4、ofuptoafewGigabyteswidelyusedcurrentlyCapacitieshavegoneupandper-bytecostshavedecreasedsteadilyandrapidly(roughlyfactorof2every2to3years)Volatile—contentsofmainmemoryareusuallylostifapowerfailureorsystemcrashoccurs.PhysicalStorageMedia(Cont.)FlashmemoryDatasurvivespowerfail

5、ureDatacanbewrittenatalocationonlyonce,butlocationcanbeerasedandwrittentoagainCansupportonlyalimitednumber(10K–1M)ofwrite/erasecycles.ErasingofmemoryhastobedonetoanentirebankofmemoryReadsareroughlyasfastasmainmemoryButwritesareslow(fewmicroseconds),eraseisslowerWidelyusedin

6、embeddeddevicessuchasdigitalcameras,phones,andUSBkeysPhysicalStorageMedia(Cont.)Magnetic-diskDataisstoredonspinningdisk,andread/writtenmagneticallyPrimarymediumforthelong-termstorageofdata;typicallystoresentiredatabase.Datamustbemovedfromdisktomainmemoryforaccess,andwritten

7、backforstorageMuchsloweraccessthanmainmemory(moreonthislater)direct-access–possibletoreaddataondiskinanyorder,unlikemagnetictapeCapacitiesrangeuptoroughly1.5TBasof2009Muchlargercapacityandcost/bytethanmainmemory/flashmemoryGrowingconstantlyandrapidlywithtechnologyimprovemen

8、ts(factorof2to3every2years)Survivespowerfailuresandsystemcrashesdiskfailurecandest