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 drive, floppy disk, optical disc, or USB flash drive. 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.
Disk image file formats may be open standards, such as the ISO image format for optical disc images, or proprietary to particular software applications.
Contents |
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.
Disk images are used heavily for duplication of optical media including DVDs, Blu-ray disks, 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 drive, tape drive, key drive, floppy drive, CD/DVD/BD/HD DVD or a network share among others. 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 ("OTFE").
Some operating systems such as Linux[1] and Mac OS X[2] have virtual drive functionality built-in (such as the loop device), while others such as Microsoft Windows require additional software.
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,[3] and also quieter). 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).
Some audiophiles argue that music played from a hard drive is superior to the same music played directly from CD,[4][5] due for example to lower data error rates.
Virtual drives may also be used as part of emulation of an entire machine (a virtual machine).
After the spread of broadband, CD and DVD images became a common medium for Linux distributions.[1] Applications for Mac OS X 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). Disk images may also be needed to transfer software to machines without a compatible physical disk drive.
Virtual hard disks are most commonly used in on the fly disk encryption ("OTFE") software such as FreeOTFE and TrueCrypt, where an encrypted "image" of a disk is stored on the PC. When you enter the disk's password, the disk image is "mounted", and made available as a new drive letter on your PC. Files written to this virtual drive are written to the encrypted image, and never stored in cleartext.
The process of making such an encrypted disk available for use is called "mounting", the process of removing it is called "dismounting" or "unmounting".
A hard disk image is interpreted by a Virtual Machine Monitor as a system hard disk drive. IT administrators and software developers administer them through offline operations using built-in or third-party tools. In terms of naming, a hard disk image for a certain Virtual Machine monitor has a specific file type extension, e.g., .vmdk for VMware VMDK, .vhd for Xen and Microsoft Hyper-V, .vdi for Oracle VM VirtualBox, etc.
Hard drive imaging is used in several major application areas:
There are two schemes predominant across all Virtual Machine Monitor implementations:
The virtual disk is implemented as either split over a collection of flat files, typically each one is 2GB in size, collectively called a split flat file, or as a single, large monolithic flat file. The pre-allocated storage scheme is also referred to as a thick provisioning [6] scheme.
The virtual disk can again be implemented using split or monolithic files, except that storage is allocated on demand. Several Virtual Machine Monitor implementations initialize the storage with zeros before providing it to the virtual machine that is in operation. The dynamic growth storage scheme is also referred to as a thin provisioning[6] scheme.
There are two modes in which a raw disk can be mapped for use by a virtual machine:
On computers running Mac OS X, disk images are now ubiquitous 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.
Software packages for Windows are also sometimes distributed as disk images including ISO images. While Windows computers (although this functionality was introduced in Windows "7") do not natively support mounting disk images to the files system, several software options are available to do this; see Comparison of ISO image software.
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, if they do not support Windows Vista, they may slightly move or realign partitions and thus make Vista unbootable (see Windows Vista startup process).
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.
Emulators 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 discs are in a format not supported by the host operating system), and allows a large library of software to be managed.
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 kilobytes, and contain just the information necessary to bypass CD-checks. Therefore, the Mini Image is a form of a No-CD crack, for pirated 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.[7][8] 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.[9]
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.
Software which can be used to write and access disk images include:
|
||||||||||||||
|
|||||||||||