One of the interesting things we discovered during tests is this: You can't overcommit physical memory resources for guests in a standalone Hyper-V environment. Hyper-V instead tallies allocated memory for all local guests, whether they're running or not, erring on the side of caution. In contrast, VMware and Xen give administrators the option of overprovisioning system resources on single hosts, on the assumption that all host virtual machines in a system won't need to run maximum resources simultaneously.
Although we don't advocate overextending your hardware, this is a useful capability for reallocating resources in emergencies. For example, if your virtual server environment has four host servers, and one dies, overcommitting enables your setup to temporarily run four hosts' worth of virtual machines on three hosts.
VMware has VMotion and Citrix has XenMotion, both of which provide for live migration of virtual machines. Microsoft, on the other hand, has Quick Migration, which isn't live and is slower than VMotion or XenMotion. Despite early feature-set promises, Hyper-V doesn't support true live migration of a virtual machine from one physical host to another on any version of Windows Server 2008. Instead, the Microsoft Quick Migration feature allows a centrally stored VM to be suspended or shut down, then restarted on a new host.
The consensus is that Microsoft opted for Quick Migration to get Hyper-V to market as soon as possible while satisfying basic functionality needs and enabling clustering and high-availability options for Win2008 Enterprise and Datacenter customers as a bundled perk. But whatever the corporate strategy, no live migration means little or no service or maintenance on Hyper-V hosts during business hours--some Hyper-V admins will need to put in time during off-hours to handle these chores. This will make Hyper-V a nonstarter for many shops, leading them toward Xen or VMware.
Quick Migration times can range from seconds to minutes for restoration of all services in a complex environment--you must take a server VM offline to move it to a second machine with Hyper-V. If you have ungainly apps or complex environments, they'll take awhile to get rolling. We expect some version of live migration in future releases of Hyper-V, so prospective hypervisor customers and vendors should keep an eye out. But for the time being, the bottom line is Windows' low cost and familiarity versus competitors' industrial strength and convenience.
Hyper-V's high availability is less of a pain point than its hobbled VM migration functionality. As with the latest version of XenServer Enterprise, Windows Server 2008 running Hyper-V bundles high-availability functionality for clustered hosts. And since clustering is now bundled with 2008 Enterprise and Datacenter, cost isn't an issue.When we conducted our "yank out the power" test, Hyper-V came through with flying colors, restarting guests on other cluster hosts with minimal disruption.It isn't an instantaneous failover, but the minimal outage window is more than acceptable given the product's zero cost.