In the majority of cases, VMware Inc.'s ESX hypervisor provides the plumbing to a virtual desktop infrastructure (VDI) implementation, regardless of which connection broker is used, said Michael Keen, an enterprise strategist at Alliance Technologies, an IT consultancy in Des Moines, Iowa, that consults on VDI implementations.
That could change as Microsoft Hyper-V gains traction and as Citrix Systems Inc. adds new features to XenServer, but in existing shops going the VDI route, ESX remains a commanding presence.
"It's what they've got, and they trust it and they don't want to mess around with another moving part," said Mike Palin, CEO at LeoStream, whose connection broker, like XenDesktop, works with VMware ESX and Citrix XenServer hypervisors.ESX management, performance trump cost
For some who use VMware as the back choice for VDI, the management and performance capabilities of VMware ESX still trump lower-cost alternatives.
Last year, Landon Winburn, a Citrix systems administrator at the University of Texas Medical Branch, chose Citrix XenDesktop 2 over VMware View 3. He said Citrix's ICA protocol, which has been updated to HDX, delivers a better end-user experience than Remote Desktop Protocol and PCoIP (or PC over IP), VMware's new protocol from Teradici. At the same time, he decided to stick to VMware ESX on the back end.
"We looked at XenServer, and even though it's free, we chose not to use it because when you start getting big, the management [capability] in XenServer is lacking," Winburn said.
Performance also plays a role in the decision to go with ESX, said Chris Wolf, senior analyst at Burton Group. ESX's memory overcommit feature provides better virtual machine (VM) densities in VDI environments than does XenServer: in the range of 25% to 40%. "That's what's been observed in the wild, by our customers," Wolf said.
For instance, a server with eight processing cores running VMware ESX might be configured with 64 single virtual CPU VDI sessions, Wolf said. The same server running XenServer might only run 30 to 40 XenDesktop sessions, he said.
IT architects could compensate for XenServer's lack of memory overcommit by adding extra memory to the server, but doing so adds cost to the configuration.
Given that VDI's total-cost-of-ownership argument is already difficult to make, "When you figure in the extra hardware cost, a lot of organizations say, 'That it's just not worth it,' " Wolf said.
Citrix has said it is working on memory overcommit in XenServer. Alliance Data's Keen said he wasn't expecting it in the upcoming XenServer 6, but in a subsequent version.
With the release of VMware View 4 this week, VMware's VM density advantage may increase further, wrote Brian Madden, an independent analyst tracking virtual desktops in his blog. That's because View 4 now supports vSphere and the new ESX 4.x hypervisor, which includes performance enhancements that can boost the number of recommended VDI sessions to 16 per processing core, up from 8 in ESX 3.x.
At the same time, VMware ESX is less compelling with Citrix's legacy XenApp, which by nature is memory intensive and thus does not use memory overcommit as heavily as VDI, said Burton Group's Wolf. In these environments, XenServer is an easier sell, "especially if there's a separate desktop support group in place," he said.VDI market still up for grabs
But given the small number of enterprise VDI deployments out there, ESX's dominance on the back end for VDI doesn't say much.
LeoStream's Palin estimates that the number of true enterprise VDI deployments stands at well under 1,500: about 500 deployments each for VMware and Citrix, and 200 for LeoStream.
"VDI's potential is staggering, but the reality is much smaller," Palin said. "There are lots and lots of [VDI] proof of concepts and trials and initial deployments of under 100 users," but when it comes to deployments of 1,000 seats or more, "it's just the tip of the iceberg."
And whether or not Citrix needs to sell XenServer to succeed with XenDesktop is unclear, he added. "They're the desktop experts, but they're also pragmatic and say, if you want to use ESX, go ahead," he said. "The lack of an installed base for XenServer doesn't indicate a lack of future success [for XenDesktop]."
Wolf concurred. "[Citrix] can lose the back end for now, and it's OK. What's important for them is to penetrate and control the front end, because in the end, that's a lot harder to switch out."
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