hUBIAnOptimizedHybridMappingSchemeforNANDFlashBasedSSDs - PDF document

1 hUBI:AnOptimizedHybridMappingSchemeforNA
hUBI:AnOptimizedHybridMappingSchemeforNANDFlash-BasedSSDs GuangjunXie BaiduInc.,Beijing,China Nankai-BaiduJointLab,NankaiUniversity,Tianjin,China Email:xieguangjun1980@163.com GuangzhiXu,GangWang,XiaoguangLiu,RuiCao,YanGao Nankai-BaiduJointLab,NankaiUniversity,Tianjin,China Email:xgz.nku@mail.nankai.edu.cn,wgzwp@163.com,liuxg74@yahoo.com.cn Abstract „NAND”ash-basedSSDshavebecomeattractive alternativestoharddiskdriversduetotheirhighrandomread performancesandlowpowerconsumptions.However,thepoor incommercialapplications. Inthispaper,weproposeanovelmappingschemecalled hybridmappingunsortedblockimages(hUBI).hUBIaimsfor (1)optimizedrandomwriteperformances,(2)lowwritelaten- cies,and(3)lowspaceconsumptions.Itoptimizestraditional hybridmappingschemestoachieve(1).InhUBI,themerge operationinvolvesonlyasingleblock.So,(2)isguaranteed.To avoidintroducingahighspacecostmaintainingthemetadata, hUBIputsitsmetadataintoout-of-band(OOB)areasofSSD pages,whichobtains(3).Ourexperimentalresultsshowthat holdslowwritelatenciesatthesametime. I.I NTRODUCTION OverthelastdecadetheCPUspeedhasincreaseddra- maticallywhiletheaccesstimeoftraditionaldiskshasonly improvedslowly[1].Thebottleneckseemsmoreapparent whenparallelworkloadsintroducedbyrapiddevelopingIT applicationsincrease.Asthetraditionalharddisksusing spinningdisksandmovableread/writeheadscannotmeet applicationsrequirements,peopleseekfornewstorage mediumthatcansolvethisproblem.Attheendoflastcen- tury,solid-statedisks(SSD)wereintroducedintothestorage traditionalharddisks[2]. SSDshavethepropertiesoflowpowerconsumptionsand highaccessspeeds.Now,mostSSDsarebasedonNAND ”ashchips.Alongwiththeattractivepropertiesoflowcosts andhighdensities,theNAND”ashchiphasaproblem thatblocksshouldbeerasedbeforebeingwrittenanderase operationsarequiteslow.Inconsiderationofwearleveling andread/writelatencies,dynamicmappingsareusedinmost SSDdrivers. Mappingschemescanbedividedintothreedimensions: Mappingschemes[3]maintainmappingsbetweenlogical blocksandphysicalblocks.Eachpageupdatewillcause ablockeraseandseveralpagecopyoperations.Block- Mappingschemesimplysimpleimplementationsandlow spacecostsfortheirmetadata.However,duetotheirhigh writelatencies,block-mappingschemeshavepoorrandom writeperformances.Toimprovethewriteperformance, page-mappingschemes[4]maintain“ne-grainedmappings betweenlogicalpagesandphysicalpages.Inpage-mapping schemes,pagewritesareperformedonfreepages.Com- paredtoblock-mappingschemes,page-mappingschemes writeperformances.However,page-mappingschemescon- sumealotofspacestostoretheirmetadata.Hybridmapping schemes[5][6][7]combinetheadvantagesofblock- mappingschemesandpage-mappingschemes.Theygroup blocksintodataareaandlogarea.Pagesinthelogarea areaddressedlikepage-mappingschemes.Pagewritesare performedonfreepagesinthelogarea.Whenthelog areahasnofreepages,amergeoperation[10]istriggered toeraselogareablocksandstoresvalidpagesindata areablocks.Bythismeans,hybridmappingschemesget ef“ciencyatthesametime.However,intraditionalhybrid mappingschemes,complexmergeoperationsmaycause highwritelatencies,whichcouldbecatastrophicintime- sensitiveapplications. Inthispaper,wepresentanovelhybridmappingscheme calledhybridmappingunsortedblockimages(hUBI).hUBI contributesinseveralaspects: € hUBIusesaoptimizedhybridmappingalgorithmgiv- inghighrandomwriteperformances. € whichleadstoaguaranteedlowwritelatency. € hUBImaintainsitsmetadataintheout-of-band(OOB) areas,whichreducesthespaceconsumptionandthe numberofI/Ooperations. ExperimentalresultsshowthathUBIprovidesanconsid- erablerandomwriteperformanceandalowtimeoutrate undermixedreadandw

