vm memory vram sizing considerations

8/2/2019 Vm Memory Vram Sizing Considerations

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VMMemory(vRAM)Sizing

ConsiderationsChallenges and considerations in determining how

much memory to allocate to virtual machines

WHITEPAPERBYDAVIDDAVISANDBRADBONN


2/17

TableofContentsIntroduction.................................................................................................................................................. 2

MemoryUsagethroughDifferentLevelsoftheVirtualizationStack........................................................... 3

MemoryBallooning............................................................................................................................... 4

TransparentPageSharing..................................................................................................................... 5

MemoryCompression........................................................................................................................... 5

VMwarePerformanceMetricstouseforVMMemoryUsageAnalysis....................................................... 5

DeterminingifaVMhasMemoryrelatedPerformanceProblems.......................................................... 7

AdditionalFactorstoKeepinMindwhenSizingVMMemory..................................................................... 8

MemoryHogApplications..................................................................................................................... 8

Limits......................................................................................................................................................... 8

Reservations..............................................................................................................................................

8

KnowinganApplicationsNeeds............................................................................................................... 9

InstitutingaVMMemorySizingProcess...................................................................................................... 9

DeterminingTimeFrameforAnalysis..................................................................................................... 10

FindingthePeakValue............................................................................................................................ 11

ComparingthePeakUsageValuetoAllocatedMemory........................................................................ 11

ForecastingUsageChangesuntilNextMemoryVMAllocationReview................................................. 12

FactorinaBufferValue.......................................................................................................................... 12

ChangetheVMSizingAllocationandDocumenttheChange................................................................ 12

SettingUpaRegularMemoryReviewBasedonanAppropriateTimeframe........................................ 13

ProvisioningaNewVMforAppropriateMemoryAllocations................................................................... 13

InitiallyAllocatetheMinimumPossible.................................................................................................. 14

MonitorMemoryMetricsIntensely....................................................................................................... 14

AddMoreMemoryinSmallIncrements................................................................................................. 15

AddtheNewVMtotheRegularMemorySizingReviewProcess.......................................................... 15

Conclusion...................................................................................................................................................

15

AbouttheAuthor.................................................................................................................................... 16

AbouttheSponsor.................................................................................................................................. 16

2011 VKernel Corporation. All rights reserved. http://www.vkernel.com 1


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IntroductionWhensizingvirtualmachinesavirtualizationadministratormustselectthenumberofvCPUs,thesizeof

thevirtualdisk,thenumberofvNICs,andtheamountofmemory.Outofallthoseresources,the

amountofmemoryallocatedtoeachvirtualmachine(VM)isoftenthemostdifficulttodetermine.This

isbecausememoryisthemostdynamicandleastpredictableofthoseresources.

VirtualizationadministratorsshouldavoidoverallocatingmemorytotheirVMsbecausedoingsocan

wasteexpensiveserverresourcesandwill,therefore,minimizethereturnontheinvestment(ROI)in

thatinfrastructure.Ontheotherhand,thebusinesscriticalapplicationsrunningonvirtualmachines

alsoneedtomaintainhighperformance,andthelastthingavirtualizationadministratorwantsisto

haveperformancecomplaintsfromendusers.

VMadministratorsoftenfacedifficultlyinbalancingtheneedtomaximizeROIoftheserverhardware

andtherequirementforapplicationstoperformwell.Fortunately,withVMware'smemoryover

commitmentcapabilitiesandtheguidancefromthiswhitepaper,virtualizationadministratorswillgaina

deeperunderstanding

about

how

to

make

VM

memory

sizing

more

straightforward

and

less

of

apainful

trialanderrorprocess.Byreadingthiswhitepaper,VMadministratorswilllearn:

Howmemoryusageworksatdifferentlevelsofthevirtualizationstack(seeFigure1)

ThechallengesindeterminingaVMsmemoryrequirements

HowtoscreenformemorybasedVMperformanceissuesbeforeattemptingtooptimizean

environment

HowtoapproachappropriatesizingofnewVMsthatneedtobedeployed

Howtoinstitutearegularmemorysizingprocessforthedatacenter

WhatVMperformancemetricsprovidethebestinsightsonmemoryusageandhowtoaccess

these

Figure1TheVirtualizationStack



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MemoryUsagethroughDifferentLevelsoftheVirtualizationStackPhysicalmemoryonthehostismanagedbythehypervisor,andthehypervisorloadsthismemoryjust

likeanyotheroperatingsystemdoes. VMsloadedinthehypervisorwillincreasethememoryutilization

incrementallyasuserscreatevirtualmachines.OncetheguestOSisrunningandtheapplicationsload,

theguestOSbelievesthatitownsallthevirtualmemorythathasbeenconfigured(justasitdoesona

physicalserver)andwilloften"pindown"largeblocksofmemorytocacheitforfutureuse.

