A system and method for creating N customized disk images from N identical disk images. A computing system creates N identical disk images from a master image. The computing system customize a first identical disk image according to a customization written in customization scripts. While customizing the first identical disk image, the computing system monitors block-level operations applied on the first identical disk image. The computing system applies the monitored block-level operations simultaneously to other N−1 identical disk according to similar customizations written in customization scripts. After applying the block-level operations, the N identical disk images become N customized disk images. Identical Virtual Machines (VMs) operating in the N identical disk images become N customized VMs by operating based on the N customized disk images.