2 riteworkload. 2011 International Joint C
riteworkload. 2011 International Joint Conference of IEEE TrustCom-11/IEEE ICESS-11/FCST-11978-0-7695-4600-1/11 $26.00 © 2011 IEEEDOI 10.1109/TrustCom.2011.1391015 Thefollowingpartsofthispaperareorganizedasfollow: Section2introducesotherrelatedworks.Section3de- scribesthedesignofhUBIindetails.Section4showsour analysisandexperimentsofhUBI.And“nallySection5 givestheconclusion. II.R ELATED W ORK UnsortedBlockImages(UBI)[11]isamappingscheme usedintheLinuxkernel.Ithasalayeredarchitectureand mostotherschemesadoptitsdesign.UBIisablock-mapping scheme.Asdiscussedabove,eachpageupdateinUBIbrings ablockeraseandseveralpagecopyoperations.Blockerase andpagecopyarebothtimeconsumingoperations.Sopage updatesinUBIhaveahightimecost. Therehavebeenresearchesonhybridmappingschemes. Kim[5]presentedalogblockschemeinwhicheachlog block(i.e.,ablockinlogarea)isdedicatedtoonedata block(i.e.,ablockindataarea).Thismaycauselowspace utilizationoflogblocks.e.g.,thereisanupdatetoonedata block.Althoughthelogareastillhaslotsoffreerooms,if thelogblockitownsisfullyoccupied,amergeoperation willstillbenecessary.Tofullyutilizethelogareaspace, Lee[7]proposedamappingschemecalledfullyassociative sectortranslation(FAST).InFAST,apageinlogblockcan bemappedtoanydatablocks.FASTmergesfewertimes than[5],butitbringscomplexmergelogic.Amergein[5] onlyinvolvestwoblocks.Whileattheworstcase,amergein FASTcaninvolveasmanyas n +1 blocks( n isthenumber ofpagesinalogblock).Park[8]introducedacompromising solutionthatmaps K logblocksto N datablocks.Heran alotofworkloadstryingto“ndpropervaluesfor K and N .Liu[9]triedtoapplyhybridassociationsbetweenlog blocksanddatablocks.Foronehotdatablock(i.e.,theblock thataccessedfrequently),aexclusivelogblockisassigned, whileothercolddatablocksusefullyassociationsdescribed inFAST.However,themergeproblemremains. PartitionbasedUBI(pUBI)isanimprovedhybridmap- pingscheme.InpUBI,blocksaregroupedintopartitions. Eachpartitionisacontiguousregionoftheaddressspace. Blocksinapartitionaredividedintodatablocks,logblocks andm-blocks.Updatestoapartitionwillbeperformedon itscorrespondinglogblocks.Inonepartition,thereisonly oneactivem-blockstoringitsmetadata.Amergeoperation iscalledwhentherearenofreepagesinthelogarea. ComparedwithUBI,pUBIprovidesabetterrandomwrite performance.However,themergeoperationinpUBImay involveseveraldataandlogblocks,thereforeitcanbringa numberofblockeraseandpagecopyoperations.Insome circumstance,suchatimeconsumingmergeoperationmay causeadataupdatetimeout. hUBIadoptsthelayeredarchitectureofUBIbutdesigns anewhybridmappingmechanism.Tosolvethetimeout probleminpUBI,hUBIfocusesonoptimizingmergeop- erations.ExperimentalresultsshowthathUBIsolvesthis         !"!   # $%# &%# Figure1.OverviewArchitecture  ' (  )) ) ) *+ ,,*- Figure2.HuaweiSSDPageLayout problemeffectivelyandprovidesacomparablerandomwrite performancewithpUBI. III.D ESIGNOFH UBI ThemajorobjectiveofhUBIisloweringthewritelaten- ciesandspaceconsumptionsofhybridmappingschemes. OurworkisbasedontheNA