Itisimportanttonotethatvirtualizationisnotapanaceaandthereismemoryoverheadassociatedwith

usingthistechnology.Guestsandtheirapplicationswillneverhaveaccesstothesameamountof

physicalmemoryinstalledonthehost.Thereisanadditionalquantityofphysicalmemorythatisused

upwitheachvirtualmachinepoweredoninthehypervisor,andtheamountofoverheadisdetermined

bythevirtualresourcesthathavebeenallocatedtotheVM(seefigure2). Largeramountsofvirtual

RAMandadditionalvCPUsincreasetheoverheadmemoryamountonthehost(seefigure3).

Figure2TheImpactstoAvailableMemoryforanApplicationfromVMandGuestOverhead



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Figure3VM

Overhead

Table

SOURCE:FROM VMWARE.COM

(HTTP://PUBS.VMWARE.COM/VSP40_ I/WWHELP/WWHIMPL/COMMON/HTML/WWHELP.HTM#HREF=RESMGMT/R_OVERHEAD_MEMORY_ON_

VIRTUAL_MACHINES.HTML#1_7_9_9_10_1&SINGLE=TRU)

Virtualizationadministratorscanthinkofmemoryasa"timeshare"whereitisprovidedtoeachvirtual

machine,asneeded,andfreedupwhenavailabletobeusedbyanothervirtualmachine,alsoas

needed.

VMwareemploysavarietyoffeaturesthatmakememoryovercommitmentpossible.Withoutsuch

featureslikememoryballooning,transparentpagesharing(TPS),andcompression,anESX/ESXiserver

wouldsimplyrunoutofmemoryandcrash.

MemoryBallooning

WhenanadministratorinstallsVMwareTools,thememctldriver(a.k.a.ballooningdriver)isinstalledin

theguestOS.ThisdrivercreatesabubbleorballoonofmemoryconsumedinsidetheguestsotheOS

seesitasbeingusedbyanapplication.ThehypervisorthentakesthephysicalRAMfreedupbyinflating

thisballoonandallocatesittootherVMsthatrequireit.

Memoryballooningintroducesasmallamountofprocessingoverhead,andifitforcesaguestOSto

beginpagingtodisk,thiscansignificantlyslowdowntheapplication(s)ontheVM. IftheVMisntusing

thememory,

then

ballooning

itself

isnt

aserious

performance

issue,

but

it

is

an

indication

that

physical

memoryonthehostisbecomingscarce.

Oneofthebiggestadvantagesofmemoryballooningoverothermethodsforhandlingmemoryover

commitmentisthatthememoryballooningdriverallowstheguestoperatingsystemtochoosewhich

pagesarerelinquishedtothehypervisorforotherVMs. Thisway,pageswhicharentinactiveusecan

besafelyfreedup,causingalmostnoperformanceimpactfromtheguestsperspective.

http://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=truehttp://pubs.vmware.com/vsp40_i/wwhelp/wwhimpl/common/html/wwhelp.htm#href=resmgmt/r_overhead_memory_on_virtual_machines.html#1_7_9_9_10_1&single=true


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TransparentPageSharing

Transparentpagesharingisthededuplicationofmemorythatpermitsidenticalvirtualmemorypages

tobecollapsedintoasinglepagewithinthehostsphysicalRAM,therebyfreeingupmemoryforother

uses.Forexample,ifmultiplevirtualmachinesonahostareallrunningthesameoperatingsystemand

application,thehypervisorwillcomparepagesofmemorythroughhashingtolocateidenticalpagesthat

canbe

freed

up

through

their

consolidation.

Ballooning

and

transparent

page

sharing

work

together

to

ensurethatovercommittedmemorydoesn'tcauseperformanceissuesfortheapplicationsintheguest

virtualmachines.

