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Chapter 1.  Designing Your Server Environment

Chapter 1.  Designing Your Server Environment

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useitasajumping-offpointinordertoexploreconceptsthatarenew

toyou.



1.1.Planning

Theinstallationprocessactuallybeginswellbeforeamachineis

bootedforthefirsttime.Itcanandshouldbeginevenbefore

themachinearrivesonsite.Planningisanoften-neglected

aspectofservermanagement,andprobablythesinglemost

importantfactorinreducingsupportcosts.Thisisespecially

trueforaserverproduct,sinceaserverinstallationbyitsvery

natureaffectsmoreendusersthanaworkstationinstall.While

it'simportanttokeepfromgettingstuckinthequagmireof

chronicbureaucraticdeliberationandmassivecommittee

decisionmaking,rushingintoaserverinstallisthelastthing

youwanttodo.Specificareasofplanningfocusshouldinclude

thehardwareplatform,storageplatform,volumearchitecture

andmanagement,andnetworking.



1.1.1.Hardware

Apple'sXserve,a1U(singlestandardrack-size)serverproduct,

haseffectivelyendedmostconversationsabouthardware

choices.Whenthenumbersarerun,theXserve,withits

includedunlimitedclientlicenseversionofMacOSXServer,is

almostalwaysabettervaluethanaPowerMacG5witha

separateServerlicensetackedon.Theonlyrealexceptionsare

verysmalldeploymentsparticularlyineducationenvironments,

[*]

environmentswithexistinghardwarethatcanbeputtowork

asaserver,orwhenthepurchaseofnewhardwarecan'tbe

justified.

[*]Intheeducationmarket,MacOSXissoldforapproximatelyhalfofitsretailprice.



Thatsaid,MacOSXServercanrunonvirtuallyanyhardware

platformthatMacOSXcan.Infact,muchofthetestingthat

wentintothisbookwascarriedoutonasetofiBooksIcarry



aroundwhiletraveling.Thisisn'ttoimplythatApplesupports

suchaconfiguration;infact,portablesarespecificallynot

supportedbyMacOSXServer.Realdeploymentsshouldalways

conformtoApple'slistofsupportedhardware,iffornoreason

otherthanthatgettingsupportforanunsupportedhardware

platformcanbequitedifficult.



AccordingtoApple'swebsite,MacOSXServerrequiresanXserve,

PowerMacG5,G4,orG3,iMac,oreMaccomputer;aminimumof128

MBRAMoratleast256MBRAMforhigh-demandserversrunning

multipleservices;built-inUSB;and4GBofavailablediskspace.These

specificationsareprobablyunderestimatedformostserverroles.



Thechoiceofasupportedplatform,though,isreallyonlythe

beginningofagoodplanningprocess.Hardwareconfigurationis

anentirelydifferentmatter.MacOSXServerisageneralpurposeserverproductwithliterallyhundredsoffeatures.This

meansthatit'sverydifficulttodrawconclusionsabout

hardwarerequirementswithoutdefiningwhattheserverwillbe

usedfor.Forinstance,aniMacwith128MBRAMcouldeasily

supportthousandsofstatic-contentHTTPqueriesaday,while

anAppleFilingProtocol(AFP)serversupportingthesame

numberofconnectionswouldneedtobesignificantlymore

capable.Thepointisthatdifferentservicesevenatthesame

scalecanhaveverydifferentrequirements,andthose

requirementsplayanimportantroleinyourchoiceofhardware.

Goingintogreatdepthregardingtheperformancebottlenecks

ofvariousservicesiswellbeyondthescopeofthischapter,but

iscoveredinsomedepthinchaptersspecifictoeachservice.

Fromthischapteryoushouldtakeaframeworkforthis

planning;theactualdetailswillcomelater.

Oneveryimportantconceptrelatestosystemarchitecture:

determiningwherebottlenecksactuallyexistinasystem(beit



asinglemachineoraverylargenetwork)inordertoeither

workaroundthosebottlenecksormoreeffectivelyoptimize

performance.AgoodexampleofthisistheSCSI320standard

(discussedlaterinthischapter).ASCSI320busistheoretically

capableof320megabytespersecond(MBS)ofdata

throughput.However,wheninstalledinMacspriortotheG5,a

SCSI320disk'sthroughputwouldmaxoutataspeedvery

similartothemuchmoreaffordableSCSI160,mainlydueto

theconstraintsoftheG4'sPCIbus.Luckily,Applepublishes

fairlydetailedhardwareschematicsinitshardwaredeveloper

technotes.See

http://developer.apple.com/documentation/Hardware/Hardware.html

fordetails.



