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Chapter 8. ONS 15454 Storage-Area Networking

Chapter 8. ONS 15454 Storage-Area Networking

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companiesconsolidatedtheirserversandstoragetohelp

overcomethismanagementproblem.TheSmallComputer

SystemInterface(SCSI)protocolhasbeenusedtoconnect

serverstostoragedevicesbecausemostserversandstorage

devicessupportedthisprotocol.However,thecablingdistance

forSCSIisseverelylimited,andotherprotocols,suchasFibre

Channel,weredevelopedtoovercomethisdistancelimitation.

Thischapterfocusesonthestorage-areanetworking(SAN)

extensionusingtheCiscoONS15454.SANextensionsprovide

thecapabilitytousethedifferentlocationsofstoragedevices.

Thisiscommonlyreferredtoashaving"islands"ofstorage

devices.YoucanlinktheseislandsusingSANextension

solutions.

Morespecifically,youcanuse15454networkstoconnectSANs

indifferentgeographicallocations.Thisisimportantbecauseof

theneedtoconsolidatedatacenterresourcesandtocreate

architecturefordisasterrecoveryandhighavailability.Service

providerscanoffermanagedSANservicesforbusinessesthat

donothavetheresourcestoextendtheirSANsontheirown.

ThischapterintroducesyoutotheSANextensioncapabilitiesof

theONS15454andaddressesthefollowingareas:

UsingtheMultiserviceProvisioningPlatform(MSPP)for

storagetransportusingSynchronousOpticalNetwork

(SONET)andwavelengths

UsingtheFC-MR-4-seriescard

Overcominglongdistancesbyusingbuffercredits







SANReview

Insimpleterms,aSANisanetworkinfrastructuredeployed

betweenserversandstorageresources.Forexample,one

solutionistoprovideastoragenetworkwhereFibreChannel

(FC)switches,suchastheCiscoMDS9000,canbe

interconnectedovera15454network.The15454network

interfaceswiththeFCswitches,asshowninFigure8-1.



Figure8-1.SANExtensionExample



[Viewfullsizeimage]



ASANisnormallyadedicatednetworkoptimizedforthe

exchangeoflargeamountsofdatawithinthedatacenterto

supportbetterutilizationofavailablestorageandtoprovidefor

disaster-recoverysolutions.Companiestypicallyhavetwodata

centers,onethatisactiveandoneinstandby,incasetheactive

datacenterfails.TheSANenablesthetwodatacenterstobe

synchronizedsothatthestandbycenterhasup-to-date

information.ASANcreatesadedicatedlinkbetweentheserver

andstoragedevicesthatmustfollowcertainrequirements,such

aslowdelay,highbandwidth,andhighreliability.FCand

EnterpriseSystemsConnection(ESCON)switchesareusedat



thelocaldatacenterstoprovideaccesstostoragediscs.



Note

Networkattachedstorage(NAS)issimilartoaSAN

butusesfilesinsteadofblocks.Blocksofdata

representamoregranularrepresentationofthe

data,comparedtofiles.It'ssimilartotheconceptof

MicrosoftWindows,inwhichfilesrepresentdata.In

addition,thedataisstoredonyourharddiskina

muchmoregranularformat,whichyoucanthinkof

asblocks.SANreplicatesthedatabetweenthe

serversandstoragedevicesattheblocklevel

insteadofthefilelevel.Databases,suchasOracle,

storedataasblocks.Thismethodofreplicatingdata

attheblocklevelmakesmoreefficientuseofthe

storagedevicesthanthefilereplicationmethodused

byNAS.



SANProtocols

ThemostcommonlyusedSANprotocolsincludethese:

SmallComputerSystemInterface(SCSI)SCSIisthe

mostcommonmethodforconnectingstoragedevicesto

servers.MultipleversionsofserialSCSIareinusetoday.

Forexample,SCSIcanbemappedontopofFC,which

enablesSCSItoovercomeitsdistancelimitationand

achievemuchgreaterspeedsandaccesstomoredevices.

SCSIiswidelysupportedbyserversandstoragedevices.



FCFCiscapableoftransferringlargeblocksofdataover

longerdistancesatveryhighdatarates,suchas4Gbps.