MemoryCompression

MemorycompressionanddiskswappingbythehypervisorarethelastditcheffortsbyESX/ESXitokeep

thehypervisorfromcrashingwhenmemoryresourcesonthehostarestretchedtothebreakingpoint.

Thecompressionofmemorypagesbythehypervisoralsocausesadditionalprocessingoverhead;

however,thisoverheadissmallincomparisontotheslowdowncausedbyswappingpagesouttoa

storagedevice. UsersofvSphere4.1andabovewillbeabletotakeadvantageofthisfeaturetoreduce

theamount

of

swapping

taking

place

when

physical

memory

resources

are

close

to

being

exhausted.

VMwarePerformanceMetricstouseforVMMemoryUsageAnalysisVirtualizationadministratorsshouldtakethetimetobecomefamiliarwithcriticalvSpherecountersthat

areusedtoanalyzememoryperformance.ThesecounterscanhelppreventVMmemoryperformance

issues,aswellashelpresolvethemwhentheyoccur.

ThetwomostcommonlyusedvSpherememorymetricsarememoryactiveandmemoryconsumed.

Normally,whenviewingmemoryusageonaVMwithintheVIclient,thememoryactivemetricis

displayed

(see

Figures

4

and

5).

The

memory

consumed

metric

is

displayed

by

default

at

the

host

level.

AverageMemoryActiveinKB(memory.active.average)ThisistheamountofmemorythattheguestVMisactivelyusing,andisestimatedbasedonrecentlytouchedpages(thesmallest

ofactive,consumed,andgranted).Monitorthismetricforahighmemoryactivethresholdthat

isapproachingtotalmemorycapacityonthehost. Thismetricdoesntshowthefullfootprint

ofaVMinmemory,soitcantbeusedreliablyfordeterminingaVMsminimummemory

requirements. However,itdoesgivevisibilityintohowmuchmemorypagingactivityistaking

place,andsocanbeusedtojudgewhethertheapplicationsonaVMareengagedoridle.

AverageMemoryConsumedinKB(memory.consumed.average)Thisistheamountofguestphysical

memory

consumed

by

the

virtual

machine

for

guest

memory,

and

is

calculated

as

the

amountofmemorygranted(configuredfortheVM)lesstheamountofmemorysavedby

memorysharingtechniques. Thismetrichasclassicallybeenthebetterchoicefordetermining

whatavirtualmachinestotalmemoryrequirementsare;however,thereareadditional

complexitiesthatcanmakeitlesshelpfulonmoderninstallations. Factorssuchaslarge

memorypages,transparentpagesharing,andcachingactivitywithintheguestcancausethis

metrictobecomelessinformative.



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Figure4AMemoryUtilizationChartfromwithinVMwarevCenter

Figure5vSphereCountersforMeasuringMemoryPerformanceWhilememoryactiveandmemoryconsumedaretypicallythetwomosttellingVMmemorymetrics,

therearemanymorethatcanbeusedtolearnaboutmemoryconsumption,allocation,andsharing.

MoreinformationonVMwarememorymetricscanbefoundhere:

http://www.vmware.com/support/developer/vc

sdk/visdk400pubs/ReferenceGuide/memory_counters.html



8/17

Forexample,whenahostorvirtualmachineisswappingouttodisk,thisisanimmediateindicatorofa

memoryproblem.StatisticslikeAverageMemorySwappedinKB(memory.swapped.average)and

AverageMemoryReclaimedbyBallooning(mem.vmmemctl.average)showthataVMishavingtoswap

oremployballooningtogetthememorythatisneeded.

Evenwhen

avirtual

machine

guest

OS

is

no

longer

connected

to

physical

hardware,

it

can

still

be

useful

introubleshootingperformanceproblems.Theguestcandeterminewhatapplicationsarerunningand

howmuchmemorythoseapplicationsareusing.Ifthoseapplicationsarebusinesscriticalandneed

morememory,theVMmemoryshouldberesized.Also,itisessentialtohaveVMwareToolsinstalledso

thatthehosthastheabilitytoemployballooningifthereisanymemorycontention.