1.1.2.Storage

Storageisoneofthebiggestvariablesinaserverinstall,

probablybecausethepriceofharddiskspacehasdropped

enoughforawidevarietyofsolutionstodevelop.Thegauntlet

ofchoiceshasonlyincreasedwithApple'sentryintothestorage

market.First,theXservewasdesignedspecificallyto

accommodatealargeinternalstorage.Later,Applereleasedthe

XserveRAID,anIDE-basedFibreChannelstoragearray.More

recently,theintroductionofXsan,Apple'sStorageAreaNetwork

(SAN)product,hasaddedanotherpiecetothepuzzle.



Twotrendsshouldbenoted.First,inboththeXserveandXserveRAID,

ApplehaschosentheeconomyofvariousIDEflavorsspecifically--ATA

100andSerialATAoverthewell-establishednatureofSCSI

connectivity.Timewilltellwhetherornotthischoiceofsizeand

redundancyoverperformanceandreliabilitywasagoodone.Inthe

meantime,itisyetanotherareainwhichApplehastakenastrong

standindefenseofsometechnologyinanewrole.Second,ona

relatednote,thechoiceoffiberandtheadoptionofSANtechnology

illustratesacommontrendseeninAppleproducts:bringingenterprise

technologyintotheworkgroupanddrivingdownthepriceofpreviously

veryexpensiveproducts.Xsanisnotcoveredinthisbook.



Confusionregardingstoragereallyarisesontwodifferent

levels:choiceofstoragetechnologyandstorageconfiguration.

We'lllookatbothoftheseissuesastheyrelatetotheplanning

process,aswellasexaminesomespecificproductsandthe

architecturaldecisionsthatwentintothem.Ourdiscussion

focusesontheXserve,sinceitisthemostcommonMacOSX

Serverplatform.Thisfocusdoesn'treallynarrowour

conversationmuch,sincetheXserveismostlyaMacinaserver

enclosure.Itdoes,though,giveussomefocus.



1.1.2.1Storagetechnologies

Atonetimeinthenot-so-distantpast,discussionsofserver

storagetechnologies(outsideofverymassivehigh-end

hardware)tendedtobefairlysimple.Serverplatformshave

traditionallysupportedSCSIdisksthatwasthatandthenyou

couldmoveintodiscussionsofvolumemanagement.Applewas

thefirstvendortochangethat,byusingATAdisksintheXserve

insuchawaythattheybecameviableforserverproducts.This

featurehasspurredconsiderabledebateand(towardtheoverall

goodoftheserverindustry)madeourdiscussionmore

complex.Thissectionexaminesvariousstoragetechnologies

andhighlightstheirstrengths,weaknesses,andrelativecost.

Discussionofgenericstoragetechnologiesmightseemtobe

beyondthescopeofthisbook,givenitsstatedgoalofrefraining

fromrehashingmaterialthat'swidelyavailableelsewhere.

However,thetopicofstoragetechnologiesisparticularly

germanetoMacOSXServersystemadministrationfora

numberofreasons:

Asmentioned,ApplehasreallypioneereduseofIDE/ATA

drivesinserverproductsinboththeXserveandXserve

RAID.Thisdesignnecessitatesanumberofdiscussionsthat



previoussystemadministrationtitleshavehadtheluxuryof

glossingover.

TheintroductionoftheXserveRAIDbringsFibreChannelto

theMacinabigway.Fibreisanexcitingtechnologyand,as

somethingnewtoalargeportionofthisbook'saudience,

somethingworthtalkingabout.

MostUnixorWindowsadministratorswouldn'tconsider

FireWiretobeaviablestorageplatformbutduetoits

popularityintheMacworld,wemustcoverit.

TherecentriseofserialATAisahugedevelopmentin

storagetechnologiesingeneral,muchlesstheMacplatform

alone,andisthereforesomethingthatwarrantscoverage.

Whatitreallyboilsdowntoisthis:wecannolongersay,"Real

serversuseSCSI."Itoversimplifiestheissue,andit'sjustnot

thecase.Ifyou'realreadyfamiliarwiththesetechnologies

(ATA,SATA,FibreChannel,SCSI,andRAID),youmightwantto

skiptheremainderofthissection.Thenagain,maybeyouwant

torevisitsomeofthemandpossiblygainanewperspective.



