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Chapter 11. Network Attached Storage (NAS) and Storage Area Networks (SANs)

Chapter 11. Network Attached Storage (NAS) and Storage Area Networks (SANs)

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Chapter11.NetworkAttachedStorage

(NAS)andStorageAreaNetworks

(SANs)

SOMEOFTHEMAINTOPICSINTHISCHAPTERARE

LocalVersusNetworkedStorageDevices165

NetworkAttachedStorage167

StorageAreaNetworks169

BasicSANs:ArbitratedLoops172

UsingaFabricSwitchedTopologyforSANs176

AMixedTopologyofLoopsandSwitches178

IPSANsandiSCSI180

WhatKindofNASorSANSolutionShouldYouUse?

182

Allthecomponentsthatmakeupacomputer,fromtheCPUto

memory,thebus,andothers,arerapidlyexpandinginspeed

andcapacityandprobablywillcontinuetodosoformanyyears

tocome.Oneareaofcomputertechnologythatwhilecontinuing

toevolveisdoingsoatasomewhatslowerpaceisstorage

devicesattacheddirectly,suchasdiskandtapedrives.For

desktopworkstationsthatuselimitedstorage,thisisnotreally

aproblem.Forserversthatofferfileservicestoother

computers,thisisaseriousissuewithwhichadministrators



mustcontend.Astheneedforstoragecontinuestogrow,itis

pushingthelimitsofthemosttypicalserverstorageinterface

today:theSCSI(SmallComputerSystemsInterface)interface.

AndbecausemostlargeserversuseRAIDarrays,evenmore

disksareneededtostoredataduetothemechanicsofdisk

mirroring,striping,andsoon.

TheSCSIparallelarchitecture,usingmultiplewiresandintricate

clocking,canworkonlyovershortdistances.Thedifferent

levelsofSCSIthatexisttodayhavedifferentdistance

limitationsaswellaslimitationsastothenumberofdevices

thatcanbeattachedtoaSCSIcable.Becauseofthesephysical

limitations,itisapparentthateventuallyyouwillrunoutofPCI

slotstoholdSCSIcards,andthatallthestorageyoucanfit

withinthelimitsofthecurrentSCSIcapabilitieswillbe

exhausted,asshowninTable11.1.

Table11.1.SCSITypesandCapabilities

Type



Bus

Width



Speed



MaxNumberof

BusLength

Devices



SCSI-1



8bits



5MBps



8



625meters



FastSCSI



8bits



10MBps



8



325meters



UltraSCSI



8bits



20MBps



8



1.525meters



Ultra2SCSI



8bits



40MBps



8



1225meters



FastWideSCSI



16bits



20MBps



16



325meters



WideUltraSCSI 16bits



40MBps



16



1.525meters



WideUltra2SCSI 16bits



80MBps



16



1225meters



Ultra3SCSI



16bits



160MBps 16



12meters



Ultra4SCSI



16bits



320MBps 16



12meters



Onepracticalsolutionwouldbetogetanotherserveranddivide

thechores.Butforsomeverylargeservers(orclusters)that

provideInternetservices,orotherapplicationsthatrequirea

largeamountofstorage,thephysicalSCSIarchitectureisnow

seeingitslastdays.Youcanonlyconnectsomuchstoragetoa

server,giventhedistancelimitationsofSCSItechnologiesand

higherdiskcapacities.Eventually,diskstoragespacethatyou

canconnecttoaserverwillbecomefinite,andabottleneck.

Thus,parallelSCSIdoeshavelimitations.ThenewSerial

AttachedSCSI(SAS)standard,whichcombinescompatibility

withSerialATA(SATA)harddiskswiththemanagement

featuresandperformanceofSCSI,isexpectedtoeventually

replaceparallelSCSIimplementationsattheserverlevel.

FormoreinformationaboutSAS,seeChapter7ofthebook

UpgradingandRepairingServers.



Newertechnologiesareneededinjustafewyearstosatisfythe

needforgrowing,reliablestorage.Oneistheemerging

InfiniBandtechnology,andtheotheristheproliferationof

StorageAreaNetworks(SANs).

Asenterprisedatabeginstobemeasuredinterabytesnow

insteadofgigabytes,theresimplymustbeabetterwayto

makedataavailabletooneormorelargenetworkservers.

Currently,thatmethodespeciallyforlargenetworksinwhich

downtimeisveryrarelytoleratedistheStorageAreaNetwork

(SAN).Anothermethodofexpandingdatastoragebyusing

networkingtechnologyoftenusedinsmalltomid-sizednetwork

environmentsistheNetworkAttachedStorage(NAS)device



(sometimescalledanetworkappliance),whichyouwillalso

learnaboutinthischapter.

