CHAPTER4 MAKINGVIRTUAL INFRASTRUCTURES …media.techtarget.com/Syndication/NATIONALS/22367IntroToVirt4_FINAL.pdfvirtualnet-workadapter Anyserver role Any application GUESTFAILOVER

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HIGH-AVAILABILITYMETHOD

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CHAPTER 4

MAKINGVIRTUALINFRASTRUCTURESHIGHLYAVAILABLE

Making virtual machinesand the applications they

run highly availablecan protect production

workloads.

p3WHICH HA STRATEGY

IS BEST?

p4CHOOSING A HIGH-

AVAILABILITY METHOD

p8CASE STUDY

p 10PREPARING AN HA

VIRTUAL INFRASTRUCTURE

p 13GUEST NLB CLUSTERS

Introduction to Virtualization E-book

MakingVIRTUALINFRASTRUCTURES

Highly Available

SERVER VIRTUALIZATION offers organi-zations an opportunity to “green”their data centers by reducing theirphysical server footprint—sometimesby a factor of 20 to 1 or more. Afterimplementing server virtualization,enterprises can expand by virtualiz-ing other layers within the data cen-ter, including desktops and applica-tions. However, once you implementserver virtualization, you becomedependent on a single solutionbecause each host server will runmultiple production workloads.Organizations can’t afford to losethat server anymore; it has to run.But if it doesn’t, it must be able tohand off its workload to a stand-byhost so that production applicationsremain available at all times.Therefore it’s important to look

at the different ways to protect pro-duction workloads and ensure thatthey’re always available. There arethree different ways to make work-

loads that are contained within virtu-al machines (VMs) highly available:

e Create host clusters.r Create guest failover clusters.t Create guest server or network

load balancing (NLB) clusters.

Each of these options can providea specific level of availability to VMsand to the applications they run. Byusing high-availability (HA) clusterconfigurations, you can address anumber of potential issues in yourenvironment.

� You can protect the system fromhost failures.When a host fails, twothings can happen—either the VMsthat it runs will be moved off to otherhosts or else applications within theVMs will be moved to other VMs.Which scenario occurs depends onwhich of the three HA strategies youuse to protect a host.

CHAPTER4Making virtual infrastructures highly available

2 INTRODUCTION TO VIRTUALIZATION

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� You must performmaintenanceon host servers. Production applica-tions will continue to run becauseeither other VMs will support themor the VMs will be transferred toanother node on the host cluster.

� In the data center, you candynamically move application work-loads from one machine to another.This ensures that applications run atoptimal performance—even duringpeak load periods.

Each of these methods is availablewhen running a hypervisor; however,which method is required for eachapplication depends on its level ofimportance. In most organizations,nearly all users need access to finan-cial systems or email, but few userswill miss a network maintenanceserver that fails for a short period oftime. Consider each application typeto identify the best HA method.

WHICH HA STRATEGYIS BEST FOR YOUR VMS?Each HA strategy provides a validapproach for VM fault tolerance. Butit’s not always apparent which methodshould be used. The table on page 4,“Choosing a VM High-AvailabilityMethod,” outlines considerationswhen choosing and implementingan HA solution for production VMs.These guidelines can be helpful;

however, you should—at the very

least—create host failover clusters.Each host runs several productionVMs. If that host fails and no HAsolution exists, then each VM onthe host will fail. This is different

when you run single workloads inindividual physical machines. In sucha case, there is no reason why youcan’t run a host-level cluster whilesimultaneously running a guest-levelHA solution such as failover cluster-ing or NLB.Use these in concert with your

organization’s existing service-levelrequirements to determine whichlevel of HA you need to configure foreach VM. You also must consider thesupport policy of the application youintend to run in the VM.

WORKING WITH SINGLE-SITEAND MULTI-SITE CLUSTERSSingle-site and multi-site clusters areavailable for host servers. Single-siteclusters are based on shared storagein various forms. VMware, for exam-ple, uses two key technologies forhost clustering: HA and the VirtualMachine File System (VMFS), whichis a sharable file system that lets

Consider eachapplication typeto identify the bestHAmethod.

multiple host servers connect to thesame storage container. VMFS usual-ly requires some form of SAN, net-work-attached storage (NAS) or aniSCSI storage target. VMware canalso perform this via the Network FileSystem (NFS), which enables smallorganizations to access HA configu-rations for host servers. The HAVMware component then managespotential host server failures. VM-ware host clusters can include upto 32 nodes.

