The amount of virtual disk space that is available for use by software executing within a virtual machine may be dynamically adjusted while the virtual machine is running in a virtual computer system. According to one aspect, a method for flexible run-time reservation of disk space from a virtual machine is provided. The method includes designating for use by a first virtual machine, a fixed portion of disk space accessible by a computing system. The first virtual machine is activated. During operation of the first virtual machine, a portion of the fixed portion of the disk space designated for use by the first virtual machine is reserved from within the first virtual machine. Use of the reserved portion is controlled from outside of the first virtual machine.