Disclosed is a method, apparatus and a computer program of a virtual execution environment. In one aspect thereof a data processor includes a read-only storage medium and a virtual machine monitor configured to execute a program in a virtual container. Prior to execution of the application a set of application components accessed during a phase change are recorded in the read-only storage medium in a manner predetermined to minimize the time needed to retrieve the set of components. At the occurrence of the phase change, or prior to the occurrence of a phase change if the files are prefetched, the virtual machine monitor retrieves the components from the read-only storage medium. In a further aspect thereof there is provided a packager of an application to be executed in a virtualization environment by a virtual machine monitor. The packager is responsive to receiving notification from a virtual machine monitor of an occurrence of contemporaneous file accesses, not associated with a known phase change of a virtual container, to evaluate the occurrence as a candidate phase change, and responsive to the evaluation indicating a presence of a previously unknown phase change, for recording the files accessed on a new read-only storage medium in the order accessed for use by the same or a different virtual machine monitor during another instance of execution of the application.