A disk image, in computing, is a computer file
containing the contents and structure of a disk volume
or of an entire data storage
device, such as a hard disk drive
, tape drive
, floppy disk
, optical disc
, or USB flash drive
. A disk image is usually made by creating a sector
-by-sector copy of the source medium, thereby perfectly replicating the structure and contents of a storage device independent of the file system
. Depending on the disk image format, a disk image may span one or more computer files.
The file format
may be an open standard
, such as the ISO image format for optical disc images, or a disk image may be unique to a particular software application.
The size of a disk image can be large because it contains the contents of an entire disk. To reduce storage requirements, if an imaging utility is filesystem-aware it can omit copying unused space, and it can compress
the used space.
Disk images were originally (in the late 1960s) used for backup
and disk cloning
of mainframe disk media. The early ones were as small as 5 megabyte
s 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
Disk images are used for duplication of optical media including DVDs, Blu-ray discs, etc. It is also used to make perfect clones of hard disks
A virtual disk
may emulate any type of physical drive, such as a hard disk drive, tape drive
, key drive
, floppy drive
, or a network share
among others; and of course, since it is not physical, requires a virtual reader device matched to it (see below). An emulated drive is typically created either in RAM
for fast read/write access (known as a RAM disk
), or on a hard drive. Typical uses of virtual drives include the mounting
of disk images of CDs and DVDs, and the mounting of virtual hard disks for the purpose of on-the-fly disk encryption
Some operating systems
such as Linux
[Although macOS's built-in DiskImageMounter software does not emulate a physical drive]
have virtual drive functionality built-in (such as the loop device
), while others such as older versions of Microsoft Windows
require additional software. Starting from Windows 8
, Windows includes native virtual drive functionality.
Virtual drives are typically read-only, being used to mount existing disk images which are not modifiable by the drive. However some software provides virtual CD/DVD drives
which can produce new disk images; this type of virtual drive goes by a variety of names, including "virtual burner".
Using disk images in a virtual drive allows users to shift data between technologies, for example from CD optical drive to hard disk drive. This may provide advantages such as speed and noise (hard disk drives are typically four or five times faster than optical drives, are quieter, suffer from less wear and tear, and in the case of solid-state drive
s, are immune to some physical trauma). In addition it may reduce power consumption, since it may allow just one device (a hard disk) to be used instead of two (hard disk plus optical drive).
Virtual drives may also be used as part of emulation of an entire machine (a virtual machine
Since the spread of broadband, CD and DVD images have become a common medium for Linux distribution
Applications for macOS
are often delivered online as an Apple Disk Image
containing a file system that includes the application, documentation for the application, and so on. Online data and bootable recovery CD images are provided for customers of certain commercial software companies.
Disk images may also be used to distribute software across a company network, or for portability (many CD/DVD images can be stored on a hard disk drive). There are several types of software that allow software to be distributed to large numbers of networked machines with little
or no disruption to the user. Some can even be scheduled to update only at night so that machines are not disturbed during business hours. These technologies reduce end-user impact and greatly reduce the time and man-power needed to ensure a secure corporate environment. Efficiency is also increased because there is much less opportunity for human error. Disk images may also be needed to transfer software to machines without a compatible physical disk drive.
For computers running macOS
, disk images are the most common file type used for software downloads
, typically downloaded with a web browser
. The images are typically compressed Apple Disk Image
(.dmg suffix) files. They are usually opened by directly mounting them without using a real disk. The advantage compared with some other technologies, such as Zip and RAR archives, is they do not need redundant drive space for the unarchived data.
Software packages for Windows
are also sometimes distributed as disk images including ISO image
s. While Windows versions prior to Windows 7
do not natively support mounting disk images to the files system, several software options are available to do this; see Comparison of disc image software
Virtual hard disks are often used in on-the-fly disk encryption
("OTFE") software such as FreeOTFE
, where an encrypted "image" of a disk is stored on the computer. When the disk's password is entered, the disk image is "mounted", and made available as a new volume on the computer. Files written to this virtual drive are written to the encrypted image, and never stored in cleartext
The process of making a computer disk available for use is called "mounting", the process of removing it is called "dismounting" or "unmounting"; the same terms are used for making an encrypted disk available or unavailable.
A hard disk image is interpreted by a Virtual Machine Monitor
as a system administrator using terms of naming, a hard disk image for a certain Virtual Machine monitor has a specific file.
