One of the new features of vSphere 5.1 was upgrade procedure of the VMware Tools on Windows VMs: when you upgrade them the first time to version 5.1 (the right version number it’s 9.x), other upgrade will not require any reboot of your Windows system. For Linux VMs there is no need to reboot the system (but I suggest in order to be sure that the kernel modules are loaded proprerly), and now also Windows VMs could have a zero downtime upgrade. But it’s really true?