Knowingwhichmetricstoevaluateisnotenoughtodetermineadequatememorysizing.Topreventand

solvevSpherememoryperformanceissues,itisimportanttonotethethresholdsforthosemetrics,

whichwhencrossed,areindicativeofVMperformanceissues.Herearefoursuchthresholdsthatare

importanttoknow:

Highhostmemoryconsumption>80%(memory.active.average)

HighVMmemoryconsumption>7590%(insidetheguestOS)

HostorVMSwapping(onthehost,memory.swapped.average)>0

VMBallooning(mem.vmmemctl.average)>0

Anotherfactortoconsideristheuseoflargememorypages.Largememorypagesincreasethe

performanceofmemorytranslationsforapplicationsthatuselargeamountsofmemory.Largememory

pagesareenabledbydefaultinESX/ESXi,butmustalsobeenabledintheguestoperatingsystem.For

moreinformationontheperformanceoflargepageswithvSphere,consultVMwaresLargePage

PerformanceStudy.FormoreinformationonvSphereMemoryResourceManagementseeVMware:

UnderstandingMemoryResourceManagementinVMwareESXServer.

DeterminingifaVMhasMemory-relatedPerformanceProblems

IfaVMisalreadyexperiencingaperformanceproblemrelatedtomemory,thentheavailableRAMis

insufficientwhetherduetovirtualmemoryassignedtotheVMorthephysicalmemoryinstalledonthe

host.Indicatorsofanimpendinghostorclusterwidememoryshortageincludewhenactivememory

approachestotalmemoryand/orwhenswappingisoccurringfrequently.Ifhostmemoryistheproblem,

eitherthereisnotenoughorthedistributedresourcescheduler(DRS)isnotbeingusedproperlyto

balancethememoryloadacrossotherhosts.

Inadditiontothephysicalhostmemoryshortagethatmanysystemadministratorsarefamiliarwithona

traditionalserver(notusingvirtualization),thereareotherwaysthatmemoryshortagescanoccurina

virtualizedenvironment.

VMmemoryshortagescanoccurifresourcecontrolsareconfiguredincorrectly(perhapsareservation

orlimithasbeensetinadequately)ortoomanymemoryintensivevirtualmachineshavebeendeployed

http://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdfhttp://www.vmware.com/files/pdf/large_pg_performance.pdf


9/17

onasingleserver.Toidentifyavirtualmachinememoryshortage,administratorsshouldanalyze

memoryresourcesettings(reservationsandlimits,whicharecoveredbelow),aswellasoverallVM

memorydemands.

AnotherpossibilityisthatVMshavebeengrosslyovercommittedandmemoryoverheadassociated

withthose

over

commitments

is

consuming

too

much

memory

resources.

The

amount

of

memory

being

usedformanagingmemory,perESX/ESXihost,isavailablebymonitoringthemem.overhead.average

counter.

Finally,applicationlevelmemoryshortagescanoccurinsidetheguestOSifthereisamemoryleakinan

applicationortherearesimplytoomanyapplicationsdemandingmemoryfortheamountofvirtual

memoryallocated.Toidentifyanapplicationlevelmemoryshortage,administratorscanlookinsidethe

guestOS,justastheywouldonatraditionalphysicalserver,startingwithWindowsTaskManager.

AdditionalFactors

to

Keep

in

Mind

when

Sizing

VM

Memory

MemoryHogApplications

Certainapplicationsaredesignedtobe"memoryhogs".No,thisisnotareferencetomalicious

applications.Instead,thereferenceistoapplicationsthatmaybequitefamiliartomostdatacenters,

suchasanExchangeServer,OracleDB,oraSQLdatabase.Theseapplications(andothers)aredesigned

toreserve(cache)allavailablememorywhentheystart.Whilethisworksfinewhentheapplicationis

runningonaphysicalserver,itisthemostinefficientwaytomanagememorywhentheapplicationis

runningasavirtualmachine. Checkwiththeapplicationvendortodeterminewhatthetruememory

needsareforthedeploymentasaVM,andsupplementthatinformationwithalookatthememory

activemetric

within

the

VM

to

determine

just

how

much

of

the

memory

is

being

actively

accessed.

Limits

Byconfiguringalimitonavirtualmachine,acapisplacedonthemaximumamountofmemorythatthe

VMcanuse.Withoutalimit,theallocatedmemoryisthelimitforthevirtualmachine.Limitsare

enforcedbyVMwareToolsthroughmemoryballooning,whichkeepstheVMfromutilizingmemory

abovethelimitsetting.

VMmemorylimitsareoftenputinplacebyadministratorwhomaynothavefullyunderstoodthepoor

performanceandwastedresourcesthatlimitscancreate.Whenlookingatmemoryutilization,these

memorylimits

skew

the

memory

metrics

and

can

cause

confusion

while

troubleshooting.

