However, each physical machine also runs an Azure Host Agent, which runs on top of Windows Server Core. While the use of Server Core helps cut down on reboots, it doesn't eliminate them.
But then, why doesn't Hyper-V just stay up while it reboots the VM containing the Host Agent? Because the virtual disk drivers are implemented for Windows Server. (Hyper-V has the concept of a management OS, which has privileged access to the hardware.)
Source: http://blogs.technet.com/b/markrussinovich/archive/2012/08/2...
That's a pretty serious disadvantage over VMWare.
But I guess the limited attack area of Windows Server Core means many people can avoid applying patches much of the time.
By hooking up Hyper-V to the Windows management OS, they get to leverage Windows device drivers. And this, in turn, ensures broader hardware compatibility.
Of course, this goal became unnecessary for Azure, where Microsoft controls the hardware. But Hyper-V had already been designed that way.
Certainly, Server Core is pretty stripped-down. The Russinovich blog says they reboot about once a month, but the Azure support blog says they reboot every 2-3 months.
http://blogs.msdn.com/b/wats/archive/2013/09/24/windows-azur...
"When this occurs, you loose connectivity..."
"While this type of proactive notification of planned maintenance is not currently provided, we encourage you to provide comments on this topic so we can take the feedback to the product teams."
Wow. They didn't notify at one point?
Azure uses HyperV[1] as their virtualization technology. That isn't the same as Windows at all, although it is included in Windows Server.
While I have had more reboots on Azure than on AWS, it isn't weekly by any means - more like every 3 months or so. It hasn't been often enough for it to be a problem (low usage environment though, so YMMV).
[1] http://en.wikipedia.org/wiki/Hyper-V#Microsoft_Hyper-V_Serve...