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But experts know there is there is room for both, and perhaps room in the market for even more than two virtualization platforms. Some customers may choose one or the other, but others may deploy both, depending on the use case.
For an example, look at Brocade, the networking vendor. Brocade has moved numerous VMware workloads to Hyper-V, but still has VMware, Citrix and Oracle technology running on its servers, mostly HP blades.
"Hyper-V is just as viable as the VMware solution," says Troy Foo, manager of Brocade's Windows engineering team, explaining that Brocade upped its use of virtualization as part of a data center consolidation project.
Even after paying for Windows Server and System Center licenses, Brocade estimates that Hyper-V is about $23,000 cheaper per physical host, because of VMware's high cost and a Windows license that allows unlimited virtual machines per server.
Yet Brocade isn't ditching VMware. While Hyper-V makes sense for Windows servers, Brocade still uses VMware's vCenter and vSphere software for Unix workloads and others.
"The Unix guys are used to the Unix operating system on [VMware] ESX," says Doug Lind, Brocade's lead Windows architect. "They really like the model. It works for them. It makes sense to have the best hypervisor for the best fit."
"The world wants to think everything runs good on one product, but in reality that's not the case," Foo says.
In addition, Brocade uses technology from Oracle (formerly Sun's Solaris virtualization), has Citrix's XenServer running in development and testing, and the Linux-based KVM in a test setup. Brocade also has a virtual desktop deployment for remote workers.
For server virtualization, VMware and Microsoft are the most widely used. Brocade has VMware deployed on 400 physical servers and Hyper-V on 445 physical servers.
Whether it's VMware or Microsoft, Lind says there are some limitations with virtualization related to memory. Putting large databases and file systems into virtual machines is difficult and may not be as cost-effective as running on bare metal. Lind says a Brocade analysis determined any application with more than 16GB of memory is better off in its own server.
Although VMware had a huge head start over Microsoft in virtualization, Hyper-V has steadily improved to the point where it's feasible for many applications, Lind says. With additions such as memory oversubscription and instant failover, Lind says "there's no feature set, from my point of view, that we're missing from Hyper-V. Before that, there were a couple of growing pains."
Although Hyper-V does support Linux in addition to Windows, it's not really known for cross-platform support.
"VMware is the hypervisor of choice for our engineering team because they need the ability to support multiple OS platforms in order to be able to develop Brocade software for each of them," a company spokesperson said in an email. "Because we run all types and versions of vendor-based OS's, VMware provides us with the flexibility to do this efficiently."