Design Tradeoffs in a Flash Translation Layer(2010,drona.csa.iisc.ernet.in(~gopi)
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DesignTradeoffsinaFlashTranslationLayer
GarthGoodson
NetApp
495EastJavaDrive
Sunnyvale,CA,USAgarth.goodson@netapp.comRahulIyer
NetApp
495EastJavaDrive
Sunnyvale,CA,USArahul.iyer@netapp.com
ABSTRACT
Althoughflashhasbetterrandomaccesscharacteristicsthan
disk,ithasanumberofundesirablecomplexities,including
theinabilitytooverwriteablockinplace.Alayer,calledthe
flashtranslationlayerisusedtovitalizeflashandpresentsit
asablockdevice.Althoughmanysuchdesignshavebeen
proposedinthepast,theyoftendonotfocusontherequire-
mentsoflarge-scaleenterprisestorage,nordotheyexamine
thetrade-offsbetweendifferentapproaches.Inthispaper,we
usetrace-basedsimulationtoanalyzethetrade-offsandde-
signsoftwopopularclassesofflashtranslationlayers.
1.INTRODUCTION
Drivenbytheperformancerequirementsoflowlatencyand
deliveringordersofmagnitudemorerandomI/Ospersecond
(IOPs),flashisbeginningtomakeseriousinroadswithincom-
mercialstoragesystems.Theever-droppingpricepergigabyte
combinedwiththeincreasingdensitiesandlowcostperIOPs
havepropelledflashfromhand-helddevicestotheenterprise
market.Althoughflashoffersahugeperformanceimprove-
ment,makingitaseasyandefficienttouseastoday’sdisk
drivespresentsachallenge.
Althoughitispossibleforstoragesystemstodirectlyaccess
rawflash,thestoragesystemmustbeawareofthecomplexi-
tiesinvolvedindoingso,inparticular,theinabilitytoupdate
datainplace.Oneapproachtointegratingflashisthrough
theuseofflash-basedfilesystems[16,15,22].Althoughthis
approachultimatelyprovidesthetightestintegrationandbest
performance,itcanrequireanextensivereworkofthestorage
system’sexistinginfrastructureandfilesystem.Theother,
morepopular,techniqueistopackageflashasasolidstate
diskdrive(SSD)thatexportsastandardblockinterface(for
example,SATA,SAS,FC).Asflashmemorydoesnotsup-
portoverwritinginplace,flashdevicesrequireatranslation
layertomaplogicalblockstotheirlocationswithinphysical
flashmemory.Thislayeriscalledtheflashtranslationlayer
Permissiontomakedigitalorhardcopiesofallorpartofthisworkforpersonalorclassroomuseisgrantedwithoutfeeprovidedthatcopiesarenotmadeordistributedforprofitorcommercialadvantageandthatcopiesbearthisnoticeandthefullcitationonthefirstpage.Tocopyotherwise,torepublish,topostonserversortoredistributetolists,requirespriorspecificpermissionand/orafee.HPCAWEST2010Bangalore,IndiaCopyright2010ACMX-XXXXX-XX-X/XX/XX...$10.00.(FTL)[7,14].UsinganFTLallowsfilesystemsandSSDs
tomaintaintheblockinterfaceofdiskswithoutsacrificingthe
tighterintegrationandcontroloverhowtheflashismanaged.
Thispaperexploresthetrade-offsindesigningaflashtransla-
tionlayerandtheimpactofproposedchangestotheexisting
blockinterfaces.
Inthispaperwedescribethedesignandimplementationof
twoFTLsandanalyzetheiroverheads.ThetwoFTLsarecho-
senbasedonthetwomostcommonclassesoftranslationlay-
ersdiscussedintheliterature.ThefirstFTLusesalog-based
approachwherenewupdatesareappendedtotheendofalog;
thesecondisbasedonmappingconsecutiverangesoflogical
blockstoconsecutivesetsofflashblocks.Weimplementboth
FTLsinatrace-basedsimulatorandevaluatetheoverheadsof
overwriting,andtheeffectivenessofcleaningandwearlevel-
ingusingsynthetictracesandrealblocktraces.Wealsoan-
alyzetheimpactofproposedchangestotheATAandSCSI
protocols[11,18]thatallowtheflashdevicetobenotifiedof
file-levelblockde-allocations.
Thepaperisorganizedasfollows:Section2givesanoverview
offlashtranslationlayersanddescribesthetwomainclasses
ofFTLs.Section3describesthedesignimplementationofthe
twoFTLsweimplementandevaluate.Section4evaluatesthe
twoFTLs.Section5comparesrelatedwork.Finally,Section6
concludes.
2.FLASHTRANSLATIONLAYERS
Thissectiondescribesthepurposeofflashtranslationlayers
andprovidesanoverviewofthedifferentgenericclassesof
FTLs.
2.1Overview
Aflashtranslationlayerhidesthecomplexityofflashby
providingalogicalblockinterfacetotheflashdevice.Since
flashdoesnotsupportoverwritingflashpagesinplace,anFTL
mapslogicalblockstophysicalflashpagesanderaseblocks.
2.1.1Mappingpagestologicalblocks
Therearemanywaystoperformthemappingfromflash
pagestologicalblocks.Itcanbealgorithmicoritcanstore
mappinginformationpersistentlyonflash.Becausealgorith-
micmappingsarestaticinnature,theyareseldomsolelyused
forhigh-performanceflashdevices.Persistentmappinginfor-
mationcanbespreadacrossthemetadataregionsofeachpage.
However,metadatastoredinthiswayrequiresascanoftheen-
tiredevicetoreconstructthefullmappingtable,whichistoo
expensiveforlargedevicesandusedonlybymuchsmaller