As

general

practice,memorylimitsarenotrecommendedbyVMwareexceptinrarecasesofseriousphysical

memorycontention.

Reservations

Memoryreservationssettheminimumamountofmemorythatavirtualmachine(orresourcepool)can

access.EventhoughthatmemorymaynotbeneededbytheVMatalltimes,areservation,ineffect,



10/17

pullsthatmemoryawayfromallothervirtualmachinesthatmayneedit.Likelimits,reservationsare

bettersetonresourcepoolsratherthanonindividualvirtualmachines,astheycanaffectthe

performanceofothervirtualmachinesandskewmemorymetrics.

KnowinganApplicationsNeeds

Takingthe

time

to

look

inside

the

virtual

machine

to

analyze

the

memory

that

an

application

uses

can

yieldagreatdealofinformationastothatVMsmemoryneeds.WhenevaluatingaWindowsoperating

system,ResourceMonitorandPerformanceMonitorcanberuntoexposewhichprocessesusememory

andhowmuchmemorytheyuse.Inaddition,consultingtheapplicationsbusinessownerhelps

determineotherpertinentinformation,includingwhenthisapplicationisused,whousesit,whatwould

happenifitwereunavailable,andhowtheuseoftheapplicationisgrowing.

Bytakingtimetounderstandthesefactors,VMadministratorscandrawfurtherinsightsintoproperly

sizingmemoryfortheVM,configuringresourcecontrols(ifneeded),andunderstandingthepriorityor

relativeimportanceoftheapplicationcomparedtoothers.

InstitutingaVMMemorySizingProcessNotonlyisavirtualenvironmentitselfdynamic,theusageoftheindividualapplicationsintheVMswill

alsobeinaconstantstateofflux.AsaVMwareadministratorworkingonacriticalproductionvirtual

infrastructurewithhundredsorthousandsofconstantlychangingvirtualmachines,theremustbea

formalprocessformemorysizingotherthanjusta"ruleofthumb"ortrialanderror.Thisprocess,

ideally,willinvolvetheapplicationadministratorsandwillbeundertakenbothwhenanewVMisbeing

createdforanapplicationandonaperiodicbasis.

Theworkflowdiagrambelowintroducesabestpracticeforhowtoexecutesuchaprocess.Whilethis

processmay

require

slight

modification

in

certain

situations,

it

will

work

as

is

for

most

VMware

administrators.ThenextsectionsfollowtheinitialNopath(notanewlydeployedVM)intheprocess

stepbystep,followedbycoverageoftheYespath.



11/17

Figure6TheVMMemorySizingWorkflow

DeterminingTime

Frame

for

Analysis

ItisratherobviousthatmemorysizingforaVMmustbedonewhenthevirtualmachineisfirstcreated,

butwhataboutatothertimesthroughoutitsuse?Wastingexpensiveserverresources,especially

memory,onoversizedvirtualmachinesdecreasesROIandmayacceleratetheneedtopurchase

additionalhardware.Ontheotherhand,slowperformingapplicationscausedbyundersizedVMswill

leadtoendusercomplaintsandcanaffectabusinessbottomline.Additionally,applicationusagecan

varydramaticallyoverthecourseofaday,weekormonth,makingitnecessarytoallocateforthepeak.



12/17

Frequentmemorysizingis,therefore,necessarybeyondanannualreview,orworse,leavingamemory

allocationasisfromthetimeofaVMscreation!

Herearethreetipsforperiodicmemorysizing:

UseRecentData Makesurethatthemostrecentmemorystatisticsarebeingused,aslastyear's

(or

even

last

months)

usage

may

no

longer

be

valid.

ChooseaTimeFrameThatCapturesPeakUsage MakesurethattheVMmemoryconfigurationvalueselectedcapturesthetimeofpeakusagethatwilloccurduringadesignated

timeperiodforthatapplication.Forexample,useadayforavirusscannerthatperformsadaily

scan,andamonthforanaccountingapplicationthatclosesthebookseverymonth.

ControlforSeasonality Forsomeorganizations,suchasuniversitiesorretailers,seasonalitymustbetakenintoaccountasdemandforresourceswillpeakduringstudentregistrationor

duringtheholidayshoppingseason,respectively.