1.1.2.2ATAttachment(ATA)andIntegratedDrive

Electronics(IDE)

ATAandIDEaregenerictermsforasetoftechnologiesand

standardsthatgrewoutoftheWintelarchitecture.Long

consideredaPC-classtechnology(ratherthanbeingapplicable

totheworkstationorservermarkets),ATAandIDEhavecome

alongwayinrecentyears,enoughthatundertheright

circumstances,theycanbeconsideredappropriateforserver

deployments.Specificimprovementsinclude:



Vastlyimprovedthroughput

TheoreticalmaximumthroughputforIDEdriveshasrisen

steadilythroughouttheevolutionoftheprotocol.Today's

IDEdriveshaveatheoreticalmaximumthroughputof133

MBS.WhilefarlessthanSCSI'scurrent320MBmaximum,

thisperformanceissufficientformostapplications,

especiallyinthecontextofawell-engineeredstorage

architecture,andespeciallygiventhefarlargercapacities

andfarlowerpricesofIDEdrives.

Onboardcachealsoaidsperformancesignificantly.Current

high-endIDEdrivessupportupto8MBofonboardcache,

vastlyincreasingreadperformance.Note,though,that

theoreticalthroughputdoesnotalwaysequatewithrealworldperformance,andmoreoftenthannot,it'sIDEthat

benefitsfromthisphenomenon

(http://www.csl.cornell.edu/~bwhite/papers/ide_scsi.pdf).



Advancedfeatures

Along-held(andlong-correct)assertionregardingIDE

driveswasthattheylackedcertainadvancedfeaturesthat

SCSIsupported;namely,taggedcommandqueuing,

scatter-gatherI/O,andDirectMemoryAccess(DMA).The

tremendoussaturationofthelow-endstoragemarket,

however,hasforcedvendorstoinnovateatatremendous

pace(infact,allofthesefeaturesarestandardamong

recenthigh-endATA-100and-133offerings).Itwouldbe

presumptuoustoassumethatthesefeaturesinmanycases

first-generationcapabilitieswereasmatureorstableastheir

SCSIcounterparts;however,initialresultshavebeenvery

promising,andthere'snoreasontoassumethatIDEwon't

continuetoevolveatarapidpace.



Qualityandreliability

Itisimportanttorealizethatthere'sactuallylittledifference

amongdrivemechanismsinherentthefactthatthedriveis

SCSIorIDE.Thesetermsrefertotheprotocoland

hardwarestandardsusedtocommunicatewiththehost

CPU,whichthereforeimpliesthatonewayoranotherthe

drivecontrollerelectronicswillhavecertaincharacteristics.

Moreoftenthannot,actualqualitydifferencesaresimply

reflectionsofSCSI'shigherpriceandmoreselectivetarget

market.Vendors,usingtheSCSItodifferentiatetheir

productlines,addlargercachesandfasterdrive

mechanismstoSCSIproducts.Fastermechanisms(SCSI

drivesregularlyspinat10,00015,000RPM,whereaseven

high-endmechanismsusedintheIDEspacespinat7,200

RPM)placegreaterstressonmechanicalsystemsingeneral

andthereforerequireahigherstandardofengineering.

TheyarealsoprimarilyresponsibleforSCSI'ssuperiorseek

times.Thereisnotechnicalreasonwhyadrivevendor

couldn'tdropanATA-133controllerontoa15,000RPM

mechanismandindeed,asillustratedearlier,severalvendors

aremovingtolargerandon-diskcachesforIDEdrives.This

trendtowardsbetter-engineered,higher-qualityATA

features,hasresultedinIDEdrivesthataremore

appropriateforserverrolesthaneverbefore.

TheSCSIprotocolitselfdoesmaintainseveralarchitectural

advantages,althoughtheyarenotnecessarilyreflectedby

performancedifferences.SCSIisageneral-purpose

communicationsprotocol,designedtobeextensibletotalktoa

numberofsystemsanddevices.ATAisadiskcommunication

protocolthathasbeenextendedinasometimesadhocwayto

includeothertypesofdeices.SCSIalsowinswhenitcomesto

physicalinterconnect(hardware)features.Inanyincarnation,

butespeciallymorerecentones,itsupportsmuchlongercable

lengthsandmanymoredevicesthanATA.Infact,ATAisstill

limitedtotwodevicesperbus,andanIDEdevicecan



sometimesmonopolizetheIDEbusinsuchawaythattheother

deviceonthebushaslimitedaccesstosystemresources.Thus,

despiteATA'sgrowth,itshouldbedeployedonlyverycarefully

inmostservers.Thereisoftenasignificantcostpremium

betweenthelowendofATAandthehighend,wheregreater

reliabilityandmoreadvancedfeaturesareshowingup.Drives

destinedforserverdeploymentshould,ataminimum,support

taggedcommandqueuingandscatterandgatherI/O.Large

cachesatleast8MBarealsoamust.Additionally,device

managementandconflictremainbigissuesintheIDEworld,so

inaserverenvironment,eachdriveshouldhaveitsown

dedicatedbus.Finally,duetodecreasedqualityassuranceand

shorterwarrantiesassociatedwithIDEproductlines,ATAdrives

should,wheneverpossible,bedeployedredundantly.