3 ND”ash-basedSSDsproduced byHuaweiInc.and
ND”ash-basedSSDsproduced byHuaweiInc.andcanbeappliedtootherSSDs.Overview architectureofhUBIisshowninFigure1. Therawmemorytechnologydevice(MTD)driverpro- videsaphysicalerasableblock(PEB)viewwithoutwear leveling.ThehUBIdriverimplementsahybridmapping algorithmandprovidesalogicalerasableblock(LEB)view. Finally,touseSSDsasnormalblockdevices,theMTD blockdriveremulatesasectorviewforupperapplications. A.MetadataLayout 1)DiskLayout: hUBIdividespagesineachblockinto logareaanddataarea.Thepage-mappinginfoforlogarea pagesisstoredondisktorecordtherelationshipsbetween logicalpagesandphysicalpages.Sincethepage-mapping infochangesalongwithpageupdates,bystoringthepage- mappingalongwiththepagedata,onlyoneI/Ooperation isneededwhenwritingboththepagedataandthepage- mappinginfo.Figure2showsthepagelayoutofHuawei MLCSSDs.Theout-of-band(OOB)areaisusedforerror controlandthe“rsttwobytesofOOBareusedasabadblock ”ag.Only2bitsofthe”agareactuallyused,thereforewe canusethesecondbytetoholdthepage-mappinginfo,as showninFigure3. Twomainadvantagesaregainedbytheabovedesign:(1) ThenumberofI/Ooperationsisreduced,asthepagedata andthepage-mappingcanbewrittentodisksinoneI/O operation.(2)ByusingtheOOBareastostorethemetadata, hUBIdoesnotconsumeextrastoragespaces. 1016  ' (  )) ) ) *+ ,,*- ( . $&!. Figure3.hUBIPageLayout 2)MemoryLayout: Therearetwoalternativestomain- tainthein-memorypage-mappingtable(PMT).Oneofthem storesthephysicalpagenumber(PPN)foreachpageinthe logarea.Foraread/writeaccess,iftherequiredpageis inthelogarea,thePPNcanbegottenbysearchingthe PMT.Otherwise,iftherequiredpageisinthedataarea,as dataareapagesarestoredintheorderoflogicaladdresses, thePPNisequaltothelogicalpagenumber(LPN).By usingthissolution,fewermemoryspacesareconsumedto maintainthemetadata.However,areadoperationmaycause asmanyas n comparisonstogetthecorrespondingPPN ( n isthetotalnumberoflogpagesinoneblock).The othermethodmaintainsthepage-mappinginfoforbothlog anddataareapages,whichissimilartothepage-mapping scheme.Bythisway,readoperationsavoidPMTsearching. But,morespaceswillbeoccupiedandthingsgetevenworse whenthesizeofstoragegrowslarger. Tobalancebetweenmemoryconsumptionsandperfor- mances,wecomeupwithacompromisingmethod.hUBI optimizesthe“rstdesignbyaddingabitmap.Onebitinthe bitmapcorrespondstoauniquelogicalpageandindicates whetheritisinthelogareaornot.Whenaccessingapage, thecorrespondingbitinthebitmapis“rsttested.Sincebit testisafastoperation,wepaylittleperformancepenaltyto savealotofmemoryspace. AnoverviewofmetadatalayoutforhUBIisshownin Figure4.hUBIextendstheeraseblockmappingtable(EMT) usedbyUBItosupporthybridmapping.EachentryofEMT storesthemetadataforalogicalblock.Itcontainsfourparts: theblock-mappinginfo,apointer,anindexandabitmap. ThepointerpointstothePMT.Theindexrecordsthenext freelogareapageinPMT.Thebitmapindicateswhethera pageisinthelogarea.e.g.,inFigure4,LEB1ismappedto PEB10.InPEB10,anupdateoperationisperformedafter amergeoperation.TheLPNofthewrittenpageis2. B.ReadandWriteAlgorithms Inthissection,wedescribethereadandwritealgorithms ofhUBIindetails. Thekeylogicofthereadalgorithmistheaddresstrans- lation.InhUBI,alogicaladdresscanbedividedintothree parts:aLEB(logicalerasableblock)number,aLPN(logical pagenumber)intheLEBandanoffsetinthepage.Usually, readrequestsarealignedbypages,thuswecanas

