Microsoft has made Release Candidate 0 (RC0) of its Windows Server 2008 "Longhorn" operating system (OS) available to download, simultaneously previewing its "Viridian" server virtualisation hypervisor in a separate package.

The Windows Server Virtualisation (WSV) features were originally earmarked to appear in the final version of the OS, but were withdrawn earlier this year. They have now been made available in a community technology preview (CTP) that can be integrated into the OS as one of the server "rolls", which are a new feature of Windows Server 2008.

“We do not recommend you put the CTP into production yet, but many customers were desperate to get their hands on it [Viridian], and it is nice to synchronise these two releases together,” said Windows Server product manager Gareth Hall.

Details of WSV remain scarce, but it is anticipated it will be limited to 16 server cores. The hypervisor will support “multiprocessor guests, large memory allocation [with] more than 32GB per machine, and integrated virtual switch support that enables IT organisations to virtualise most workloads", Microsoft senior technical product manager Ward Ralston wrote on a Windows Server Division blog.

Hall said that the features included in Windows Server 2008 RC0 will be identical in the final version, expected in the first half of next year.

“The best way to describe it is feature complete. There will be no changes to the feature set, but it is not quite finished in terms of quality,” he said. “There is still some tidying up to be done and work happening with previous customers feeding back.”

Also last week, Microsoft released the beta version of its first service pack for Windows Vista. The pack will contain fixes, patches and updated device drivers for the desktop operating system. It is scheduled to be complete early in 2008.