Disk image

Disk images were originally (in the late 1960s) used for backup and disk cloning of mainframe disk media. Early ones were as small as 5 megabytes and as large as 330 megabytes, and the copy medium was magnetic tape, which ran as large as 200 megabytes per reel. Disk images became much more popular when floppy disk media became popular, where replication or storage of an exact structure was necessary and efficient, especially in the case of copy protected floppy disks. Disk image creation is called disk imaging and is often time consuming, even with a fast computer, because the entire disk must be copied. In Unix or similar systems the dd program can be used to create raw disk images. A virtual burner, by definition, appears as a disc drive in the system with writing capabilities (as opposed to conventional disc authoring programs that can create virtual disk images), thus allowing software that can burn discs to create virtual discs. ==Uses==

Digital forensics Forensic imaging is the process of creating a bit-by-bit copy of the data on the drive, including files, metadata, volume information, filesystems and their structure. Further, there are less issues with wear and tear. A hard disk drive or solid-state drive in a virtual machine is implemented as a disk image (i.e. either the VHD format used by Microsoft's Hyper-V, the VDI format used by Oracle Corporation's VirtualBox, the VMDK format used for VMware virtual machines, or the QCOW format used by QEMU). Virtual hard disk images tend to be stored as either a collection of files (where each one is typically 2GB in size), or as a single file. Virtual machines treat the image set as a physical drive. Rapid deployment of systems Educational institutions and businesses can often need to buy or replace computer systems in large numbers. Disk imaging is commonly used to rapidly deploy the same configuration across workstations. This image is then written to a computer's hard disk (which is sometimes described as restoring an image). Network-based image deployment Image restoration can be done using network-based image deployment. This method uses a PXE server to boot an operating system over a computer network that contains the necessary components to image or restore storage media in a computer. This is usually used in conjunction with a DHCP server to automate the configuration of network parameters including IP addresses. Multicasting, broadcasting or unicasting tend to be used to restore an image to many computers simultaneously. Digital preservation Libraries and museums are typically required to archive and digitally preserve information without altering it in any manner. Emulators frequently use disk images to emulate floppy disks that have been preserved. This is usually simpler to program than accessing a real floppy drive (particularly if the disks are in a format not supported by the host operating system), and allows a large library of software to be managed. Emulation also allows existing disk images to be put into a usable form even though the data contained in the image is no longer readable without emulation. == Limitations ==

Disk imaging is time consuming, the space requirements are high and reading from them can be slower than reading from the disk directly because of a performance overhead. When imaging multiple computers with only minor differences, much data is duplicated unnecessarily, wasting space. Speed and failure Disk imaging can be slow, especially for older storage devices. A typical 4.7 GB DVD can take an average of 18 minutes to duplicate. Floppy disks read and write much slower than hard disks. Therefore, despite their small size, it can take several minutes to copy a single disk. In some cases, disk imaging can fail due to bad sectors or physical wear and tear on the source device. Unix utilities (such as dd) are not designed to cope with failures, causing the disk image creation process to fail. When data recovery is the end goal, it is instead recommended to use more specialised tools (such as ddrescue). ==See also==