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39-A Case for Redundant Arrays of Inexpensive Disks.pdf

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39-A Case for Redundant Arrays of Inexpensive Disks.pdf

NajuG.Mancherilnajucmu.edu15721DatabaseManagementSystemsFebruary27,2005AilamakiThissummarycanbefoundonlineathttp//www.andrew.cmu.edu/user/ngm/15721/summaries/ACaseforRedundantArraysofInexpensiveDisksDavtdA.Patterson,GarthGibson,andRandyH.KatzBylettingtheDBMSmanagethebufierspageskeptinmemory,wecanachievebetterperformancethanwithatraditional\workingsetvirtualmemorymanagement.Abufierpoolmanager\understandsitsdatabasesystemscommonoperations,soithastheabilitytopredictfuturereferencebehavior.Themostinnovativeideasinthispaperare†SLEDscannotkeepup.Whileprocessingpowerandmemoryaccesstimesaredoublingeveryfewyears,singlelargeexpensivedisksaremechanicaldevicesdominatedbyseekandrotationaldelays.Transferratesareimprovedbyincreasingstoragedensity,butrawseektimeincreasesatabout7peryear.†Inexpensivedisksareprettygood.ThenumberofI/Os/sec/actuatoriswithinafactorof2whencomparedwithexpensivedisks.Theyaresmaller,sotheyuselesspower.Standardsallowmanufacturerstoprovideadvancedbufieringfeaturesthroughcheapembeddedcontrollers,whereasnonstandardmainframedisksrequireasystemspeciflccontroller.†Asthenumberofdisksincreases,redundancyisrequiredtomaintainreliability.Simplyincreasingthenumberofdisksjustensuresthatthesystemwillfail.AssuminganexponentialdistributiononthemeantimetofailureMTTFofasinglediskandindependenceoffailures,wehaveMTTFarrayMTTFsingledisknwherenisthenumberofdisksinthearraylimn1MTTFarray0†Wecanusecheckdiskstostoreparityinformation.Fullredundancyrequirestwiceasmanydisks.ECCcodesaresmallerthanthedata,lettingusdetecterrorswithoutcomparingtwofullcopiesofthedata.AlthoughthisachievesbetterperformanceperdiskthanRAID0,smallwritesrequirereadingGsectors,mergingdata,computingnewECC,andwritingnewdataECCbacktodisk.RAID3usesthediskcontrollertoidentifywhichdiskfailed,sowecanlimitthenumberofcheckdisksto1pergroup.†Bottlenecks.Wecangetridofthegroupwritebottleneckbyputtingeachtransferableunitonitsowndisk,ratherthanstripingitacrossseveraldisksRAID4.Wecangetridofthecheckdiskbottleneckbystripingthecheckdiskinfooveralldisks.Thispaperisverythoroughinitsanalysisofcostandperformance.Unlikemostpapers,theauthorsexplicitlyincludealistofoverlookedissues.Herearesomeoftheimportantones†HowdodefectivesectorsafiectRAIDTheauthorsassumeafailfastmodelofdiskcontrollersinRAID3.IsthisareliableassumptionHowdodisksfailundercurrenttransactionprocessingsystems†HowdoestheslowdownfactorSactuallybehaveAlthoughtheauthorsusethisfactortobalanceworstandbestcaseread/writetime,itisagrosssimpliflcationoftheactuallyread/writetime.Howaccurateisthissinglefactormodel†HowdoyouscheduleI/Otolevel5RAIDstomaximizewriteparallelismThisisaclassicschedulingproblemwherealotofworkhasprobablyalreadybeendone.Theauthorsofthispaperhavedoneaverythoroughjobanalyzingthecostandperformanceoftheirsystem.TheypresentaclearargumentastowhyRAIDswilloutstripSLEDsanditseemsthatenoughpeoplebelievedthem.IdontknowofanycompanytodaystillmanufacturingSLEDs,whereasmanycommercialdatabasesystemsrequireatleastsomemeasureofstorageredundancytoensurereliablebehavior.Itisunclearhowtheperformanceargumentsgivenherewouldchangeiftheworldwastosuddenlyswitchtoadifierenttechnology,likesolidstatedisks,butthereliabilityconcernsraisedwithprobablystillbeapplicable.RAIDsredundancyandcheckmodelwillprobablypersistincomputerstorageforquitesometime.1

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