As Mark Russinovich WinHEC 2007 keynote confirms, Windows Server 2008 (formerly codename Longhorn) introduces some remarkable kernel changes so that new Microsoft operating system now supports hot add of hardware without any downtime.
This feature, called Dynamic Hardware Partitioning (DHP), depends on Windows itself and it’s designed to work with processors, memory and some PCI Express cards like network cards (depending on availabily of DHP-aware drivers), regardless if added hardware is physical or virtual.
DHP is available inside OS since Windows Server 2003 Service Pack 1, where it’s limited to memory hot add. And same limitation will be present in Windows Server 2008 32bit versions.
Another big limitation (valid for all operating systems, 32 or 64bits) is Microsoft still doesn’t support hot remove of hardware (planned for future Windows versions), so that system administrators will still have to turn off the physical or virtual machine before removing a component.
So, in theory, every virtualization product on the market able to run a 64bit Windows Server 2008 guestOS will also be able to offer hot add capabilities Microsoft presented talking about upcoming Windows Server Virtualization hypervisor (codename Viridian).
And since first version of Viridian will not expose anymore hot add capability as previously announced, VMware and other virtualizatio providers have a chance to offer it before Microsoft itself.