Manage Upgrades to the Azure Guest Operating System (Guest OS)
Updated: May 20, 2013
The following topics describe how to manage Windows Azure Guest operating systems (Guest OS) updates for your cloud services; explain which Windows Azure SDK versions are compatible with each Windows Azure Guest OS version; and provide a reference to Windows Azure Guest OS releases.
The Guest OS is the operating system that runs on the virtual machines (VMs) that host your services in Windows Azure. The Guest OS is updated periodically. You can allow the Guest OS for your service to update automatically each time a new update is released, or you can manually update it at a time of your choosing. All role instances defined in a particular service deployment will run on the Guest OS version that you specify. Microsoft recommends you allow the Guest OS to be updated automatically.
In addition, your service may be subject to updates to the underlying Windows Azure host operating system (Host OS) from time to time. These updates are mandatory and cannot be postponed.
All updates are applied using a new OS image to prevent any possible configuration drift. Updates recycle your running role instances, taking them through their shutdown sequences and then restarting them. They honor the update domains associated with your service. As long your service has at least two instances, the update will not cause any service downtime. However, an update will reduce the capacity your service can handle as each instance taken offline and restarted.