Luckily,Applehastakencareofmostoftheserequirementsin

itsserverproducts,showingthatgoodoverallsystem

architecturecanovercomemanyofIDE'sshortcomings.The

bottomlineisthatIDEmightgoalongwaytowardsmeeting

yourstorageneeds,especiallywhenmanagedappropriately.

WhileSCSImightbearchitecturallysuperiorandostensiblya

betterperformer,itsperformanceandfeature-to-priceratiois

typicallymuchlowerthanIDE's,andIDE'svastlylowercost

makesforeasyredundancyandaffordableexpansion.



Asofthiswriting,currentXserves(whichusetheG5processor)utilize

serialATAandsupportthree(ratherthanfour)drives.TheXserve

RAID,however,stillusesATA100.



1.1.2.3SerialATA(SATA)

SerialATA,orSATA(firstincludedbyAppleinthePowerMac



G5),isabackwards-compatibleevolutionoftheATAprotocol.

Thefundamentalarchitecturaldifferencebetweenthetwois

thatSerialATAis,asthenameimplies,aserialratherthana

paralleltechnology.Thismightatfirstseemcounter-intuitive;

afterall,sendingabunchofdatainparallelseemsfasterthan

sendingthesamedataserially.However,serialtechnologies

haveproventheirviabilityinrecentyears,notablyintheform

ofUSB,FireWire,andFibreChannel.Ingeneral,serial

technologiesrequirefewerwires--insteadofmultipleparallel

channels,theyoperateontheconceptofhavingonechannel

eachway,toandfromthehosttothedevice.Thisreductionin

thenumberofwiresinthecablemeansthattheelectronic

noiseassociatedwiththemisreduced,allowingformuchlonger

wiresand(theoretically)muchfasteroperation.

SATAhasallthebenefitsofATA,alongwithseveraladded

bonuses:

Becauseit'saPoint-to-Point,ratherthanamultidevicebus,

therearenodeviceconflictsormaster-slaveissues.

SATAisalsohot-pluggable(similartoFireWire),and

requiresamuchthinnercablethantheIDEribbon.

Itpromisesmorewidespreadsupportforadvanceddrive

featuresthatareotherwisejustmakingtheirwayintothe

ATAspace,liketaggedcommandqueuingandoverlapping

commands.

RightnowSATAdoesn'treallyofferasignificantperformance

increaseoverATA,butassystembusperformanceimproves,

thatdifferencewillbecomemoreandmorepronounced.While

SATAisn'trevolutionaryinanysenseoftheword,itisavery

completeevolutionoftheATAstandardanditdoesgoalong

waytowardslegitimizingATAintheserverspace.Nonetheless,

therequirementsplacedonIDEshouldalsobeenforcedbefore



deployingSATAdrivesinaserverrole.



1.1.2.4FibreChannel

FibreChannelisaserialstorageprotocolthatiscommonin

datacenterandenterpriseenvironments.Itisahighperformanceflexibletechnologythatisespeciallynotabledueto

Apple'smovetowardsitwiththeXserveRAIDandXsan

products.FibreChannelwasdesignedtomeetseveralofSCSI's

limitations;specifically,thoserelatedtoarchitecture,cable

lengthanddevicesupport.

AneasywaytounderstandthisistolookatthenameSCSI.It's

anacronymforSmallComputerSystemsInterface,and

althoughithasturnedouttobeextremelyextensible,its

evolutionfromasystemdesignedforsmall-scalecomputer

systemsisundeniable.SCSIisasingle-mastertechnology.

Generally,asingleCPU(bywayofaSCSIadaptor)hassole

controlovermultipleSCSIdevices.Thisisadifficult

architecturetomaintainintoday'sworldofcentralizedStorage

AreaNetworks(SANs).Similarly,eveninitsmostcapable

iteration,SCSIislimitedtoacablelengthof25meters,and

cannotdealwithmorethan16devices.Althoughthisisavast

simplification,theprimaryadvantagesofFibreChannelareas

follows.



Architecture

FibreChannelsupportsthreetopologies:Point-to-Point,

ArbitratedLoop,andSwitchedFabric.ThesimplestisPointto-Point,whichconnectsasinglehosttoasingledevice;for

instance,anXserveRAIDtoanXserve.Point-to-point

connectionsresembleSCSIorFireWireconnectionsfroma

conceptualstandpoint,andareusuallynotdifficulttograsp.



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