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

NajuG.M15-721:DatabaseManagementSystemsFebruary27,2005AilamakiThissummarycanbefoundonlineat/user/ngm/15-721/summaries/ACaseforRedundantArraysofInexpensiveDisksDavtdA.Patterson,GarthGibson,andRandyH.KatzBylettingtheDBMSmanagethebufiers(pageskeptinmemory),wecanachievebetterperformancethanwithatraditionalworkingsetvirtualmemorymanagement.Abufierpoolmanagerunderstandsitsdatabasesystemscommonoperations,soithastheabilitytopredictfuturereferencebehavior.Themostinnovativeideasinthispaperare:SLEDscannotkeepup.Whileprocessingpowerandmemoryaccesstimesaredoublingeveryfewyears,singlelargeexpensivedisksaremechanicaldevicesdominatedbyseekandrotationaldelays.Transferratesareimprovedbyincreasingstoragedensity,butrawseektimeincreasesatabout7%peryear.Inexpensivedisksareprettygood.ThenumberofI/Os/sec/actuatoriswithinafactorof2whencomparedwithexpensivedisks.Theyaresmaller,sotheyuselesspower.Standardsallowmanufacturerstoprovideadvancedbufieringfeaturesthroughcheapembeddedcontrollers,whereasnon-standardmainframedisksrequireasystem-speciflccontroller.Asthenumberofdisksincreases,redundancyisrequiredtomaintainreliability.Simplyincreasingthenumberofdisksjustensuresthatthesystemwillfail.Assuminganexponentialdistributiononthemeantimetofailure(MTTF)ofasinglediskandindependenceoffailures,wehaveMTTFarray=MTTFsingledisknwherenisthenumberofdisksinthearray=)limn!1MTTFarray=0Wecanusecheckdiskstostoreparityinformation.Fullredundancyrequirestwiceasmanydisks.ECCcodesaresmallerthanthedata,lettingusdetecterrorswithoutcomparingtwofullcopiesofthedata.AlthoughthisachievesbetterperformanceperdiskthanRAID0,smallwritesrequirereadingGsectors,mergingdata,computingnewECC,andwritingnewdata+ECCbacktodisk.RAID3usesthediskcontrollertoidentifywhichdiskfailed,sowecanlimitthenumberofcheckdisksto1pergroup.Bottlenecks.Wecangetridofthegroupwritebottleneckbyputtingeachtransferableunitonitsowndisk,ratherthanstripingitacrossseveraldisks(RAID4).Wecangetridofthecheckdiskbottleneckbystripingthecheckdiskinfooveralldisks.Thispaperisverythoroughinitsanalysisofcostandperformance.Unlikemostpapers,theauthorsexplicitlyincludealistofoverlookedissues.Herearesomeoftheimportantones:HowdodefectivesectorsafiectRAID?Theauthorsassumeafail-fastmodelofdiskcontrollersinRAID3.Isthisareliableassumption?Howdodisksfailundercurrenttransactionprocessingsystems?HowdoestheslowdownfactorSactuallybehave?Althoughtheauthorsusethisfactortobalanceworstandbestcaseread/writetime,itisagrosssimpliflcationoftheactuallyread/writetime.Howaccurateisthissinglefactormodel?HowdoyouscheduleI/Otolevel5RAIDstomaximizewriteparallelism?Thisisaclassicschedulingproblemwherealotofworkhasprobablyalreadybeendone.Theauthorsofthispaperhavedoneaverythoroughjobanalyzingthecostandperformanceoftheirsystem.TheypresentaclearargumentastowhyRAIDswilloutstripSLEDsanditseemsthatenoughpeoplebelievedthem.IdontknowofanycompanytodaystillmanufacturingSLEDs,whereasmanycommercialdatabasesystemsrequireatleastsomemeasureofstorageredundancytoensurereliablebehavior.Itisunclearhowtheperformanceargumentsgivenherewouldchangeiftheworldwastosuddenlyswitchtoadifierenttechnology,