FindingthePeakValue

Thisis

one

of

the

most

challenging

aspects

of

the

analysis.

Determining

aVMs

peak

memory

usage

cannotbereliablyaccomplished100%ofthetimeusingVMwaresmetricsalone. Thememoryactive

metricisanestimatedvalue,andevenwhenmeasuredatapeak,onlyindicatesthelevelofpage

activity. Itis,therefore,asignificantunderestimationofwhatavirtualmachinesmemoryneedsmay

actuallybe. Thememoryconsumedmetric, bycontrast,couldpotentiallybemorehelpful,butinmost

modernvirtualdeploymentsitoftenproducesthesamevalueasthememorygrantedmetric,resulting

inlittleornohelpfulinsight. Whetherduetolargememorypagesskewingthesemetrics,oragreedy

guestOSand/orapplication(s),memoryutilizationoftenrisestoabove95%nearlyallofthetime,and

increasingaVMsmemoryallocationwillnotdecreasethepercentageutilization.

Sowhat

peak

value

does

avirtualization

administrator

choose?

This

is

an

on

going

issue

for

VM

administratorseverywhere,andtheanswerisneverthesameinallcases. LookinginsidetheguestOS

canpotentiallygivedeeperinsight,butonlyifindividualprocessesaremonitored,andonlyiftheOS

deliversmetricsthatdistinguishbetweenwired,active,free,swap,etc. Sincepresentlythereisnoeasy

waytodothiswiththemetricsavailableinvCenteralone,performingthisassessmentatscalecan

becomequitechallenging. Untilmoremeaningfulmetricsareavailable,orVMwareToolsgaindeeper

OSmemoryinformation,determiningthepeakmemoryusageperVMwilllikelycontinuetobemoreof

anartthanascience.

Forthetimebeing,agoodpracticeistoexaminepeakmemoryconsumed. Ifthatvalueseems

abnormally

high

and

grows

equally

with

the

allocation

of

additional

memory,

then

check

within

the

OS

foranyactivediskswapping(independentlyofESXswapping).Ifnoneexists,thememoryconsumed

metricisnotveryinformative,andOSlevelanalysiswillberequired.

ComparingthePeakUsageValuetoAllocatedMemory

Thetruepeakvalueobservedshouldthenbecomparedtothememoryconfiguredforthevirtual

machine.Theprocessshouldtobe:



13/17

Takeapercentofthememoryusageatthepeakvs.allocated

Iflessthan75%,thenchangeslikelyneedtobemade

Ifgreaterthan75%,thenstartevaluatingapplicationgrowth

Forexample,iftheVMisconfiguredfor8GBand,basedonanalysis,thetypicalmemoryactiveforthat

VMis

around

4GB

(even

taking

long

term

peaks

into

account),

then

50%

memory

utilization

of

the

allocatedmemoryisthepeakvalue.Withthatratio,itislikelythatchangesneedtobemadetothe

configuration.However,additionalanalysismaybeneededtodetermineexactlywhattheallocation

shouldbeasdetailedinthestepsbelow.

ForecastingUsageChangesuntilNextMemoryVMAllocationReview

BeforedecidingtodownsizeaVM'smemoryconfiguration,itisimportanttoconsiderthefactthatthe

memorydemandsoftheVM(and,morespecifically,itsapplication)couldincreasebetweennowand

thenexttimethememorysizingprocessisconducted.

Hastheapplicationownerbeencontacted?Wasanincreasingtrendinmemorydemandnotedbasedon

historicalanalysis?Itisimportanttoensurethatsufficientmemoryisprovisionedforbothcurrentusage

andestimatedusagegrowth,oraVMcouldfacememoryrelatedperformanceissues.

FactorinaBufferValue

Besidesfactoringinanexpectedgrowthrate,addingabufferisimportanttoensurethatthememory

allocationissufficient.Whilehistoricalpeakshavelikelybeentakenintoaccount,itisstillagood

practicetofactorinabuffertoensurethatthevirtualmachine'smemoryhassomeheadroomtoavoid

problems.Forlesscriticalapplications,abuffermaynotbenecessary,andcanbeavoidedtopreserve

morememoryformoreimportantapplications.Forthemorecriticalapplicationsthatdohavedynamic

memoryrequirements,addinga25%additionalbufferontheamountofmemoryallocatedfortheVMis

recommended.Thisway,intheeventthatanunexpected,businesscriticaldemandisplacedon

memory,theadditionalmemoryisavailabletofulfillthatneed.