NASdevicesareattachedtoaLANalongwithclientandserver

computers.SANsareusuallyconnectedtothelargerservers

usingaseparatenetwork.Thereareexceptionstothisrule,of

course.SomeenterprisenetworksstilluseNASbecauseitwas

"therefirst."Thereisnoneedtoreplacewhatworkswitha

moreexpensivetechnologyjustbecauseitexists.Inthis

chapteryouwillfindcompellingreasonsforusingNAS,SANs,

orpossiblybothinyournetwork,dependingonyour

requirements.Bothofthesetechnologieshavespecificfeatures

thatmakethemusefulindifferentenvironments.Youwillfind

thatsomenetworkscanmakeuseofbothtechnologies,again,

dependingonthedatarequirementsforthenetwork.

First,let'sexaminethemostwidelyusedtechnologyfor

attachingstoragedevicestoaserver:theSmallComputer

SystemsInterface(SCSI).We'llalsocoverthelimitationsofthis

legacytechnologyinlargeenterprisenetworks.







LocalVersusNetworkedStorageDevices

BothSCSIcardsandnetworkadaptercardsconnecttothePCI

businyourcomputer,butthesimilarityendsthere.The

networkcardusesaserialcommunicationsmethod(usingtwo

wiresinmostcasesforsendandreceiveformodernEthernet

networks).

YoucanlearnmoreabouttraditionalEthernetnetworkadaptersby

readingChapter7,"NetworkInterfaceCards."



SCSIusesaparallelmethod,involvingmanywires(thenumber

variesaccordingtotheSCSIspecification).Serial

communicationstechniquescanbeusedtocovergreater

distancesthanparallelmethods.ThatiswhysimpleEthernet

networkscanspanmuchlargerdistancesthancanthelocally

attachedSCSIdevicesforaserver.

TodaytherearemanyversionsofSCSImorethanthenumberof

differentcablesusedtomakeconnectionsbetweenSCSI

devices(orcontrollersofthesedevices)andthePCIcardsthat

providethelinkbetweentheserverandstorage.Formore

informationonthedifferenttypesofSCSIanditseventual

replacement,SAS,checkoutUpgradingandRepairingServers.



Note

TheSCSIparallelinterfacewasfirstusedbyApple

Computers,andyouwillstillfinditinuseforsome

Appleproducts.JustlikeFireWire(whichwasalso

developedbyApple),theseadvancedtechnologies

havebeenadoptedbytheIntelcommunity,and



havealsobeenadoptedasspecificationsbythe

IEEE.NowIntel-basedcomputersrunningWindows

orLinuxcaneasilysupportbothoftheseprotocols.

Keepinmind,however,thatthesetechnologiesdo

notprovidethedistancethatFibreChannelorIP

SANscanprovide,usingtheirrespective

technologies.



Usingfiber-opticcabling,serialcommunicationscancoververy

largedistances,andallowfastaccesstoanetworkcomposedof

manydevices.

JustasaLANcanbeusedtoallowaservertoprovidestorage

filesharestoclientworkstations,therearetwoothermajor

typesofnetworkedstoragethatcanovercomethedistance

limitationsimposedbySCSIdevices:NetworkAttachedStorage

andStorageAreaNetworks.



DefiningNetworkAttachedStorage(NAS)

Onewaytoattachadditionalstorageistoplacethestorage

itselfonthenetworkfortheserverorclientstoaccess.This

techniqueiscalledNetworkAttachedStorage(NAS),becauseit

sharesthesamenetworkastheclients.Whenthestorage

device(suchasadiskarrayortapedrive,oramixtureofboth

inthesamebox)isaccessed,thedatatransferoccursonthe

sameLANasyourclientsandservers.Thus,ifyournetworkis

alreadyexperiencingheavyutilization,usingNASmayfurther

causedegradationinnetworkperformance.Inasmallnetwork,

suchasinaSOHOsettingorasmalldepartmentthatneeds

localaccesstoasmallnumberoffiles,aNASdevicecanbea

perfectfit,especiallyifthemajorityofcommunicationsare



betweentheNASdeviceandserversorclientsofthelocalLAN

segment.Yetifalargeamountofnetworktrafficissent

betweenserversorclientsontheLAN,theNASdevicecan

severelyimpactthebandwidthontheLAN.Allofthisis,of

course,becausetheservers,clients,andNASdevicessharethe

samenetwork.Thisisoneofthemajordifferencesbetween

NetworkAttachedStorageandStorageAreaNetworks.The

latteroptionusesaseparatenetworkanddoesnotcauseany

bandwidthdegradationontheLAN.

Formuchlargernetworks,whichusehundredsofserversand

manythousandsofclients,thechoicebetweenNASandSANsis

alittledifferent.BecauseSANsresideonanetworkseparate

fromtheproductionnetwork,datatransfersbetweentheserver

andstoragecanbegreatlyincreased.