NotallSANprotocolsarecapableofconnectingoverlong

distances.

FiberConnection(FICON)FICONistheIBMversionof

FC.FICONconnectsmainframesdirectlywithcontrolunits

orESCONaggregationswitches.Thetransportrequirements

forFICONarethesameasthoseforFC.

FibreChanneloverIP(FCIP)isanotheroptionof

extendingstoragedata.FCIPtunnelsFibreChannelover

TransmissionControlProtocol/InternetProtocol(TCP/IP).

FCIPencapsulatestheFCprotocolwithintheInternet

Protocol(IP)packet.Asaresult,existingIP-basednetworks

canbeusedtointerconnectFC-basedstoragenetworks.

ESCONSimilartoFC,ESCONiscommonlyusedbymany

businesses.ESCONhasalargeinstalledbasebecauseithas

beenusedformanyyears.UnlikeFC,ESCONisalow-speed

IBMprotocolthathasdistancelimitationsofupto40km.

Youcanusedifferentnetworktechnologies,suchasDense

WaveDivisionMultiplexing(DWDM),SONET,andIP,to

transporttheseSANprotocols.



SONETorDWDM?

MultiplemethodsexistfortransportingSANprotocolsbetween

geographicallocations.YoucanuseaSONET-based

infrastructureinwhichyoucanaddastorageextensioncardin

theexistingequipmentlocatedatthedatacenters.Thisis

possiblewhentheSONETringshavebeenbuiltoutusingthe

ONS15454.Inthisscenario,youcanusetheFC-MR-4cardto



extendtheFCorFICONbetweenthelocationsthatcontainthe

serversandthestoragedevices.

Alternatively,youcanuseaDWDM-basedinfrastructure.Thisis

applicablewhenhighdensityandhighbandwidthareneeded

betweentheserverandstoragedevicelocations.TheONS

15454supportsthisenvironmentwhentheONS15454isused

astheDWDMtransmissionnetworkandisusedtoaddthe

wavelengthservice,suchasSANextension,ontheDWDM

network.Inthisscenario,youselecttheappropriateCisco

15454transpondercardthatsupportstheSANprotocol,the

numberofSANports,andthebandwidthrequiredbetweenthe

desiredlocations.Thesection"ASingle-ChassisSANExtension

Solution:ONS15454,"laterinthischapter,liststheavailable

transpondercardsusedforSANextensiononthe15454.



Note

Bothsolutions,SONETandDWDM,offerquick

protectionontheorderof50ms.



DataStorageMirroring

Thetimeittakestobackupstoragedataandthetimeittakes

torecoverfromafailurearecriticalcomponentstomany

companies,especiallyonesinthefinancialmarketplace.Inthe

past,tapeswereusedtoreplicatethestoragedataandwere

physicallymovedoff-site.Thisprocessofbackingupdatais

unacceptabletomanycompaniestodaybecauseoftheslow

recoverytime.Today,largecompaniessuchasbanksrequirean

almostimmediaterecoveryoftheirservices.Thiscanbe

accomplishedbymirroringthedatabetweenlocations,known



asdatareplicationordatamirroring,anditistheprimary

reasonbusinesseswanttotransportstoragedatabetweendata

centers.

Twomaintypesofdatareplicationexist:synchronousand

asynchronous.

Insynchronousdatareplication,dataiswrittentothelocal

storagedeviceandtheremotestoragedeviceinrealtime,

ensuringthataremotecopyofthedataisidenticaltothe

primarycopy.

Inasynchronousdatareplication,dataiswrittentoprimary

storageandthenextoperationcontinues;thereisnopause

forconfirmationthatthedatahasbeensuccessfullywritten

tothesecondarysite,aswithsynchronousreplication.



