Disk image

A disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, tape drives, floppy disk, CD/DVD/BD and key drive, although an image of an optical disc may be referred to as an optical disc image. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfectly replicating the structure and contents of a storage device.

Some disk imaging utilities omit unused file space from source media, or compress the disk they represent to reduce storage requirements, though these are typically referred to as archive files, as they are not literally disk images.

Use

History

Disk images were originally used for backup and disk cloning of floppy disk media, where replication or storage of an exact structure was necessary and efficient.

Software distribution

On Macintosh computers running Mac OS X, disk images are now ubiquitous for software downloads. Typically, when a user initiates a download of a software package (often an application or printer/scanner driver), the web browser will download a compressed disk image file (.dmg suffix), which is saved automatically to the user's Downloads folder. Usually the disk image file will then automatically open, creating a mounted volume which appears on the user's Desktop - this volume contains the desired installer or other software. (If the disk image does not automatically open and mount, this can be initiated by double-clicking the disk image file.) Frequently, the installation is done by drag-and-drop, where the software is displayed beside an alias to the destination folder; the user completes the installation by dragging the software to the alias. Sometimes an installer package on the mounted volume will auto-launch (the OS prompts for an administrator password before proceeding with the installation). Once the installation is done, the mounted volume can be "ejected". The disk image file itself can then be deleted, or retained for future use.

While Windows computers (at least those running Windows XP) do not natively support mounting disk images to the files system, several software options are available to do this.

System backup

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. For this reason, it is also used for backing up CDs and DVDs.

Non-software type files 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 software files, file-based backup solutions may fail to reproduce all necessary characteristics, particularly with Windows systems. For example, in Windows certain registry keys use short filenames, 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 with dd, available on nearly all live CDs.

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, they may slightly move/realign partitions and thus make Windows Vista unbootable, if they do not support Vista.

Rapid deployment of clone systems

Large enterprises often need to buy or replace new computer systems in large numbers. Installing operating system and programs in to 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 unique distinctions that each system must bear.

Imaging process

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).

Hard drive imaging

Hard drive imaging is used in several major application areas:

Forensic imaging or acquisition is the process where the entire drive contents are imaged to a file and checksum values are calculated to verify the integrity (in court cases) of the image file (often referred to as a “hash value”). Forensic images are acquired with the use of software tools. (Some hardware cloning tools have added forensic functionality.)
Drive cloning, as previously mentioned, is typically used to replicate the contents of the hard drive for use in another system. This can typically be done by software-only programs as it typically only requires the cloning of file structure and files themselves.
Data recovery imaging (like forensic imaging) is the process of imaging every single sector on the source drive to another medium from which required files can be retrieved. In data recovery situations, one cannot rely on the integrity of the file structure and therefore a complete sector copy is mandatory (also similar to forensic imaging). The similarities to forensic imaging end there though. Forensic images are typically acquired using software tools such as EnCase and FTK. However, forensic imaging software tools have significantly limited ability to deal with drives that have hard errors (which is often the case in data recovery and why the drive was submitted for recovery in the first place).

Data recovery imaging must have the ability to pre-configure drives by disabling certain attributes (such as SMART and G-List re-mapping) and the ability to work with unstable drives (drive instability/read instability can be caused by minute mechanical wear and other issues). Data recovery imaging must have the ability to read data from “bad sectors.” Read instability is a major factor when working with drives in operating systems such as Windows. A typical operating system is limited in its ability to deal with drives that take a long time to read. For these reasons, software that relies on the BIOS and operating system to communicate with the hard drive is often unsuccessful in data recovery imaging; separate hardware control of the source hard drive is required to achieve the full spectrum of data recovery imaging. This is because the operating system (through the BIOS) has a certain set of protocols or rules for communication with the drive that cannot be violated (such as when the hard drive detects a bad sector). A hard drive’s protocols may not allow “bad” data to be propagated through to the operating system; firmware on the drive may compensate by rereading sectors until checksums, CRCs, or ECCs pass, or use ECC data to recreate damaged data.

Data recovery images may or may not make use of any type of image file. Typically, a data recovery image is performed drive to drive and therefore no image file is required.

File formats

In most cases, a file format is tied to a particular software package. The software defines and uses its own, often proprietary, image format, though some formats are widely supported by competing products. An exception to proprietary image formats is the ISO image for optical discs, which collectively includes the ISO 9660 and Universal Disk Format (UDF) formats, both defined by open standards. These formats are supported by nearly all optical disc software packages.

External links

Back-up-Any-Hard-Drive

v t e Disk image file formats
Comparison of disc image software
Optical discs	BIN+CUE DAA IMG+CCD+SUB ISO ISZ MDF+MDS MDX NRG UIF
Hard disks	DMG FVD IMG ISO NDIF QCOW UDIF VDI VHD VMDK WIM
Floppy disks	ADF, ADZ DC42, DART DMS IMG, IMA, IMZ VFD
CDDA	Disc Description Protocol
Convention: Any item in this table that has the form of "A+B" or "A+B+C" indicates a disk format that spans multiple files, where A contains the bulk of the data, and B and C are sidecar files.

Use