Citrix XenServer can also rely onshared storage—usually in the formof NFS, NAS, SAN or even an iSCSItarget to provide HA for host servers.In a Citrix host server environment,admins can create highly availableconfigurations by building host serverresource pools. While other hypervi-sors rely on management databasesto control multi-host configurations,each Citrix XenServer host stores itsown copy of the resource pool config-uration data. This removes a potential



VMCHARACTERISTICS

Operating systemedition (i.e., Win-dows Server)

Number ofguest nodes

Requiredresourcesin the VM

Potential serverrole

Internal VMapplication

HOST SERVERCLUSTERING

Web, Standard,Enterprise, orData CenterEditions

Single nodesonly

At least onevirtual net-work adapter

Any serverrole

Anyapplication

GUEST FAILOVERCLUSTERING

Enterprise or Data Center Editions

Generally two, but up to 16

iSCSI disk connectors and aminimum of three virtual networkadapters: cluster public, clusterprivate and iSCSI

Stateful application servers, file andprint servers, storage componentsfor collaboration servers, networkinfrastructure servers such as dy-namic host configuration protocols

SQL or database servers, Exchangeservers, message queuing servers,file servers, print servers

GUEST NLB CLUSTERING

Web, Standard, Enterprise,or Data Center Editions

Up to 32

Minimum of two virtual net-work adapters: cluster publicand cluster private

Stateless application servers,dedicatedWeb servers, front-end collaboration servers, front-end terminal servers

Web farms, Exchange clientaccess servers, Internet securityand acceleration server (ISA),VPN servers, streaming mediaservers, unified communicationsservers

CHOOSING A VMHIGH-AVAILABILITYMETHOD


single point of failure from resourcepool configurations. Citrix resourcepools can also include up to 32 hostnodes.Microsoft Hyper-V relies onWin-

dows Server 2008 Failover Clusteringto create host clusters. Single-siteHyper-V host clusters require sharedstorage in the form of SANs or iSCSItargets. No other storage format issupported. Hyper-V single-site clus-ters can include up to 16 host nodes.Hyper-V can also support multi-site

clusters, which span more than onesite to support disasters that mightaffect an entire site. Because of this,the Hyper-V multi-site cluster doesnot require shared storage and canrely on the much faster direct-attached storage (DAS) to operate.However, to provide VM high avail-ability, these DAS repositories mustbe synchronized at all times with athird-party replication tool.No matter which hypervisor you

use, it’s best to create host clusterswhen possible to provide two differ-ent levels of service continuity:

� Host clusters support continuousVM operation. If a host fails or indi-cates that it is failing, all VMs runningon that host will be transferred auto-matically to another node on thecluster.

� Host clusters support VM opera-tion during maintenance. If you needto work on one cluster node to install

software updates, for example, youcan move VMs off of the node duringoperation. Move VMs back to thenode once the operation is complete.Repeat this process if other clusternodes also require maintenance.

In either case, service will be inter-rupted while VMs are being moved.When the cluster detects that a nodeis failing, the cluster service causesVMs to fail over to another node. Inthis case, it will use a migration pro-cess to move the VM from one nodeto another. Depending on whichhypervisor you use, this may causea service interruption. VMware andCitrix, for example can perform livemigrations—those that occur whilethe VM is running. The first releaseof Hyper-V cannot perform livemigrations. However, Hyper-V R2,which is due out later this year, willsupport them.When a node completely fails,

the cluster service moves the VM by


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No matter whichhypervisor you use,it’s best to create hostclusters when possibleto provide two differentlevels of servicecontinuity.

(Continued on page 7)



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THERE’S A LOT of hubbub about live migration—the ability to move a VM from onehost server to another while it’s still running. Live migration is used in one oftwo instances:

q During operator-initiated live migrations, administrators can performmain-tenance on a host server.

q Dynamic live migrations can occur if a resource monitor detects that a VMis peaking, and the current VM host doesn’t have enough spare resources torespond to this peak. In this case, the resource monitor scans all hosts for amachine that has enough spare resources. It then moves the VM to this newhost.