4 sumethe offsetinthepagetobezeroanduseatu
sumethe offsetinthepagetobezeroanduseatuple&#xLEB,;&#x-369;LPN $&  !/0 1(2!  $ &   3 4  + .+ . . 555 555 555 555 &1  & 6 331 0            &%  4 + .   + . $% 1(      &     $&  !/0 $&! + .  7  4 47 4  " $ "  - 8 * *7  9   Figure4.hUBIMetadataLayout torepresentalogicaladdress.Inthefollowingparts,we usethisformoflogicaladdresstodemonstratethereadand writealgorithms. Algorithm1showsthereadalgorithmofhUBI.Toread apage,thealgorithm“rstlocatesthepage-mappingtable foragivenLEB.Thenthecorrespondingbitinthebitmap istestedtocheckwhethertherequiredpageisinthelog area.Forareadrequesttologareapages,thealgorithm willsearchthepage-mappingtablesequentiallyto“ndthe correspondingPPN.Otherwise,therequiredpageisinthe dataarea,thePPNisequaltotheLPN. Algorithm1: hUBIReadAlgorithm Input : leb , lpn begin emt _ entry =get_entry_from_emt( leb ); peb = emt _ entry.peb ; pmt = emt _ entry.pmt ; bitmap = emt _ entry.bitmap ; if is_in_log_area( lpn , bitmap )== true then ppn =get_ppn_from_pmt( lpn , pmt ); else ppn = lpn ; read_page( peb , ppn ); end Algorithm2showsthewritealgorithmofhUBI.The algorithm“rstwritesthedataandthepage-mappinginfoto afreelogareapage.ThenanewentryisaddedtothePMT andthecorrespondingbitinthebitmapisalsoset.When therearenofreepagesinthelogarea,amergeoperationis calledtocleanthelogarea. Algoritm3describesthemergelogic.Whenthereareno freelogpagesinthelogareaofcurrentPEB P (LEB L ),a newPEB P  isallocatedfromthefreePEBlist.Thenvalid 1017 Algorithm2: hUBIWriteAlgorithm Input : leb , lpn , data begin emt _ entry =get_entry_from_emt( leb ); peb = emt _ entry.peb ; pmt = emt _ entry.pmt ; bitmap = emt _ entry.bitmap ; ppn =get_next_free_log_page( pmt ); set_oob_mapping( oob , lpn ); write_physical_page_with_oob( peb , ppn , oob , data ); set_bitmap( lpn , bitmap ); set_pmt( lpn , pmt ); if nofreelogpages then merge( leb ); end end pagesarecopiedfrom P to P  .Afterthat, P willbeput ontheeraselistwaitingtobeerasedbyagarbagecollector. Finally,theEMTwillbeupdatedtomap L to P  .e.g.,in Figure4,accordingtoalgorithm2,theseconddataupdate toPEB3willtriggeramergeoperation,thenanewPEB K willbeallocatedandvalidpageswillbecopiedtoPEB K .Thenentry0inEMTwillbeupdatedtomapLEB0to PEB K . Algorithm3: hUBIMergeAlgorithm Input : leb begin emt _ entry =get_emt_from_emt( leb ); peb = emt _ entry.peb ; peb  =alloc_free_peb(); bitmap = emt _ entry.bitmap ; pmt = emt _ entry.pmt ; foreach ppninpmt do copy_page( peb  , peb , ppn ); end foreach ppninDataarea do if is_in_log_area( ppn , bitmap )== false then copy_page( peb  , peb , ppn ); end end emt _ entry.peb = peb  ; add_to_erase_queue( peb ); end IV.A NALYSISAND E XPERIMENTS A.Analysis Byusingbitmap,readalgorithmsofhUBIbringtinyextra timecost.Sotheanalysisbelowisfocusedonthewrite algorithm. Twoperform

5 ancemetricsareconcernedinevaluatingthe w
