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LiveMigrationofVirtualMachines ChristopherClark,KeirFraser,StevenHand,JacobGormHansen y,EricJul y,ChristianLimpach,IanPratt,AndrewW UniversityofCambridgeComputerLaboratory yDepartmentofComputerScience 15JJThomsonAvenue,Cambridge,UKUniversityofCopenhagen,Denmark firstname.lastname@cl.cam.ac.ukfjacobg,ericg@diku.dkAbstractMigratingoperatingsysteminstancesacrossdistinctphys- icalhostsisausefultoolforadministratorsofdatacenters andclusters:Itallowsacleanseparationbetweenhard- wareandsoftware,andfacilitatesfaultmanagement,load balancing,andlow-levelsystemmaintenance. BycarryingoutthemajorityofmigrationwhileOSescon- tinuetorun,weachieveimpressiveperformancewithmin- imalservicedowntimes;wedemonstratethemigrationof entireOSinstancesonacommoditycluster,recordingser- vicedowntimesaslowas 60ms.Weshowthatthatour performanceissuftomakelivemigrationapractical toolevenforserversrunninginteractiveloads. Inthispaperweconsiderthedesignoptionsformigrat- ingOSesrunningserviceswithlivenessconstraints,fo- cusingondatacenterandclusterenvironments.Weintro- duceandanalyzetheconceptof writableworkingset ,and presentthedesign,implementationandevaluationofhigh- performanceOSmigrationbuiltontopoftheXenVMM. 1Introduction Operatingsystemvirtualizationhasattractedconsiderable interestinrecentyears,particularlyfromthedatacenter andclustercomputingcommunities.Ithaspreviouslybeen shown[1]thatparavirtualizationallowsmanyOSinstances torunconcurrentlyonasinglephysicalmachinewithhigh performance,providingbetteruseofphysicalresources andisolatingindividualOSinstances. Inthispaperweexploreafurtherallowedbyvir- tualization:thatofliveOSmigration.Migratinganen- tireOSandallofitsapplicationsasoneunitallowsusto avoidmanyofthediffacedbyprocess-levelmi- grationapproaches.Inparticularthenarrowinterfacebe- tweenavirtualizedOSandthevirtualmachinemonitor (VMM)makesiteasyavoidtheproblemof`residualde- pendencies'[2]inwhichtheoriginalhostmachinemust remainavailableandnetwork-accessibleinordertoservice certainsystemcallsorevenmemoryaccessesonbehalfof migratedprocesses.Withvirtualmachinemigration,on theotherhand,theoriginalhostmaybedecommissioned oncemigrationhascompleted.Thisisparticularlyvaluable whenmigrationisoccurringinordertoallowmaintenance oftheoriginalhost. Secondly,migratingatthelevelofanentirevirtualma- chinemeansthatin-memorystatecanbetransferredina consistentand(aswillbeshown)effashion.Thisap- pliestokernel-internalstate(e.g.theTCPcontrolblockfor acurrentlyactiveconnection)aswellasapplication-level state,evenwhenthisissharedbetweenmultiplecooperat- ingprocesses.Inpracticalterms,forexample,thismeans thatwecanmigrateanon-linegameserverorstreaming mediaserverwithoutrequiringclientstoreconnect:some- thingnotpossiblewithapproacheswhichuseapplication- levelrestartandlayer7redirection. Thirdly,livemigrationofvirtualmachinesallowsasepa- rationofconcernsbetweentheusersandoperatorofadata centerorcluster.Usershave`carteblanche’regardingthe softwareandservicestheyrunwithintheirvirtualmachine, andneednotprovidetheoperatorwithanyOS-levelaccess atall(e.g.arootlogintoquiesceprocessesorI/Opriorto migration).Similarlytheoperatorneednotbeconcerned withthedetailsofwhatisoccurringwithinthevirtualma- chine;insteadtheycansimplymigratetheentireoperating systemanditsattendantprocessesasasingleunit. Overall,liveOSmigrationisaextremelelypowerfultool forclusteradministrators,allowingseparationofhardware andsoftwareconsiderations,andconsolidatingclustered hardwareintoasinglecoherentmanagementdomain.If aphysicalmachineneedstoberemovedfromservicean administratormaymigrateOSinstancesincludingtheap- plicationsthattheyarerunningtoalternativemachine(s), freeingtheoriginalmachineformaintenance.Similarly, OSinstancesmayberearrangedacrossmachinesinaclus- tertorelieveloadoncongestedhosts.Inthesesituationsthe combinationofvirtualizationandmigration improvesmanageability. NSDI Õ05:2nd Symposium on Networked Systems Design & Implementation USENIX Association273
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