A virtual file system is formed configured to enable the dynamic composition of immutable file system images. A file system containing a software distribution is divided into a plurality of mutually exclusive sub-trees. Each sub-tree includes a portion of the software distribution. An immutable file system image is formed for each sub-tree. During the booting of an operating system, a virtualization engine intercedes in the boot process to mount the immutable file system images to independent directories of the root file system. Upon request the virtualization engine, during run-time, combines virtual entries corresponding to immutable file system images so as to resemble the original software distribution.