In all cases, VMs are moved by migrating in-memory contents from one hostto another. Host servers must use similar processors. Otherwise, in-memorycontents won’t work on the target host.

Although live migration is quite useful in a dynamic data center, it’s completelyuseless in a host failure situation. This is because it’s impossible to obtain a host’sin-memory contents when it fails. During host failovers, VMs actually stopbecause the host has failed. VMs running on the failed host are then restartedon another host with spare resources. Therefore, HA is not live migration. �

LIVE MIGRATION VS. HIGH AVAILABILITY



restarting it on another node. Inthis case, VM downtime increasesbecause all of the VMs on that nodeare turned off. When you need to per-form maintenance on a node, use themigration process to move a VMfrom one host node to another.Remember that you must have

spare resources on each host serverin a cluster to support the move(FIGURE 1). Ideally, each host serverwill possess enough spare resourcesto support the failure of at least oneother node in the cluster.

WORKING WITH GUESTFAILOVER CLUSTERINGAny VM can be made highly availablewhen added as an application within

a host cluster. However, a VM is notlike a traditional application. Eventhough the VMwill always run—or

runs as much as possible—whenoperating on a host cluster, thismodel won’t apply to every workloadin your production network. This isbecause host server clustering doesnot affect applications containedwithin the VM. These applications

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(Continued on page 8)

FIGURE 1

Hosts in a cluster need spare resources to assume failing node workloads.

Any VM can be madehighly available whenadded as an applicationwithin a host cluster.However, a VM is not likea traditional application.

(Continued from page 5)

IRTUALIZATION USERSwhohave deployed high-availabil-ity (HA) software or clusteringto protect their virtual machines(VMs) from downtime know its

value, but also must be aware of itspitfalls.If you have 20 VMs running on a sin-

gle physical server, you don’t want all ofthem failing because of a host hardwarefailure. Implementing some type of HAsafeguard is an absolute must, accord-ing to John Humphreys, program vicepresident at IDC's Enterprise PlatformGroup."With virtualization, you are putting

five, six, 10 mission-critical servers onone physical machine,” Humphreys said.“When you do that, you need a servicethat promises high availability. Peoplewill be willing to pay a premium to guar-antee their VMs will be up and runningat all times.”At the basic level, HA software moves

VMs off of a crippled machine during acomponent failure and onto a designat-ed failover machine to avoid seriousdowntime. There are different levels ofHA that can be achieved through differ-ent types of software or through tradi-tional clustering.While HA software is simple to install

and run, it has some drawbacks, said

Rick J. Scherer, a systems administratorfor the non-profit San Diego Data Pro-cessing Corp. The IT organization usesVMware virtualization software in pro-duction and for disaster recovery acrossits two data centers. Scherer imple-ments both VMware HA andMicrosoftCluster service for HA.His production environment consists

of 29 host servers with enough CPU,memory and I/O to support a lot ofVMs, he said. As of April, Scherer wasrunning 400 VMs on those 29 hosts—about 14 VMs per host. He plans toadd 100 more virtual machines by theyear’s end.For the lowest level of HA, Scherer

runs VMware HA software within ESXclusters.With this HA solution, though, ad-

ministrators must monitor resources onhost servers to ensure there are enoughresources to host more VMs duringfailover. “We try to always have a 10%to 20% buffer on our systems so weknow we have enough resources,”Scherer said.Whether using an HA software

product or clustering method, it’simportant to eliminate single points offailure within the virtual environment.

—Bridget Botelho



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CASE STUDY:

HA is amust for virtualenvironments

V

are not aware of the host’s HA fea-ture—unlike applications that areinstalled directly into a clusterthrough guest failover clustering, forexample. Host server clustering does

ensure that the VMwill run if a hostfails (FIGURE 2). This HA model worksfor most applications, despite the factthat they aren’t aware of it when trans-fers occur from one node to another.Some state-sensitive applications,



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(Continued from page 7)

FIGURE 2

Internal applications are not aware of the status of a highly available VM.

HOSTSERVERS

HA TRANSFER

HOST CLUSTER

such as Microsoft Exchange, do notbehave properly under this model andmay lose data when a transfer occurs.