DefiningaStorageAreaNetwork(SAN)

StorageAreaNetworks(SANs)aresimilartoNASintopology,

withtheexceptionthataSANismadeupofdevicesona

networkthatisnotthesamenetworkusedfortheLAN.Figure

11.1showsthebasicdifferencebetweenNASandSANs.



Figure11.1.NASandSANsusedifferentnetwork

topologies.



[Viewfullsizeimage]



TheNAS-basednetwork,shownontheleftsideofFigure11.1,

hasnetworkclientsandserversaswellasNASdevicesattached

totheLAN.ThefileserverusesthesameLANtoserviceclient

requestsasitusestogetdatafromNASdevices.Inmany

cases,itispossibletoconfigureNASdevicestoworkasifthey

areservers,andthemostpopulardiskformatsaresupported

onthemainplayersinthismarket.Inaswitchedenvironment

asshowninFigure11.1,trafficwillbeminimizedsomewhatdue

totheswitchesinvolved.However,thesinglenetworkadapter

cardonthefileserverisabottleneck.Youcouldputinanother

NICanduseittoattachtotheNASdevices.Thiswouldbe



similartotheStorageAreaNetwork(SAN)shownontheright

sideofFigure11.1.

IntheSAN,atypicalNICattachesthefileservertotheLAN.A

separatecardcalledahostbusadapterisusedtoconnectthe

fileservertoaFibreChannelbasedSAN.Communicationswith

devicesontheSANdonotcauseanytrafficonthenetwork's

LAN.Theseexamplesaresimplifiedexplanationsofmost

installedNASandSANnetworks,buthowevercomplexthe

design,thebasicsremainthesame.







NetworkAttachedStorage

AsanalternativetoSANs,especiallyforsmallernetworks,

considerNetworkAttachedStorage(NAS).NAStechnologyis

nowavailableforalllevelsofnetworks,includingSOHO

networks.

NAShassomeadvantagesoverSANtechnology.Consequently,

insomesituationsitmaybeabettersolutionforyournetwork

storageneeds:

YournetworktrafficontheLANisminimalandtheimpact

ofoneormoreNASserverswillnotaffectperformance

greatly.AnexampleofthisisusingNAStostoredatathat

isnotusedbythenetworkclientsfrequently,suchas

referencematerial,softwareinstallationkits,orarchived

files.

Yourbudgetdoesn'tallowforthepurchaseofanexpensive

SAN.SomeNASdevicessellforafewthousanddollars.

SomeSANsdevicessellforafewhundredthousanddollars

(anddon'tforgetthemaintenancecosts),andsomeare

overthemillionmark.YoucanalwaysupgradeyourNAS

devicestonewertechnologyinthefuturewhenyour

businesscanaffordit.SANsaredefinitelynotforthehome

orSOHOmarket.

Yourneedforstorageisn'tasgreatasthatwhichaSANcan

provide.SmalleradditionstostoragecanbedonewithNAS

devicesthatcanbesharedamongmorethanoneserver.If

youneedonlyasmallamountofspace,NASdevicesmight

becheapertouse.Forlargerstorageneeds(intheterabyte

range)you'llprobablydobetterwithaSAN.



Asyoucansee,NAScanbeidealforthesmalloffice

environment,forasmalldepartmentaldeploymentthatneedsa

largeamountofstorage,orevenforhomeuserswithlarge

storagedemands.



NetworkAppliances

ManyNASsolutionscomeasapackagedproduct.The"network

appliance"willhaveitsownoperatingsystemandnetwork

connection,andinmanyinstancesisprettymuchaplug-andplaytypeofoperation.Justconnectittothenetworkandmake

someminorconfigurationtweaks,andyou'vegotstorageon

thenetworkthatyoucanrestricttospecificserversorother

computers.



Note

NetworkNASdevicescanbecomplexdevices.They

donotjustoffersimplefilesharesforthedisk

storagetheyprovide.Instead,usingmanagement

softwareforthesedevices,youcanconfigureRAID

sets,suchasmirroreddisksorstripesets,thatcan

beusedtoincreaseup-timebyenablingadisk

subsystemthatcantoleratethelossofoneormore

diskdrives.Thesefunctionsarealsoperformedby

SANs.Thepointisthatyoudon'thavetopaymore

toobtainthislevelofdataintegrity.Formore

information,RAIDisdiscussedlaterinthischapter.



You'llfindmanyvendorsofNAS,fromHPtoIBMtomanyother

smallerfirms.SomeSOHOnetworkhardwarevendors,suchas

Linksys,alsoofferNASdevicesespeciallydesignedforSOHO



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