Wolf says customers may still have trouble moving applications out of VMware-based clouds, although the same could be said of many cloud services.
"It's easy on the surface to say 'you can move the VM to somewhere else,'" Wolf says. "Sure, but what about all of the storage? I might have terabytes of storage that are attached to a single cloud provider and that could be extremely problematic."
Determining how to get data out of one provider's cloud and onto another service provider platform has to be part of the service level agreement, Wolf says. "Otherwise that could be an enormous element of lock-in and makes it that much more difficult when you think about changing providers."
True interoperability among cloud platforms is perhaps several years away, says Gartner analyst Cameron Haight. There are several technical concerns, such as figuring out how "one cloud provider [can] consume the metadata associated with a virtual machine coming from another vendor," and thus understand its service requirements, he says.
VMware opening up the vCloud API makes it conceivable that a third party could superimpose the API onto a competing virtualization framework, although this might be difficult to achieve in practice, Haight says.
"We have a long way to go in the industry [with respect to cloud interoperability]," Haight writes in an e-mail. "Some current large cloud providers (Amazon, etc.) are not that involved in standards efforts. I tell Gartner clients that we are [expecting] at least a multi-year scenario for not only agreements on semantics and APIs to occur, but also for actual implementation to be realized."
Citrix CTO Simon Crosby says customers should be able to bridge the gap between internal and external clouds without worrying what type of hypervisor is being used. Enterprise customers shouldn't have "to ask the cloud vendor whose virtual infrastructure product was used to build the cloud," Crosby says.
Citrix and Xen.org, the maker of the open source Xen hypervisor, are pushing the "Xen Cloud Platform," an initiative designed to allow flexibility in which hypervisors are used to build virtual machines running on cloud services.
While VMware has a large number of cloud vendors on board, Xen.org can claim some momentum of its own with prominent cloud platforms such as Amazon and Rackspace using Xen. But Xen.org says it won't matter what hypervisor is in use, saying its goal is to achieve broad interoperability across virtualization platforms with standards such as the Open Virtualization Format, allowing "virtual appliances ... packaged in a hypervisor-independent format for easy transport between internal and external clouds with no vendor lock-in."
VMware CEO Paul Maritz has no plans to let VMware software manage hypervisors created by rival vendors, he said during a Q&A session with media. Maritz was speaking about vSphere software, rather than VMware's cloud partner program, but he noted that there is a tradeoff between supporting existing VMware customers and encouraging a more heterogeneous IT environment, he said.
Supporting rival virtualization products is "something we think about," he said. But "at this point in time we think our challenge is to help our existing customers, all 150,000 of them, do more with their environments and lift their management up."
VMware isn't opposed to changing course if the market demands it, however.
"If [supporting multiple hypervisors] became a significant customer demand, we'd look at it," Maritz said.
The different approaches taken by Citrix and VMware may come down to simple market realities. VMware is the dominant vendor in the x86 virtualization market, claiming 970 of the Fortune 1000 companies as customers. If service providers find it easier to support just one virtualization platform, they are likely to choose the most widely implemented technology -- VMware's. Citrix is a much smaller player and must support multiple hypervisors, including its own, Microsoft's, and VMware's, to create a successful cloud initiative.
"There's a business need for them to support multiple hypervisors that VMware doesn't have," Wolf says.