Transactional applications, especiallythose that support very high-speedtransactions, do not work well with



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EVEN THOUGH THERE are three potential HA scenarios for VMs, you still shouldinvest in host clusters. Your level of investment will depend on the hypervisoryou choose. Both VMware and Citrix hypervisors can rely on NFS storage to cre-ate host clusters. Microsoft Hyper-V, however, requires the use of either a SANor an iSCSI target. While SANs may be too costly for small to medium-sizedorganizations, iSCSI is not. For example, you could easily create a shared stor-age subsystem in support of any hypervisor with a standard server—including alarge amount of storage configured in a RAID. By adding iSCSI target software,you can turn the storage server into a low-cost, shared storage infrastructurethat can support a host failover cluster.

When your shared storage infrastructure is ready, follow the steps outlinedin the table to create a host cluster using one of three hypervisors: Citrix Xen-Server, Microsoft Hyper-V and VMware ESXi. VMs can be created and installedeither before or after you create the host cluster. But it’s important to remem-ber that the files that comprise a VMmust be located on the shared storagecontainer to enable each host in the cluster to access them. �

PREPARING AN HA VIRTUAL INFRASTRUCTURE

STEP Citrix XenServer Microsoft Hyper-V VMware ESXi

1 Install XenServer Install Windows Server Install ESXi Server2008 Server Core andenable the Hyper-V role

2 Connect hosts to Connect hosts to Connect hosts tothe shared storage the shared storage the shared storage

3 Create the resource Enable and configure the Create the HA clusterpool failover clustering feature

4 Make VMs highly Make VMs highly Make VMs highlyavailable available available

this model because they are designedto behave in a particular way whenfailover occurs. These applicationscannot behave as planned when aVM has been failed over.

Because of this, you should consid-er building highly available VMs—creating clusters within the VMlayer—to produce application-awareclusters. These clusters ensure con-tinuous availability and stability of theapplications you move into the virtuallayer of a resource pool. Failover clus-ters only work for stateful workloadsor workloads that record data fromuser sessions.Stateless workloads, or workloads

that provide read-only services, canrely on NLB. Like failover clustering,NLB is an HA solution that’s fully sup-ported in the virtual layer.To ensure the HA of stateful appli-

cations within virtual workloads,most organizations opt to run single-site clusters. These clusters are ofteneasiest to create in the virtual infra-structure and don’t require a replica-

tion engine, which often must be pro-cured from third-party sources. Whenyou create single-site guest clusters,you’ll need to consider a few keypoints:

� Use anti-affinity rules. If you cre-ate a two-node VM cluster to run ontop of a host cluster, you must makesure that the two nodes of the VMcluster aren’t located on the samenode as the host cluster. If both VMcluster nodes reside on the samenode as the host cluster and thatnode fails, then your entire VM clus-ter will fail. This will nullify any benefitof having created the VM cluster.Control VM locations on host nodesusing anti-affinity rules or by placingeach node of the VM cluster on dif-ferent host clusters. In Windows,anti-affinity rules are set using theCluster.exe command. Other hypervi-sors use different methods to setthese values.

� Rely on virtual LANs (VLANs).Rely on a hypervisor’s guest VLANcapabilities to segregate intra-clustertraffic required for the guest clusterfrom other network traffic. Each virtu-al network adapter in a VM can use adifferent VLAN setting.

� Rely on iSCSI storage. To targetshared storage for guest clusters, useiSCSI storage. This lets you createshared storage infrastructures thatrely on network interfaces to access



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Consider buildinghighly available VMS—creating clusters withinthe VM layer—toproduce application-aware clusters.

http://msdn.microsoft.com/en-us/library/aa369651(VS.85).aspx

the storage. VMs can easily consumeiSCSI shared storage since they onlyneed network adapters to access it.

