Users of Suse Linux Enterprise Server (SLES) 9 and the upcoming Version 10 will be able to modify the SUSE kernel to allow a machine to run multiple "guest" operating systems. That is a coup for start-up Virtual Iron, which claims that its paravirtualization technology is faster than software emulators from companies such as market leader VMware Inc. -- though it requires users to make changes to the operating system and run the company's hypervisor software.

Virtual Iron's chief rival, XenSource Inc., still has an advantage because the processor tweaks needed to run its Xen hypervisor come installed with versions of Linux from Novell and Red Hat Inc. Xen will also be built into the kernels of SUSE Enterprise Linux Server 10 and Red Hat's Enterprise Linux 5, both of which are due out this year.

Both of those operating systems will also bundle the free Xen 3.0 hypervisor software, which allows for the creation of multiple virtual machines on a single piece of hardware. Virtual Iron's hypervisor, meanwhile, is not being bundled with SLES, though it can be purchased through Virtual Iron or its partners.

Lowell, Mass.-based Virtual Iron, which claims six unnamed enterprise customers, is betting that improved distribution combined with features absent from competitors will attract enterprise customers. Virtual Iron not only runs multiple virtual machines on a single box but can grow that virtual machine on demand to run on several boxes at once in a cluster or gridlike fashion, all connected via InfiniBand.

Mike Grandinetti, chief marketing officer at Virtual Iron, said the technology means applications don't have to be rewritten as they are moved to new machines -- perfect for J2EE applications that have spiky, peaky workloads.