ancemetricsareconcernedinevaluatingthe writeperformanceofSSDs. MeanTimeToPageWrite(MTTPW) showstheaverage timecosttowritebackapage.Fromthismetric,theaverage latencyofwriteoperationscanbederived. MeanTimeToMerge(MTTM) showsthetimecostof onemergeoperation.Sincewriterequestsareblockedwhen merging,theworstcaseofwritelatencycanbederivedfrom thismetric. Assumethetimecostofreadingapageas t r ,thetime costtowriteapageas t w ,thetimecosttoeraseablockas t e andthetimecosttodeliverapagefromupperapplication toSSDstorageas t x .Forasingleblock, d standsforthe numberofpagesinthedataareaand l standsforthenumber ofpagesinthelogarea.SinceinhUBI,every l continuous writerequestswilltriggeramergeoperation,andamerge operationneedsablockcopyandanblockeraseoperations, wehave: MTTM h = d ( t r + t w +2 t x )+ t e (1) MTTPW h = MTTM h l + t w + t x (2) WecomparehUBIwithtwoUBI-basedmappingschemes, UBIandpUBI. UBIisatypicalblock-mappingscheme.Apageupdate inUBIwillcauseamergeoperation,whichthenneeds ablockread,ablockwriteandablockeraseoperations. Assumethereare k pagesinablock,thenthe MTTPW and MTTM ofUBIare: MTTM u = k ( t r + t w +2 t x )+ t e (3) MTTPW u = MTTM u + t w + t x (4) pUBIisahybridmappingversionofUBI.InpUBI,blocks aregroupedintopartitions.Blocksinapartitionaredivided intodatablocks,logblocksandm-blocks.Writerequests ofapartitiongotoitslogblocksanddatablocksstorethe mergeddata.Eachpartitionstoresitsmappinginfoinam- block.ToanalyzetheperformanceofpUsBI,weintroduce twometrics. Localityratio  :Supposethatthereare D datablocks, L logblocksinapartitionand k pagesinablock.AspUBI usesFAST[7]onitslogblocks,every L × k writerequests causeamergeoperation.Wede“ne  astheaverageratioof differentLEBscoveredby L × k continuouswriterequests toalldatablocksinapartition. Sequentialratio  :Whenmerging,ifalogblockcovers exactlyoneLEB,pUBIwillturnthislogblockintoadata block.Wede“ne  astheaverageratiooflogblocksthatcan beturnedintodatablockstoalldatablocksinapartition. 1018 Weknowthatevery L × k writerequestscauseamerge operation.Fortheserequests,thereare D ×  differentLEBs covered,and D ×  LEBscanbeturnedintodatablocks directly.Therefore,inamergeoperation, D × k × (  Š  ) pagesarecopiedand D × (  Š  )+ L blocksareerased. Inaddition,aseverypartitioninpUBIstoresitsmapping infoinam-blockpageandmappingsarechangedduring merging,eachmergeoperationwillcauseapagewriteto am-block,whichthenimpliesa 1 /k eraseoperation.From theaboveanalysis,wecangetthe MTTW and MTTPM ofpUBI: MTTM p = D ( kt r + kt w +2 kt x + t e )(  Š  )+ Lt e + t w + t e k (5) MTTPW p = MTTM p kL + t w + t x (6) Tooptimizethesequentialwriteperformanceofmapping schemes,MTDblockdriverimplementsawrite-backcache. ThesizeofthecacheisequaltothesizeofaLEB.For awriterequest,iftherequiredLEBiscontainedinthe buffer,thewriteoperationisperformeddirectlyonthe pagecache.Otherwise,thecachedLEBisdiscarded,dirty pagesare”ushedtothediskandtherequiredLEBis fetched.Tofullyutilizethiscache,hUBIoptimizesitswrite algorithmintwoways:(1)Whenthenumberof”ushed pagesisbelowthesizeofalogarea,eachwriterequestis performedusingthewritelogicdescribedinAlgorithm2. (2)Whenthenumberof”ushedpagesisnotlessthan thesizeofalogarea,therequiredblockwillbemerged andvalidpageswillbedirectlycopiedtothedataareaof anewlyallocatedblock.Fromtheaboveanalysis,under sequentialwriteworkloads,the MTTPMs ofthethree mappingschemescanbechangedinto: MTTPW h = d ( t w +2 t x + t r )+ t e d (7) MTTPW p = Lt e + t w + t e k kL + t w + t x (8) MTTPW u = k ( t w +2 t x + t r )+ t e k (9) Theperformancemetricsfors

