Virtual machine preserving host updates for zero day patching in public cloud

Host software updates are critical to ensure the security, reliability and compliance of public clouds. Many updates require a virtualization component restart or operating system reboot. Virtual machines (VMs) running on the updated servers must either be restarted or live migrated off. Reboots can result in downtime for the VMs on the order of ten minutes, and has further impact on the workloads running in the VMs because cached state is lost. Live migration (LM) is a technology that can avoid the need to shutdown VMs. However, LM requires turn space in the form of already-patched hosts, consumes network, CPU and other resources that scale with the amount of and level of activity of VM, and has variable impact on VM performance and availability, making it too expensive and disruptive for zero-day security updates that must be applied across an entire fleet on the order of hours. We present a novel update technology, virtual machine preserving host updates (VM-PHU), that does not require turn space, consumes no network and little CPU, preserves VM state, and causes minimal VM blackout time that does not scale with VM resource usage. VM-PHU persists the memory and device state of all running guest VMs, reboots the host and virtualization components into updated code, restores the state of the VMs, and then resumes them. VM-PHU makes use of several techniques to minimize VM blackout time. One is to use kernel soft reboot (KSR) to directly transition to an updated host operating system, bypassing firmware reset of the server and attached devices. To minimize resource consumption and VM disruption, VM-PHU leaves VM memory in physical memory pages and other state in persisted pages across the soft reboot, and VM-PHU implements a mechanism called fast close to enable a reboot to proceed without waiting for the completion of in-flight VM I/Os to remote storage devices. We have implemented VM-PHU in Microsoft Azure hosting millions of servers and show results of several zero-day updates that demonstrate VM blackout times on the order of seconds. VM-PHU provides significant benefits to both customers and public cloud vendors by minimizing application downtime while enabling fast and resource efficient updates, including zero-day patches.