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In this section of our guide on installing and managing Hyper-V in Windows Server 2008, we explore one of Hyper-V's standout management features: Live Migration, which enables the movement of virtual machines from one host to another without downtime.
Hyper-V Live Migration
Running virtual machines atop a Hyper-V host is great for optimizing your physical resources. Without virtualization, many Windows servers use only a mere portion of the hardware resources they're assigned. This means it is very likely that you're buying hardware that never gets used. Hyper-V changes all that by enabling many virtual machines to share a single host.
But while that sharing of resources means better optimization, it also presents the potential for greater loss when those hosts go down. If you've got 10 virtual machines running atop a Hyper-V host, losing that host means losing 10 of your business's critical services.
That's one of the reasons why Live Migration exists. Its process of migrating virtual machines allows an administrator to relocate VMs before having to take down a host for maintenance, patching, updates, or other work. Correctly configured, those same server migration tools can automatically reboot virtual machines onto surviving equipment after an unexpected Hyper-V host failure.
For the new virtual administrator, the under-the-covers actions during this live virtual machines migration process are not well understood. When you click to migrate a virtual machine, what actually gets transferred from one host to another?
First, using Live Migration requires the installation of the Windows Failover Clustering feature to each participating Hyper-V host. This service creates a multinode cluster between each host and installs the components that watch for failure, arbitrate for VM ownership between nodes, and generally ensure that Live Migration can occur.
The other piece that must generally be in place is an area of shared storage that can be accessed by each Hyper-V host in the cluster. This shared storage can be connected via Fibre Channel host bus adapters or through iSCSI connections using traditional network interface cards. In either case, it is most often created as an exposed LUN from a locally-available storage area network (SAN).Live Migration transfers the memory and running state of the virtual machine, but not its disk files. These files remain on the central shared storage and do not move during the Live Migration process. Using this process, the ownership of a virtual machine can be transferred in a small number of seconds and without impacting the operation of the virtual machine.
Once created and properly connected, the Windows Failover Clustering feature on each Hyper-V host then works with the others to monitor running virtual machines. At any point, an administrator can click within either the Failover Cluster Manager console or via the System Center Virtual Machine Manager (VMM) console to migrate running virtual machines from one host to another.
Greg Shields is an independent author, instructor, Microsoft MVP and IT consultant based in Denver. He is a co-founder of Concentrated Technology LLC and has nearly 15 years of experience in IT architecture and enterprise administration. Shields specializes in Microsoft administration, systems management and monitoring, and virtualization. He is the author of several books, including Windows Server 2008: What's New/What's Changed, available from Sapien Press.