Is it possible to pause a cycles render?

I wanted to render a very large resolution image in cycles but the render times were upwards of 20 hours for just the one frame. Is it possible to render for a few hours, freeze it by saving the rendered data and then continue rendering it at a later time?

Not as a native Cycles feature currently. Suspending the Blender process through the task manager / system monitor could provide a solution in some cases.