The subject matter of this specification can be implemented in, among other things, a method that includes receiving, from within a guest operating system hosted by a host operating system at a computer system, requests to access sparse files within a guest file system of the guest operating system. The sparse files each correspond to an external data file outside the guest file system. Each of the requests to access the sparse files within the guest file system is directed to the corresponding external data file outside the guest file system. The method includes identifying a frequency with which each of the sparse files is accessed. The method includes moving an external data file from a first type of storage device to a second type of storage device based on the frequency with which the external data file is accessed relative to others of the external data files.