6 sdsusedinourexperiment areshowninTableI
sdsusedinourexperiment areshowninTableI TableI H UAWEI MLCSSDP ERFORMANCE M ETRICS Operation Time(us) 256KBBlockErase t e 1500 2KBPageRead t r 50 2KBPageWrite t w 800 2KBDataTransfer t x 50 FromTableIandeqs.(1)-(9).wecomputethetheoretical writeperformancesforUBI,pUBIandhUBI(TableII). Assumeeachblockcontains128pages( k =128).ForpUBI, eachpartitionhas16datablocks( D =16)and4logblocks ( L =4).ForhUBI,eachblockcontains100pagesfordata area( d =100)and26pagesforlogarea( l =26),twopages areremainedfortheblockmetadata. TableII T HEORETICAL W RITE P ERFORMANCE UBI pUBI hUBI SW(SequentialWrite)RW(RandomWrite) SWMTTPW 0.961ms 0.863ms(  =  ) 0.965ms RWMTTPW 123.1ms 4.71ms(  =1 , =0 ) 4.561ms MTTM 123.1ms 6.65ms-1976.32ms 96.5ms AswecanseefromTableII,undersequentialworkloads, allthreeschemeshavesimilarperformances.AndashUBI mergesmoretimesthanpUBI,pUBIgetsahighersequen- tialwriteperformance.Whileunderrandomworkload,the complexmergeoperationofpUBIeffectsitsperformance andhUBIachievesalittlebetter.Moreover,intheworst case,pUBIwilltakenearly2stoperformamergeoperation, whichisunacceptableforsometimesensitiveapplications. Inaword,hUBIprovidesaconsiderablewriteperformance andholdsasteadywritelatency. B.Experimentalresults Aoverviewofourexperimentenvironmentisgivenin TableIII. TableIII E XPERIMENT E NVIRONMENT OS 64-bitRedhatLinuxAS5withkernel2.6.18 Host CPU IntelXenon5250 × 2 Memory DDR2FBD16GB HDD SAS146GB × 8RAID5 SSD Manufacturer HuaweiInc. ChipType 2GBNANDMLC BlockSize 256K pageSize 2K Tools Iometer Ver2006_07_27(x86_64) MySQL Ver14.7forRedhat-Linux-GNU(i686) First,weuseiometer[24]toevaluatethereadperfor- manceofhUBI(seeFigure5andFigure6).Wecanseethat hUBIperformsmuchbetterthantraditionalHDDs.Andas readoperationsinthethreeschemesdonottriggertime 1019 + 8+ .++ .8+ ++ 8+ 4++ 48+ -++ -8+ -:.*4*-.:8*8..+- ;  3  / 3/   �=/ ' Figure5.SequentialReadPerformance + .++++ ++++ 4++++ -++++ 8++++ *++++ -:.*4*-.:8*8..+- ;  3  ,& �=/ ' Figure6.RandomReadPerformance consumingwrite/eraseoperations.Theirreadperformances donotshowmuchdifference. Then,wesimulatedthreecommonscenariostoevaluate thewriteperformanceofhUBI:(1)asinglethreadedwget program[25]downloads“lesfromalibraryofwebindexes, (2)amodi“edmulti-threadedwgetprogramdownloads“les fromalibraryofwebindexesand(3)aprogramperforms randomupdatestoaMySQLdatabase.(1)isasequential writescenario,while(2)and(3)providerandomwrite workload.For(1)and(2),thetotalsizeofthelibraryis 80G,thereare76“leswith1Gsizeandalotofsmall“les. For(3),thereare5millionrecordsinthedatabase.The averagelengthofrecordsis497bytes.Resultsareshownin Figure7. FromFigure7,wecanseethatundersequentialworkload- s,thethreemappingschemeshavesimilarperformances. ThisisbecauseawritecacheisusedintheMTDblock driverandtheresultsarewithinourexpectation.While inrandomwritescenarios,hUBIandpUBIperformmuch better,becausehUBIandpUBImergemuchlesstimesthan UBI. Then,weuseamixedreadandwriteworkloadtoevaluate + + -+ *+ :+ .++ .+ .-+  6.  6-/3(   3  ?    Figure7.WriteBenchmark + 8++ .++

