Resuming virtual machines is an important management operation in the virtualized data center. (Re)-instantiating VMs as quickly as possible helps to combat flash crowds and provide true on-demand services. Unfortunately, resuming a VM currently takes tens of seconds, depending on the VM's size and available disk I/O bandwidth. Because it is not strictly necessary to read the entire state from disk to continue executing the VM instance, we propose an on-demand resume feature. Instead of indiscriminately reading the entire state from disk, we utilize the host OS's support for lazily reading pages from disk only when they are needed. We have implemented our idea as part of the open-source, virtual machine emulator qemu. Our idea required to modify less than 100 lines of code. The experimental evaluation of our on-demand resume feature reveals that it works as expected: resume times no longer depend on the VM's total state but rather on the actively used state. We are able to resume instances with multiple gigabytes of state in as little as 3 seconds, i.e., we reduced the resume time to less than 1/10th compared to the default implementation.