ItisimportanttokeepinmindthatcertainVMwarefeatures,suchastransparentpagesharingand

ballooning,helptooptimizememoryusagewithinapplicationsandclaimmemorywhenabsolutely

necessary.Thus,thereissomeleewayindeterminingwhattheallocationshouldbetobalancebetween

notwastingmemoryandnotunderprovisioningacriticalVM.

ChangetheVMSizingAllocationandDocumenttheChange

Nowthatthetimeframeforanalysishasbeendetermined,peakutilizationhasbeendeterminedand

comparedto

the

allocated

amount,

and

an

estimate

of

future

memory

usage

has

been

made,

it

is

time

totakeaction:adjustorrightsizethevirtualmachine'smemorysizing.

UnlessthecorrectguestOSisinstalledandmemoryhotaddisenabled(assumingthememory

allocationneedstobeincreased),theguestOSmustbeshutdowntoaddorremovememoryfroma

virtualmachine.



14/17

OncetheguestOSisshutdown,avirtualmachine'smemorycanberesizedbyrightclickingontheVM

(inthehostsandclustersinventory)andclickingonProperties.Atthatpoint,thewindowbelowappears

(seeFigure7)wherememorycanberesizedontheHardwaretab.

Figure7 ConfiguringMemoryonavSphereVM

OncetheVM'smemoryisresized,andtheVMispoweredbackon,thischangeshouldbedocumented

(alwaysagoodpractice!).

SettingUpaRegularMemoryReviewBasedonanAppropriateTimeframe

Becauseapplicationusagechanges,memoryallocationsmustbecontinuouslyevaluatedtoensurethat

performancewillnotbeimpactedduetoadditionalchangesinthedynamicvirtualizedinfrastructure.A

regularmemoryreviewprocessshould,thereforebesetatappropriateintervals.Therecommendation

hereisthatthisprocessoccuratleastoncepermonth;environmentsthataregrowingquicklymaywant

todothismorefrequently.Oncearightsizingprocesshasbeenconductedafewtimesonthevirtual

machinesinanyenvironment,thestepswillbecomemorefamiliarandadministratorscanthenbuildon

thisexperiencetostreamlinetheprocess.

ProvisioningaNewVMforAppropriateMemoryAllocationsThissectionbeginsthestepbystepcoverageoftheYespath(provisioninganewlydeployedVM)in

thememorysizingworkflowinFigure6.Theremaynotbeanyhistoricalperformancedataforanew

virtualmachine,unlesstheapplicationisverysimilartoanexistingVM.Forthisreason,thememory

configurationmadeforanewVMislikelytobemuchlessaccurate,andtheVMwillneedtobe

monitoredmuchmoreclosely.Still,followingthesizingprocedureoutlinedbelowwithfrequent

monitoringimmediatelyaftertheVMiscreatedwillprovidethenecessaryinsightstosizethememory

ofthatVMasaccuratelyaspossible.



15/17

InitiallyAllocatetheMinimumPossible

Inmanycases,therewillbesomeideaofwhatthememoryallocationforanewVMshouldbe.Perhaps

othersimilarVMsexistintheenvironment,oranotheradministratorwhohasrunthatapplicationina

virtualinfrastructurecanprovidelessonslearned.Evenifthisinformationisbothavailableand

dependable,theprocessofmonitoringandreviewingdescribedbelowshouldstillbefollowed.

WhencreatingabrandnewVMforanapplicationthathasnotbeenpreviouslydeployed,andthereis

noindicationaboutwhatthememoryallocationshouldbe,vSpherewillmakeamemory

recommendationwhentheVMiscreated,basedontheguestOSspecified.ThisisavailableinFigure7

above,whereitsays"MinimumrecommendedforthisguestOSis:512MB".Here,intheabsenceof

informationtothecontrary,anadministratormayaswellaccepttakethedefaultmemoryconfiguration

fortheguestOS.

Whileitmaybetemptingtogowitha"gutfeel"orusethesameamountofRAMthatphysicalservers

areconfiguredwith(e.g.justuse4GBor8GB),it'slikelythattheallocationmaybeoverallocatingthe

amount

of

memory

actually

required

owing

to

the

vSphere

memory

conservation

techniques

mentioned

above.

