Without a doubt, the introduction of Live Migration with Windows Server 2008 R2 was that release's most important virtualization feature. Live Migration lets you move a running virtual machine (VM) between Hyper-V hosts with no downtime. However, as with all new technologies, there are some questions and misconceptions about this new feature. Let's tackle some of the frequently asked questions about Live Migration.
Q: Is the new Cluster Shared Volumes (CSV) feature required for Live Migration?
A: No. Contrary to a popular misconception, Live Migration doesn't require the use of CSV. As Microsoft likes to phrase it, CSV facilitates Live Migration. It essentially allows multiple Hyper-V VMs to access the same set of VM files located on shared storage—typically a LUN on a local SAN. That makes it easy to set up Live Migration, because you can simply give each VM access to the shared storage. Performing live migrations with CSV is fast because both VMs can access the same stored files and only the memory needs to be moved between the Hyper-V hosts. Performing live migrations without CSV takes a bit longer because the storage must be moved between the nodes.
Q: Will Live Migration work with Linux VMs?
A: Yes. Live Migration works at the Hyper-V host level, so it's independent of the guest OS running in the VM. Live Migration works just as well with Linux-based Hyper-V guests as it does with Windows-based guests. In the lab, I've successfully used Live Migration with both the Ubuntu and openSUSE Linux distributions.
Q: Do you need to use Microsoft System Center Virtual Machine Manager (VMM) with Live Migration?
A: No. You don't need to use VMM, which is a great tool for managing multiple Hyper-V (and even VMware ESX Server) hosts and their respective VMs. You can use Server 2008 R2's Failover Cluster Manager with Live Migration. To do so, expand the cluster containing the Hyper-V nodes. Then expand Nodes and right-click the VM node you want to work with. Select the Move virtual machine(s) to another node option from the pop-up menu, as shown below.
Q: How does Live Migration handle different processor architectures?
A: It's important to realize that for Live Migration to work, the physical processors in the Hyper-V host servers need to be compatible. You can't perform a live migration between Hyper-V hosts that utilize different processor manufacturers. In other words, to perform a live migration, both Hyper-V hosts must have Intel processors or both must have AMD processors—you can't perform a live migration if one Hyper-V host has an Intel processor and the other Hyper-V host has an AMD processor.
The processors in the different Hyper-V hosts don't have to be identical, however. For example, one Hyper-V host can have an Intel Core 2 E6400 running at 2.13GHz and the other Hyper-V host might have an Intel Core 2 E8500 running at 3.16GHz. A Hyper-V feature called processor compatibility mode lets you perform live migrations between Hyper-V hosts with different processor families. The processor compatibility mode is turned off by default, but you can turn it on by opening the Hyper-V Manager and examining the VM's CPU properties. Select the Migrate to a physical computer with a different processor version check box, as shown below.
You can learn more about Hyper-V's processor compatibility in the Microsoft white paper "Windows Server 2008 R2 Virtual Machine Processor Compatibility Mode".