The amount of virtual disk space available for use by software executing within a virtual machine (VM) may be dynamically adjusted while the VM is running in a virtual computer system. A method for reservation of disk space from a virtual machine is provided. A request is received at a first VM relating to reserving a portion of a virtual disk used by the first VM. In response, the first VM allocates additional storage in the virtual disk to a guest file stored in the virtual disk, wherein the guest file is not used to store meaningful data and then communicates sectors of the virtual disk corresponding to the additional storage to the virtualization layer. The virtualization layer provides to a second VM access to sectors of the physical storage space that correspond to the sectors of the virtual disk that were allocated as additional storage to the guest file.