Through these three approaches,you can configure single-site guestfailover clusters and enable them torun in the virtual layer of the resourcepool (FIGURE 3). Since you need a net-work for private cluster traffic, youalso need a network for iSCSI storage.A network for public end-user traffic

inside each VM in the cluster is alsorequired, and you must configure sev-eral virtual network adapters in theVM and on host servers.When a failure occurs on a host

server in which one node of the guestcluster is running, the second nodewill discover the guest VM failure. Itthen will automatically transfer theapplication within the VM to theother node in the guest cluster. Endusers won’t experience downtime



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FIGURE 3

Creating single-site guest failover clusters.

during the transfer (FIGURE 4).When an application running in

a failover cluster is moved from onenode to another, end users may expe-rience a delay in response time. How-ever, this delay usually lasts only afew microseconds—depending onthe application—and often goesunnoticed.

WORKINGWITH GUEST NLB CLUSTERSAlthough NLB is an HA solution, it isdifferent than failover clustering. In afailover cluster, only one node in thecluster runs a given service. Whenthat node fails, the service is passed

on to another node, which thenbecomes the owner of the service.This is because of the structure ofthe failover cluster model. Becauseof this model, only one node canaccess a given storage volume at atime. Therefore, the clustered appli-cation can only run on a single nodeat one time.In NLB or server load balancing

clusters, each member of the clusteroffers the same service. Users aredirected to a single IP address whenconnecting to a particular service.The NLB service then redirects usersto the first available node in the clus-ter. Because each member in the clus-



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FIGURE 4

Guest application failover during a host failure.

VMA is hosted onServer A

Server A fails;workload from VWAis failed over to VM B

on Server B

Virtual workloadsare highly availablebecause of built-inWindows features

ter can provide the same services,they are usually in read-only modeand considered stateless.NLB clusters are fully supported in

VMs because the hypervisor networklayer provides a full set of networkingservices, one of which is NLB redirec-tion. This means that you can createa multi-node cluster—up to 32 NLBnodes—to provide HA for the state-less services available in productionVMs. However, each computer par-ticipating in an NLB cluster shouldinclude at least two network adapters—one for management traffic andanother for public traffic. This can bedone in VMs by adding another virtu-al network adapter.When you run production services

in VMs, make sure that the config-uration is supported. Otherwise, youmay need to convert the VM into aphysical machine if issues arise. Afterthat, obtain support from the vendor.Resource pool administrators shouldconsider these configurations whenpreparing VMs.Supported configurations run

from standalone implementations onhost failover clusters to HA configu-rations at the guest level. Don’t forgetto keep a product’s licensing require-ments in mind when configuringVMs. Read support articles closelyto create the correct configurationsfor your network. If no support arti-cles exist, read product configurationdocumentation. �



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ABOUT THE AUTHORS

Danielle Ruest and Nelson Ruest are IT experts focused on continuous serv-ice availability and infrastructure optimization. They are authors of multiplebooks, including Virtualization: A Beginner’s Guide andWindows Server 2008,The Complete Reference for McGraw Hill Osborne, as well as theMCITP Self-

Paced Training Kit (Exam 70-238): Deploying Messaging Solutions with Microsoft Exchange Server 2007. Contactthem at [email protected].

Cathy Gagne, Editorial Director, [email protected]; Christine Casatelli, Managing Editor,[email protected];Michelle Boisvert, Features Editor, [email protected];MarthaMoore, Copy Editor, [email protected]; Linda Koury, Art Director of Digital Content,[email protected];Marc Laplante, Publisher, [email protected]©2009 TECHTARGET. ALL RIGHTS RESERVED.

mailto:[email protected]




http://www.microsoft.com/learning/en/us/books/10938.aspx

http://www.microsoft.com/learning/en/us/books/10938.aspx

http://www.amazon.com/Microsoft-Windows-Server-2008-Reference/dp/0072263652

http://www.amazon.com/Microsoft-Windows-Server-2008-Reference/dp/0072263652

http://www.amazon.com/Virtualization-Beginners-Guide/dp/007161401X

qDell™ High Availability Solutions Guide for Microsoft® Hyper-V™

qUsing System Center Virtual Machine Monitor 2008to Initiate VMware Vmotion

q System Center Virtual Machine Monitor with PRO Packs

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Documents

CHAPTER4 MAKINGVIRTUAL INFRASTRUCTURES …media.techtarget.com/Syndication/NATIONALS/22367IntroToVirt4_FINAL.pdfvirtualnet-workadapter Anyserver role Any application GUESTFAILOVER