Officials at VMware have poured scorn on IDC figures that suggest that VMware has lost some of its industry-leading market share to Microsoft. Instead, VMware claims that Redmond's Hyper-V product has barely made a dent in sales.

IDC stated last week that VMware's share of new x86 virtualisation software shipments was 44 percent in the second quarter, with Microsoft clocking in at 23 percent. VMware's lead in last year's second quarter was 51 percent to 20 percent, and its lead in the first quarter of 2008 was 42 percent to 18 percent, according to IDC.

IDC attributed Microsoft's growth to the general availability of Hyper-V, a notion VMware officials scoffed at. Hyper-V was made generally available on 26 June, with only a couple business days left in the second quarter, Mike DiPetrillo, a principal systems engineer at VMware, wrote in a blog posting that analyses the IDC report.

"So did Hyper-V really ship enough units in 2 days to get 23 percent market share? I doubt it," DiPetrillo writes.

DiPetrillo accused IDC of basing the 23 percent figure only on "unit shipments from the OEMs," but this does not appear to be true. The IDC survey did examine OEM vendors like HP and Dell who offer servers that have already been virtualised, but that was a separate comparison and there's no indication in IDC's report that the VMware/Microsoft market share comparison was limited to OEM sales.

IDC analyst Brett Waldman explained IDC's methodology in a phone interview Wednesday, saying the analyst firm determined market share by surveying more than 2,500 virtualisation users from 35 countries, examining public filings and having conversations with vendors. IDC's user survey looked at all new server virtualisation licences, regardless of whether they were sold through OEMs or other sources, he said.

On DiPetrillo's point that Hyper-V has only been generally available since 26 June, Waldman said numerous Microsoft customers were using Hyper-V in production before then through an early adoption program. Most of Microsoft's virtualisation market share comes from its pre-existing product, Virtual Server 2005, but Waldman said he expects Hyper-V's market share to continue rising.

DiPetrillo also questioned how IDC counted shipments of Microsoft's Virtual Server 2005, given that it is a free download off Microsoft's website. Waldman said the IDC survey only counted new licences if they were actually put into production.

Waldman denied another assertion by DiPetrillo, who claimed the IDC survey - which is part of the firm's Worldwide Quarterly Server Virtualization Tracker - was sponsored by a "competitor in Redmond" - i.e. Microsoft.

"The Server Virtualization Tracker is not a sponsored product. It is available on a subscription basis to all of our clients," Waldman said.

DiPetrillo, in a phone interview, said VMware didn't subscribe to the Server Virtualization Tracker and thus were not given a chance to offer input before numbers were published. "IDC has never contacted VMware for its actual shipment numbers," DiPetrillo said. "Since we didn't subscribe to the Virtualization Tracker we don't get input into it."

The subscriptions essentially are the same as a sponsorship, he argued. But DiPetrillo acknowledged he doesn't know for a fact that Microsoft subscribed.

In his blog, DiPetrillo also claims that IDC failed to count usage of VMware Server, a freely downloadable product. This appears to be untrue, as IDC specifically stated that VMware's market share includes both the ESX hypervisor and VMware Server. DiPetrillo said "I'll be happy to post corrections on the blog if there are corrections to be made."

DiPetrillo's blog states that it contains his personal opinions, and is not hosted by VMware or vetted by anyone at the company.

VMware's internal public relations team urged reporters to read his blog post, however, and said VMware disagrees with IDC's findings.