SynchronousDataReplication

Forsynchronousdatareplication,thedataisreferredtoas

being"mirrored"betweenthesetwosites.Whenyouwantto

achievehighavailability,youmustimmediatelyswitchovertoa

backupstoragedevicethatcontainstheexactdataasthe

primarydatadevice.Youusesynchronousdatareplicationto

achievethis.However,itisimportanttounderstandthatlatency

betweenthedatacentersiscriticaltothedata-replication

application.Synchronousdatareplicationissensitivetoend-toendlatencybecauseanapplicationrunningontheservermust

waituntilitreceivesanacknowledgmentthatthedatahasbeen

replicatedontheprimaryandthebackupstoragedevices.The

data-replicationapplicationwillnotworkproperlyifthistimeis

toolong.Figure8-2showsanexampleofsynchronousdata

replication.



Figure8-2.ExampleofSynchronousData

ReplicationUsingSONETorDWDM



Anothertypeofsynchronousmirroringoccurswhenyoumirror

theapplicationinadditiontothestoragedevice.GDPS,a

protocoldevelopedbyIBM,implementsthiscapability.GDPS

requiresthatspecialcontrolandtimerlinksbeestablished

betweenlocations,inadditiontotransportingthestorage

protocol.Ciscotranspondercardssupportthisenvironment.

Forsynchronousreplication,thefollowingeventsoccur:

1. Theserverpausesapplicationprocessingwhilewritingdata

tothelocalstoragedevice.

2. Thelocalstoragesystemsendsdataandwritestherequest

tothebackupstoragedevice.

3. Thebackupstoragedevicewritesdataandsendsan



acknowledgmenttothelocalstoragedevice.

4. AnI/Ocompletesignalissenttotheserver,andapplication

processingresumes.



Figure8-3showshowthesefourstepsarecarriedout.



Figure8-3.SynchronousDataReplicationEvents



AsynchronousDataReplication

Asynchronousdatareplicationdisconnectstheprimarywrite

operationfromtheremotewriteoperation.Inotherwords,the

applicationwritesthedatatoprimarystorageandcontinues

withthenextoperationwithoutpausingtowaitforconfirmation

thatdatahasbeensuccessfullywrittentothesecondarysite.

Asynchronousreplicationistypicallyusedwheresynchronous

datareplicationisnotpractical,especiallyinlong-haul

applicationsinwhichtheend-to-enddelayistoomuchfor

synchronousmirroring.Unlikesynchronousdatareplication,



asynchronousisnotlatencysensitiveandcanaccommodatea

certainamountofdelayinthenetworkanddataloss.

Therefore,youcanuseasynchronousdatareplicationacross

longerdistances.

Youmustdeterminethedistancesthatthestoragedevicescan

supportusingsynchronousorasynchronousapplications.This

helpsdeterminethetypeofnetworkthatcansupportSAN

extension.

Forasynchronousreplication,thefollowingeventsoccur:

1. Theserverpausesapplicationprocessingwhilewritingdata

tothelocaldisk.

2. Thelocalstoragedevicequeuesacopyofthedatafor

transmissiontotheremotestoragedeviceandsendsanI/O

completesignaltothefileserversothatapplication

processingcanresume.

3. Thelocalstoragedevicesendsdatatothebackupstorage

device.

4. Thebackupstoragedevicesendsawriteacknowledgeback

tothelocalstoragedevice.



Figure8-4showshowthesefourstepsarecarriedout.



Figure8-4.AsynchronousDataReplicationEvents







ASingle-ChassisSANExtensionSolution:ONS

15454

TheONS15454iscapableofsupportingbothsynchronousand

asynchronousdatareplication.TheONS15454providesstorage

extensionoverSONETandDWDMnetworks.Some

characteristicsofusingtheONS15454MSPPsolutionforSAN

extensionfollow:

Commonlyusedforasynchronousapplicationsbutcanbe

usedforsynchronousapplicationsaswell

Achieveconnectionsoverlargedistancesupto10,000km

IntroduceshigherlatencythanONS15454DWDM

HasalowerstorageprotocolportdensityversusONS

15454DWDM

SomecharacteristicsofusinganONS15454DWDMsolutionfor

SANextensionfollow:

Commonlyusedforsynchronousdatareplicationtoup200

km

AchieveshighdensityofSANextensionlinksoverfiber

WhenselectingtherightSANextensionsolution,itisimportant

toanswerthefollowingquestions:

• Howmuchbandwidthisneeded?



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