7 + .8++ +++ 8++ ; 
+ .8++ +++ 8++ ;  3    @/3(  "  31 /  Figure8.MixedReadandWriteBenchmark theaveragewritelatencyofhUBI.Weperformconcurrent selectandupdatequeriestotheMYSQLdatabaseusedin theabovebenchmark.Figure8showstheaverageresponse timeofselectqueries.Intheexperiment,thenumberof selectqueriestentimesthenumberofupdatequires.Aswe cansee,theperformanceofUBIfallssharplywhenupdate quiresinvolvewhileUBIandpUBIremainasteadyresponse time. ToevaluatetheworstcaseofwritelatenciesforhUBI,we observethedistributionsoftheresponsetimeintheabove experiment.Figure9andTableIVshowthedetailedstatistics fortimeoutrequests.Aswecansee,foronehundredthou- sandselectqueries,pUBIhasaboutfourhundredqueries thattakeover20ms.So,thetimeoutrateofpUBIisabout 0.4%.However,inhUBI,thetimeoutrateisonly0.001%. ThisisbecausethemergeoperationsinhUBIinvolvesonly oneblock.WhileinpUBI,amergeoperationcaninvolve severalblockeraseandanumberofpagecopyoperations. V.C ONCLUSION Nowadays,NAND-basedSSDsarewidelyusedbothin thecommercialareaandthepersonalcomputing.However, thepoorrandomwriteperformanceofSSDshasprovento beaseriousproblem.Onewaytosolvesuchaproblemis 1020 + .++++ ++++ 4++++ -++++ 8++++ *++++ 9++++ :++++ A++++ .+++++ +B. C.B8 C8B.+ C.+B+ C+B8+ C8+B.++ D.++ 6  6  @/3(  36  36  @/3(  6  6  @/3(  =/  31 1   Figure9.ResponseTimeDistribution TableIV S TATISTICSOF T IMEOUT R EQUESTS [20,50) [50,100) �100 UBI-select 3 0 0 UBI-select+update 1089 921 389 pUBI-select 0 0 0 pUBI-select+update 123 15 250 hUBI-select 0 0 0 hUbI-select+update 1 0 0 optimizingthemappingalgorithmsofSSDs.Manyworks haveexploredthisarea.Inthispaper,weintroducea novelhybrid-mappingschemeforNAND-basedSSDscalled hUBI.hUBIcontributesinthreeaspects: € hUBIusesaoptimizedhybridmappingalgorithm achievinghighrandomwriteperformance. € ThemergeoperationinhUBIisperformedonasingle block,whichprovidesaguaranteedlowwritelatency. € hUBIstoresthemappinginfoinOOBareasofSSD pageswhichholdsspace-ef“ciency. Inaword,forNAND-basedSSDs,hUBIimprovestheir randomwriteperformancesandprovidesguaranteedlow writelatencies.However,asahybridmappingscheme,the logareaofhUBIcausesawasteofstoragespaces.Our futureworkwillfocusonreducingthespacecostofmapping schemesandprovidingabetterwriteperformanceatthe sametime. A CKNOWLEDGMENTS ThisworkwassupportedinpartbytheNationalHigh TechnologyResearchandDevelopmentProgramofChina (2008AA01Z401),NSFCofChina(60903028,61070014, 61170301),ScienceandTechnologyDevelopmentPlanof Tianjin(08JCYBJC13000),andKeyProjectsintheTianjin ScienceandTechnologyPillarProgram. R EFERENCES [1]MendelRosenblum,JohnK.Ousterhout. TheDesignandIm- plementationofaLog-StructuredFileSystem InProceedings ofthe13thACMSymposiumonOperatingSystemsPrinciples, 1991. [2]F.Douglis,R.Caceres,F.Kaashoek,K.Li,B.Marsh,JA. A. Tauber:StorageAlternativesforMobileComputers In Proceedingsofthe1stSymposiumonOperationSystems DesignandImplementation,1994. [3]A.Ban. FlashFileSystem UnitedStatesPatent,No. 5,404,485,April1995. [4]A.Gupta,Y.Kim,andB.Urgaonkar. DFTL:AFlashTrans- lationLayerEmployingDema

8 nd-basedSelectiveCachingof Page-levelAdd
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