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