A disk image is a snapshot of a storage device's structure and data typically stored in one or more computer files on another storage device. Traditionally, disk images were bit-by-bit copies of every sector on a hard disk often created for digital forensic purposes, but it is now common to only copy allocated data to reduce storage space. Compression and deduplication are commonly used to reduce the size of the image file set. Disk imaging is done for a variety of purposes including digital forensics, cloud computing, system administration, as part of a backup strategy, and legacy emulation as part of a digital preservation strategy. Disk images can be made in a variety of formats depending on the purpose. Virtual disk images (such as VHD and VMDK) are intended to be used for cloud computing, ISO images are intended to emulate optical media and raw disk images are used for forensic purposes. Proprietary formats are typically used by disk imaging software. Despite the benefits of disk imaging the storage costs can be high, management can be difficult and they can be time consuming to create.