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. [1] [2]
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. [3] [4] Compression and deduplication are commonly used to reduce the size of the image file set. [3] [5]
Disk imaging is done for a variety of purposes including digital forensics, [6] [2] cloud computing, [7] system administration, [8] as part of a backup strategy, [1] and legacy emulation as part of a digital preservation strategy. [9] 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, [10] [11] ISO images are intended to emulate optical media [12] and raw disk images are used for forensic purposes. [2] Proprietary formats are typically used by disk imaging software.
Despite the benefits of disk imaging the storage costs can be high, [3] management can be difficult [6] and they can be time consuming to create. [13] [9]
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. [14] 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. [13] Typically, disk imaging requires a third party disk imaging program or backup software. The software required varies according to the type of disk image that needs to be created. For example, RawWrite and WinImage create floppy disk image files for MS-DOS and Microsoft Windows. [15] [16] In Unix or similar systems the dd program can be used to create raw disk images. [2] Apple Disk Copy can be used on Classic Mac OS and macOS 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. 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. [17] [18] 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. [19]
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. [2] Often, these images are also hashed to verify their integrity and that they have not been altered since being created. Unlike disk imaging for other purposes, digital forensic applications take a bit-by-bit copy to ensure forensic soundness. The purposes of imaging the disk is to not only discover evidence preserved in digital information but also to examine the drive to gather clues of how the crime was committed.
Creating a virtual disk image of optical media or a hard disk drive is typically done to make the content available to one or more virtual machines. Virtual machines emulate a CD/DVD drive by reading an ISO image. This can also be faster than reading from the physical optical medium. [20] 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.
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. [8] Disk imaging software is used to create an image of a completely-configured system (such an image is sometimes called a golden image). [21] [22] This image is then written to a computer's hard disk (which is sometimes described as restoring an image). [23]
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. [24] 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. [24] [23] These approaches do not work well if one or more computers experience packet loss. [23] As a result, some imaging solutions use the BitTorrent protocol to overcome this problem.
Network-based image deployment reduces the need to maintain and update individual systems manually. Imaging is also easier than automated setup methods because an administrator does not need to have knowledge of the prior configuration to copy it. [23]
A disk image contains all files and data (i.e., file attributes and the file fragmentation state). For this reason, it is also used for backing up optical media (CDs and DVDs, etc.), and allows the exact and efficient recovery after experimenting with modifications to a system or virtual machine. Typically, disk imaging can be used to quickly restore an entire system to an operational state after a disaster. [25]
Libraries and museums are typically required to archive and digitally preserve information without altering it in any manner. [9] [26] 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. [12]
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. [3]
Other limitations can be the lack of access to software required to read the contents of the image. For example, prior to Windows 8, third party software was required to mount disk images. [27] [28] When imaging multiple computers with only minor differences, much data is duplicated unnecessarily, wasting space. [3]
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. [9] 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. [12] Unix utilities (such as dd) are not designed to recognize or cope with failures causing the disk image creation process to fail. [26] When data recovery is the end goal, it is instead recommended to use more specialised tools (such as ddrescue).
Computer data storage or digital data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
Disk storage is a data storage mechanism based on a rotating disk. The recording employs various electronic, magnetic, optical, or mechanical changes to the disk's surface layer. A disk drive is a device implementing such a storage mechanism. Notable types are hard disk drives (HDD), containing one or more non-removable rigid platters; the floppy disk drive (FDD) and its removable floppy disk; and various optical disc drives (ODD) and associated optical disc media.
A floppy disk or floppy diskette is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk. The three most popular floppy disks are the 8-inch, 5¼-inch, and 3½-inch floppy disks. Floppy disks store digital data which can be read and written when the disk is inserted into a floppy disk drive (FDD) connected to or inside a computer or other device.
A live CD is a complete bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than loading from a hard disk drive. A live CD allows users to run an operating system for any purpose without installing it or making any changes to the computer's configuration. Live CDs can run on a computer without secondary storage, such as a hard disk drive, or with a corrupted hard disk drive or file system, allowing data recovery.
