Disaster recovery has become table stakes in the world ofserver virtualization. Any good virtualization platform these days will find away to restart a virtual machine in the event of a hardware failure. But whichvendor excels more than any other at getting critical applications back onlineafter failures, and making sure the most important virtual machines are givenpriority in the restart process?
Debate has broken out over this topic since the Burton Groupresearch and analysis firm declared that Microsoft’s Hyper-V is notenterprise-ready because it lacks a specific feature found in both the VMwareand Citrix hypervisors. But Microsoft contends that Hyper-V does meet the corefeatures customers are looking for, and even the Burton Group concedes thatMicrosoft has surpassed its rivals in certain types of disaster-recoveryscenarios.
The feature in question is restart priority. According tothe Burton Group, enterprise-class virtualization products must let ITadministrators assign a restart priority to virtual machines, ensuring that themost critical workloads restart before any others in the event of a physicalserver outage.
Microsoft insists that its virtualization management toolsallow this type of prioritization, if perhaps in a roundabout way, but theBurton Group has refused to give Hyper-V a final seal of approval, saying onlyVMware and Citrix allow this functionality.
The VM restart priority setting in VMware’s HighAvailability software lets IT assign VMs a priority of low, medium or high,with the high priority VMs starting first. But this is not a perfect tool, asadministrator cannot set a restart order within the “high priority”bucket.
Citrix’s XenServer provides a greater level of control andis therefore the best platform for this type of disaster recovery scenario,according to Burton Group analyst Chris Wolf.
“The idea behind priority is to ensure that missioncritical workloads come up first,” he says of VMware’s system. “Onlythose types of systems should be given high priority. Even if I had 10 VMs withhigh priority set, those 10 would all come up before any VMs set to medium orlow priority. That’s the point. Customers would like greater granularity withVMware’s priority metric (XenServer’s is better) and we’ve called that out inour vSphere assessment. Still VMware’s behavior meets the minimum expectation ofour criteria, while the XenServer implementation is the most ideal.”
VMware counters that its Site Recovery Manager software doesprovide “strict ordering of VM restart,” while conceding that itsHigh Availability software does not.
In any case, Wolf says his team at the Burton Group hasdiscussed the restart priority issue with Microsoft, and that Microsoftofficials “understand the use case and they understand why it’simportant.”
Microsoft tells a somewhat different story. “I’ve goneback and forth with Burton Group about this specific feature,” says EdwinYuen, a virtualization director at Microsoft. “Certainly we havealternatives, or ways around it.”
Hyper-V lets IT delay the restart of certain virtualmachines by a set time, 15 seconds, 30 seconds, or whatever amount of time ischosen. Delaying the restart of lower-priority virtual machines effectivelyallows the highest-priority ones to start up first, he says.
Customers can go even further in Microsoft’s System CenterVirtual Machine Manager, which lets IT write scripts defining which VMs restartfirst in the case of failure. Customers can also set rules preventing restartof certain virtual machines while back-end services are restarted. For example,if a Web application running in one virtual machine requires a SQL databasethat runs in another VM, Microsoft admins can require that the database startup before the application.
Moreover, Microsoft’s virtualization management tools canevaluate whether applications running inside virtual machines are healthy,instead of simply looking at whether the virtual machine itself is online.
Wolf agrees that “Where Microsoft has a decidedadvantage is application-aware high availability. That’s something we highlightas a real strength in the Microsoft solution that neither Citrix nor VMware canoffer.” VMware treats the virtual machine as a black box, so if anapplication inside a VM stalls, the company’s high availability product won’tdetect the problem unless there is a complete failure of the operating system,according to Wolf.
As Yuen puts it, Microsoft “can look at the VMs, theoperating systems and the services. We literally can tell ‘is the SQL databaseup and running? Has the mailbox service started?’ We can do a level ofdetection that VMware can’t do.”
This, combined with the other features Yuen described,should meet the requirements of customers as much, if not more than thespecific restart priority feature that the Burton Group considers to becrucial, Yuen says.
“I don’t believe that the feature requirement forrestart priority fulfills what the customer wants to do anyway. That’s myopinion,” Yuen says.
A few Network World readers expressed dissatisfaction withboth the Microsoft and VMware approaches in comments posted to a recent articletitled “It’s time to virtualize Microsoft Exchange (but not withHyper-V).”
“Setting a ‘high, medium, low’ priority is just as weakand unmanageable as setting a startup delay. Neither gives any sort ofguarantee that the service that you are relying on is actually available,”one reader commented. “Both ways are fragile and prone to failure and needto improve.”