Hard drive imaging is used in several major application areas:
*Forensic imaging is the process that involves copying the contents and recording an image of the entire drives contents (imaging) into a single file (or a very small number of files). A component of forensic imaging, indeed, involves verification of the values imaged to ensure the integrity of the file(s) imaged. Forensic images are created using software tools that can be acquired. Some tools have added forensic functionality previously mentioned; it is typically used to replicate the contents of the hard drive for use in another system. This can typically be done by software programs as it only structure are files themselves.
imaging is the process of imaging each sector, systematically, on the source drive to another destination storage medium from which required files can then be retrieved. In data recovery situations, one cannot always rely on the integrity of their particular file structure and therefore a complete sector copy is mandatory to imaging end there though. Forensic images are typically acquired using software tools compatible with their system. Note that some forensic imaging software tools may have limitations in terms of the software's ability to communicate, diagnose, or repair storage mediums that (often times) are experiencing errors or even a failure of some internal component.
Some backup programs only back up
user files; boot
information and files locked by the operating system, such as those in use at the time of the backup, may not be saved on some operating systems. A disk image contains all files, faithfully replicating all data, including file attribute
s and the file fragmentation
state. For this reason, it is also used for backing up optical media
s and DVD
s, etc.), and allows the exact and efficient recovery after experimenting
with modifications to a system or virtual machine
, in one go.
There are benefits and drawbacks to both "file-based" and "bit-identical" image backup methods. Files that don't belong to installed programs can usually be backed up with file-based backup software, and this is preferred because file-based backup usually saves more time or space because they never copy unused space (as a ''bit-identical'' image does), they usually are capable of incremental backups, and generally have more flexibility. But for files of installed programs, file-based backup solutions may fail to reproduce all necessary characteristics, particularly with Windows systems. For example, in Windows certain registry keys use short filename
s, which are sometimes not reproduced by file-based backup, some commercial software
uses copy protection
that will cause problems if a file is moved to a different disk sector
, and file-based backups do not always reproduce metadata
such as security attributes. Creating a bit-identical disk image is one way to ensure the system backup will be exactly as the original. Bit-identical images can be made in Linux
, available on nearly all live CD
Most commercial imaging software is "user-friendly" and "automatic" but may not create bit-identical images. These programs have most of the same advantages, except that they may allow restoring to partitions of a different size or file-allocation size, and thus may ''not'' put files on the same exact sector. Additionally, if they do not support Windows Vista
, they may slightly move or realign partitions and thus make Vista unbootable (see Windows Vista startup process
Rapid deployment of clone systems
Large enterprises often need to buy or replace new computer systems in large numbers. Installing operating system and programs into each of them one by one requires a lot of time and effort and has a significant possibility of human error. Therefore, system administrators use disk imaging to quickly clone the fully prepared software environment
of a reference system. This method saves time and effort and allows administrators to focus on each systems unique idiosyncrasies
they must bear.
There are several types of disk imaging software available that use single instancing technology to reduce the time, bandwidth, and storage required to capture and archive disk images. This makes it possible to rebuild and transfer
information-rich disk images at lightning speeds, which is a significant improvement over the days when programmers spent hours configuring each machine within an organization.
Legacy hardware emulation
s frequently use disk images to simulate the floppy drive of the computer being emulated. 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.
Copy protection circumvention
A ''mini image'' is an optical disc image file in a format that fakes the disk's content to bypass CD/DVD copy protection
Because they are the full size of the original disk, Mini Images are stored instead. Mini Images are small, on the order of kilobyte
s, and contain just the information necessary to bypass CD-checks. Therefore; the Mini Image is a form of a No-CD crack
, for unlicensed games, and legally backed up games. Mini images do not contain the real data from an image file, just the code that is needed to satisfy the CD-check. They cannot provide CD or DVD backed data to the computer program such as on-disk image or video files.
Creating a disk image is achieved with a suitable program. Different disk imaging programs
have varying capabilities, and may focus on hard drive imaging (including hard drive backup
, restore and rollout), or optical media
imaging (CD/DVD images).
A ''virtual disk writer'' or ''virtual burner'' is a computer program that emulates an actual disc authoring device such as a CD writer or DVD writer. Instead of writing data to an actual disc, it creates a virtual disk image. 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.
* Apple Disk Image
* IMG (file format)
* VHD (file format)
* VDI (file format)
RawWrite and WinImage are examples of floppy disk image
file writer/creator for MS-DOS
and Microsoft Windows
. They can be used to create raw image files from a floppy disk
, and write such image files to a floppy.
or similar systems
program can be used to create disk images, or to write them to a particular disk. It is also possible to mount and access them at block level using a loop device
Apple Disk Copy
can be used on Classic Mac OS
systems to create and write disk image files.
Authoring software for CDs/DVDs such as Nero Burning ROM
can generate and load disk images for optical media.
*Comparison of disc image software
*El Torito (CD-ROM standard)
, an archive file of an optical media volume
*Protected Area Run Time Interface Extension Services
External linksSoftware repository including RAWRITE2
Category:Compact Disc and DVD copy protection
Category:Computer file formats
Category:Disk image emulators
Category:Optical disc authoring