Network World — Disaster recovery has become table stakes in the world of server virtualization. Any good virtualization platform these days will find a way to restart a virtual machine in the event of a hardware failure. But which vendor excels more than any other at getting critical applications back online after failures, and making sure the most important virtual machines are given priority in the restart process?

VMware vs. Microsoft vs. Citrix

Debate has broken out over this topic since the Burton Group research and analysis firm declared that Microsoft's (MSFT) Hyper-V is not enterprise-ready because it lacks a specific feature found in both the VMware (VMW) and Citrix hypervisors. But Microsoft contends that Hyper-V does meet the core features customers are looking for, and even the Burton Group concedes that Microsoft has surpassed its rivals in certain types of disaster-recovery scenarios.

The feature in question is restart priority. According to the Burton Group, enterprise-class virtualization products must let IT administrators assign a restart priority to virtual machines, ensuring that the most critical workloads restart before any others in the event of a physical server outage.

Microsoft insists that its virtualization management tools allow this type of prioritization, if perhaps in a roundabout way, but the Burton Group has refused to give Hyper-V a final seal of approval, saying only VMware and Citrix allow this functionality.

The VM restart priority setting in VMware's High Availability 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, as administrator cannot set a restart order within the "high priority" bucket.

Citrix's XenServer provides a greater level of control and is 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 mission critical workloads come up first," he says of VMware's system. "Only those types of systems should be given high priority. Even if I had 10 VMs with high priority set, those 10 would all come up before any VMs set to medium or low priority. That's the point. Customers would like greater granularity with VMware's priority metric (XenServer's is better) and we've called that out in our vSphere assessment. Still VMware's behavior meets the minimum expectation of our criteria, while the XenServer implementation is the most ideal."

VMware counters that its Site Recovery Manager software does provide "strict ordering of VM restart," while conceding that its High Availability software does not.

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