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1e-20 1e-15 1e-10 1e-05 1 0 2 4 6 8 10 12 14 Bit Error Rate Thousands of Erasure Cycles Erasure Limit RBER UBER (ECC=4) Figure1. RBER(RawBitErrorRate)andUBER(Uncor- rectableBitErrorRate,with4bitsofECC)forMLCash ratedat10,000cycles;datatakenfrom[6]. RAIDsupportsarbitraryparityassignments,providinga ne-grainedtrade-offcurvebetweenthroughputandrelia- bility.Second,Diff-RAIDreshufesparityondrivereplace- mentstoensurethattheoldestdeviceinthearrayalways holdsthemaximumparityandagesatthehighestrate.This ensuresthattheagedifferentialcreatedthroughunevenpar- ityassignmentpersistswhennewdevicesreplaceexpired onesinthearray. Diff-RAID’sabilitytomaskhighBERsonagingSSDs confersmultipleadvantages.First,itoffershigherreliabil- itythanRAID-5orRAID-4whileretainingthelowspace overheadoftheseoptions.Second,itopensthedoortous- ingcommoditySSDspasttheirerasurelimit,protectingthe dataonexpiredSSDsbystoringitredundantlyonyounger devices.Third,itpotentiallyreducestheneedforexpensive hardwareErrorCorrectionCodes(ECC)inthedevices;as MLCdensitycontinuestoincrease,thecostofsuchECCis expectedtoriseprohibitively.Thesebenetsareachievedat thecostofsomedegradationinthroughputandcomplexity indevicereplacement. WeevaluateDiff-RAIDperformanceusingasoftware implementationrunningona5-devicearrayofIntelX25-M SSDs,onacombinationofsyntheticandrealservertraces. WealsoevaluateDiff-RAIDreliabilitybyplugginginreal- worldasherrorratesintoasimulator.WeshowthatDiff- RAIDprovidesuptofourordersofmagnitudemorerelia- bilitythanconventionalRAID-5forspecicfailuremodes. Theremainderofthispaperisorganizedasfollows:Sec- tion2describestheproblemofcorrelatedSSDfailuresin detail.Section3describesDiff-RAID.Section4evaluates Diff-RAIDreliabilityandperformance.Section5summa- rizesrelatedwork,Section6describesfuturegoalsforDiff- RAID,andSection7concludes. 2.ProblemDescription 2.1FlashPrimer ThesmallestunitofNAND-basedashstoragethatcanbe readorprogrammed(written)isa page (typically4KBin size).Allbitsinablankpagearesetto1s,andwriting datatothepageinvolvessettingsomeofthebitswithin itto0s.Individualbitswithinapagecannotberesetto 1s;rewritingthepagetoadifferentbitpatternrequires anintermediate erase operationthatresetsallbitsbackto 1.Theseerasuresareperformedoverlargeblocks(e.g., of128KB)spanningmultiplepages.Blocks wearout as theyareerased,exhibitingincreasingBERsthatbecome unmanageablyhighoncetheerasurelimitisbreached. Asaresultofthesefundamentalconstraintsonwrite operations,earlyash-baseddevicesthatperformedin-place pagemodicationsufferedfromverypoorrandomwrite latencies;writingtoarandomlyselected4KBpagerequired theentire128KBeraseblocktobeerasedandrewritten.In addition,imbalancedloadsthatupdatedsomepagesmuch morefrequentlythanotherscouldresultinunevenwear acrossthedevice. Tocircumventtheseproblems,modernSSDsimplement alog-basedblockstore,exposingalogicaladdressspace thatisdecoupledfromthephysicaladdressspaceonthe rawash.TheSSDmaintainsamappingbetweenlogical andphysicallocationsatthegranularityofaneraseblock. Awritetoarandom4KBpageinvolvesreadingthesur- roundingerasureblockandwritingittoanempty,previ- ouslyerasedblock,withnoexpensiveeraseoperationsin thecriticalpath.Inaddition,themappingoflogicaltophys- icalblocksisdrivenby wear-leveling algorithmsthataim forevenwear-outacrossthedevice.SSDstypicallyinclude morerawashthanadvertisedinordertocontinuelogging updatesevenwhentheentirelogicaladdressspacehasbeen occupied;forexample,an80GBSSDcouldincludeanextra 10GBofash. SSDscomeintwoavors,dependingonthetypeofash used:Single-LevelCell(SLC)andMulti-LevelCell(MLC). Acellisthebasicphysicalunitofash,storingvoltage levelsthatrepresentbitvalues.SLCashstoresasinglebit ineachcell,andMLCstoresmultiplebits.SLCprovides tentimestheerasurelimitasMLC(100,000cyclesversus 10,000),butiscurrently3-4timesasexpensive.Current industrytrendspointtowardsMLCtechnologywithmore bitspercell. FlashErrorModes: MTTF(MeanTimeToFailure) valuesaremuchhigherforSSDsthanharddrivesduetothe absenceofmovingparts.Asaresult,thedominantfailure modesforSSDsarerelatedtobiterrorsintheunderlying ash.Biterrorscanariseduetowrites( programdisturbs ), reads( readdisturbs )andbit-rotovertime( retentionerrors ) [6,10].Allthesefailuremodesoccurwithgreaterfrequency
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