HerearethreeadditionalreasonsforprovisioningnewVMswithasmallermemoryallocationinitially:

Itistechnicallyeasiertoaddmemorythantotakeitaway Withcertainguestoperatingsystems,ifmemoryhotaddisenabled,RAMcanbehotadded(butthereisneveranoptionto

"hotremoveRAM).

TheOSandhypervisorwilloftenprovidedifferentmemoryusagedataTheOScanonlyreportonitsusageofthememorythatthehypervisorisproviding.Thismeansthatthememory

usagerateswilloftenbedifferentforthehypervisorandtheOS.Thesedifferencesmakeit

difficultto

pinpoint

exactly

how

much

memory

is

needed,

however,

and

can

cause

application

ownerstobecomeoverlyconcernedthereareinsufficientresourcestomaintainperformance.

Byslowlyaddingmemoryafterbeginningwithaminimalallocation,itwillbeeasiertoassess

theimpactsofchangestoreachtheoptimalamount.

Applicationownerconcerns,PartIIApplicationownersmaybecomeuneasyifmemoryisbeingtakenawayfromanapplication,therebymakingitmoredifficulttoremovethememory,

evenifthedatashowthattheallocationiscurrentlyoverprovisioned.

MonitorMemoryMetricsIntensely

OnceanewVMsmemoryhasbeenallocated,thatmemoryusageshouldbemonitoredintensely.Just

howfrequent

and

in

depth

this

monitoring

should

be

depends

on

the

criticality

of

the

VM.

AdministratorsmayalsowanttousevSpherealarmstoalertthemastoifandwhenanewVMis

runninglowonmemory.Whenmonitoring,lookbothatthevSphereVMmemorylevelandinsidethe

guestOStoseehowmuchmemoryisbeingutilized.IstheVMmaxingouttheminimumamountof

memoryconfigured?Ifso,thenmorememoryisneeded.



16/17

AddMoreMemoryinSmallIncrements

IfthenewVMisrunninglowonmemory,morememoryshouldbeaddedinsmallincrements.

Remember,memoryisnormallythemostconstrainedresourceaserverhas.AddingoneormoreGBof

RAMonaVMmaybewastefulandunnecessary.Whenmorememoryisneeded,itisbettertouse

incrementsof256MBor512MB.

AddtheNewVMtotheRegularMemorySizingReviewProcess

EverynewVMshouldbeincludedintheperiodicmemoryreviewprocess(discussedabove)afterthe

memoryusagehasbeeninitiallystabilizedwithanappropriatememoryallocation.

ConclusionMemoryistypicallythemostconstrainedcomputingresourceinavirtualizeddatacenter.Byleveraging

theinformationpresentedinthiswhitepaperandimplementingtheworkflowdetailedabove,

virtualizationadministratorscanmoreefficientlyusethisprecious(andcostly)computingresource.

Rightsizingmemory

allocations

makes

more

memory

available

for

other

VMs,

resulting

in

increased

VM

densityanddeferredpurchasesofhardware.Additionally,regularmonitoringofmemoryusageenables

VMadministratorstoproactivelydetectproblemareasinVMsthatareunderprovisionedtoavoid

performanceissues.Ultimately,accuratelysizingmemoryinvirtualmachinesresultsinabetterreturn

oninvestmentforanyvirtualizationinitiative,andapplicationownershavegreaterconfidenceinthe

performancethatvirtualizationprovidestheirapplications.



17/17

AbouttheAuthor

DavidDavis is the author of the bestselling VMware vSphere video training library from

Train Signal. He has written hundreds of virtualization

articles on the Web, is a vExpert, VCP, VCAPDCA, and CCIE

#9369 with more than 18 years of enterprise IT experience.His personal Website is VMwareVideos.com.

AbouttheSponsor

VKernel is the number one provider of performance and capacity management products for

virtualized data centers and cloud environments. The companys awardwinning, easy touse and powerful products simplify the complex and critical tasks of realtime VMware

performance monitoring, capacity planning, resource optimization, reporting and

chargeback for virtual environments. Used by over 50,000 virtualization administrators,

VKernels products have proven their ability to immediately identify and resolve VM

performance problems, maximize capacity utilization, and reduce virtualization costs.

http://www.trainsignal.com/VMware-Training.aspxhttp://www.vmwarevideos.com/http://www.vmwarevideos.com/http://www.trainsignal.com/VMware-Training.aspx

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