processor’scapacityisdividedamongthevirtualprocessorsascomputationsare
required.Thisworksinasimilarmannertothetimeslicingthatoccursbetween
applicationsinanoperatingsystemsharingtheprocessors.Asvirtualprocessorsneed
toperformcomputations,theyarescheduledonanavailablelogicalprocessor.
PriortoWindowsServer2012,therewasasupportedratioof 8 virtualprocessorsfor
every 1 logicalprocessor(8:1)forallworkloadsexceptforWindowsVDI
environments,wherearatioof12:1wassupported.Thiswasstipulatedtoensurethat
hostswerenotovercommittedintermsofvirtualprocessorsassignedtovirtual
machines.Forexample,witharatioof8:1,ifasystemhadatotalof 8 logical
processors,thenupto 64 vCPUscouldbeassignedintotalforallofthevirtual
machinesrunningonthathost.Notethatasinglevirtualmachinecanneverbe
assignedmorevirtualprocessorsthanthenumberoflogicalprocessorspresentinthe
server.Takingthesame 8 logicalprocessors,thismeansthatasinglevirtualmachine
couldnothavemorethan 8 virtualprocessorsassigned.However,Icouldhave 8
virtualmachines,allwith 8 virtualprocessors(oranyothersmallercombinations,
providingthetotaldoesnotexceed 64 virtualprocessors).Thesupportabilityratioof
virtualprocessorstologicalprocessorswasremovedinWindowsServer2012.Ifyou
testtheenvironmentanditworks,thenitwillbesupportedbyMicrosoft.Youstill
cannothavemorevirtualprocessorsinavirtualmachinethanlogicalprocessorsthat
existintheserver.AHyper-Vhostsupportsupto2,048virtualprocessors.
Eventhoughthesupportedratiohasbeenremoved,thisdoesnotmeanthatcareful
planningisnotrequiredwhenarchitectingyourHyper-Venvironment.Virtualization
cannotmagicallyenablemoreprocessingresourcesthanarephysicallyavailable.For
virtualmachineswithverylowCPUutilization,suchasaround 10 percent,planning
on 8 virtualprocessorsto 1 logicalprocessorwouldbefineandwouldyieldanaverage
utilizationofaround 80 percentonthephysicalcore.Ifvirtualmachineshavehigh
processorutilization,aratioof8:1wouldyieldpoorperformance,becausevirtual
machinesconstantlywaitforcyclesonthephysicalcores.
Someapplications,suchasSQLServerandExchange,havetheirownsupportedratios
ofvirtualprocessortologicalprocessor,whichcanbeaslowas1:1.Icoverthisin
moredetaillaterinthischapter.Becauseofthefairlylowadditionalperformancethat
hyperthreadingactuallyyields,though,Iprefertocountprocessorcoresonlywhen
thinkingaboutmyvirtual-to-physicalratios.IfIhaveaHyper-Vhostwith 4 processor
cores,Iwouldconsider 32 mymaximumnumberofvirtualprocessors,evenif
hyperthreadingwasenabled.Figure2.9showsahigh-levelviewofmappingof
physicalprocessorstocorestologicalprocessorstovirtualprocessors. Technet24.ir