Devirtualizable virtual machines enabling general, single-node, online maintenance

Maintenance is the dominant source of downtime at high availability sites. Unfortunately, the dominant mechanism for reducing this downtime, cluster rolling upgrade, has two shortcomings that have prevented its broad acceptance. First, cluster-style maintenance over many nodes is typically performed a few nodes at a time, mak-ing maintenance slow and often impractical. Second, cluster-style maintenance does not work on single-node systems, despite the fact that their unavailability during maintenance can be painful for organizations. In this paper, we propose a novel technique for online maintenance that uses virtual machines to provide maintenance on single nodes, allowing parallel maintenance over multiple nodes, and online maintenance for standalone servers. We present the Microvisor, our prototype virtual machine system that is custom tailored to the needs of online maintenance. Unlike general purpose virtual machine environments that induce continual 10-20% over-head, the Microvisor virtualizes the hardware only during periods of active maintenance, letting the guest OS run at full speed most of the time. Unlike past attempts at virtual machine optimization, we do not compromise OS transparency. We instead give up generality and tailor our virtual machine system to the minimum needs of online maintenance, eschewing features, such as I/O and memory virtualization, that it does not strictly require. The result is a very thin virtual machine system that induces only 5.6% CPU overhead when virtualizing the hardware, and zero CPU overhead when devirtualized. Using the Microvisor, we demonstrate an online OS upgrade on a live, single-node web server, reducing downtime from one hour to less than one minute.