An optical disc image is a disk image that contains everything that would be written to an optical disc, disk sector by disc sector, including the optical disc file system. ISO images contain the binary image of an optical media file system, including the data in its files in binary format, copied exactly as they were stored on the disc. The data inside the ISO image will be structured according to the file system that was used on the optical disc from which it was created.
A flash drive is a data storage device that includes flash memory with an integrated USB interface. A typical USB drive is removable, rewritable, and smaller than an optical disc, and usually weighs less than 30 g (1 oz). Since first offered for sale in late 2000, the storage capacities of USB drives range from 8 megabytes to 256 gigabytes (GB), 512 GB and 1 terabyte (TB). As of 2023, 2 TB flash drives were the largest currently in production. Some allow up to 100,000 write/erase cycles, depending on the exact type of memory chip used, and are thought to physically last between 10 and 100 years under normal circumstances.
Optical disc authoring, including CD, DVD, and Blu-ray Disc authoring, is the process of assembling source material—video, audio or other data—into the proper logical volume format to then be recorded ("burned") onto an optical disc. This act is sometimes done illegally, by pirating copyrighted material without permission from the original artists.
In computing, mass storage refers to the storage of large amounts of data in a persisting and machine-readable fashion. In general, the term "mass" in "mass storage" is used to mean "large" in relation to contemporaneous hard disk drives, but it has also been used to mean "large" relative to the size of primary memory as for example with floppy disks on personal computers.
In information technology, a backup, or data backup is a copy of computer data taken and stored elsewhere so that it may be used to restore the original after a data loss event. The verb form, referring to the process of doing so, is "back up", whereas the noun and adjective form is "backup". Backups can be used to recover data after its loss from data deletion or corruption, or to recover data from an earlier time. Backups provide a simple form of disaster recovery; however not all backup systems are able to reconstitute a computer system or other complex configuration such as a computer cluster, active directory server, or database server.
Disk cloning is the process of duplicating all data on a digital storage drive, such as a hard disk or solid state drive, using hardware or software techniques. Unlike file copying, disk cloning also duplicates the filesystems, partitions, drive meta data and slack space on the drive. Common reasons for cloning a drive include; data backup and recovery; duplicating a computer's configuration for mass deployment and for preserving data for digital forensics purposes. Drive cloning can be used in conjunction with drive imaging where the cloned data is saved to one or more files on another drive rather than copied directly to another drive.
In computing, data recovery is a process of retrieving deleted, inaccessible, lost, corrupted, damaged, or formatted data from secondary storage, removable media or files, when the data stored in them cannot be accessed in a usual way. The data is most often salvaged from storage media such as internal or external hard disk drives (HDDs), solid-state drives (SSDs), USB flash drives, magnetic tapes, CDs, DVDs, RAID subsystems, and other electronic devices. Recovery may be required due to physical damage to the storage devices or logical damage to the file system that prevents it from being mounted by the host operating system (OS).
In computing, external storage refers to non-volatile (secondary) data storage outside a computer's own internal hardware, and thus can be readily disconnected and accessed elsewhere. Such storage devices may refer to removable media, compact flash drives, portable storage devices, or network-attached storage. Web-based cloud storage is the latest technology for external storage.
IMG, in computing, refers to binary files with the .img
filename extension that store raw disk images of floppy disks, hard drives, and optical discs or a bitmap image – .img
.
Acronis True Image is a proprietary backup, imaging, cloning and cybersecurity suite developed by Acronis International GmbH. It can back up files, data, clone storage media and protects the system from ransomware. In 2021, the product was renamed to Acronis Cyber Protect Home Office before being renamed back to True Image in 2024.
Virtual disk and virtual drive are software components that emulate an actual disk storage device.
Notable software applications that can access or manipulate disk image files are as follows, comparing their disk image handling features.
This glossary of computer hardware terms is a list of definitions of terms and concepts related to computer hardware, i.e. the physical and structural components of computers, architectural issues, and peripheral devices.
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