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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.
The file size of a raw disk image is always a multiple of the sector size. For floppy disks and hard drives this size is typically 512 bytes (but other sizes such as 128 and 1024 exist). More precisely, the file size of a raw disk image of a magnetic disk corresponds to: Cylinders × Heads × (Sectors per track) × (Sector size)
One of the main characteristics of qcow disk images is that files with this format can grow as data is added. This allows for smaller file sizes than raw disk images, which allocate the whole image space to a file, even if parts of it are empty. This is particularly useful for file systems that do not support sparse files, such as FAT32. [5]
Notable software applications that can access or manipulate disk image files are as follows, ... Compressed file system images: Ext2, Ext3, Reiserfs, ...
This is a 32-bit value that is intended to identify uniquely the disk medium (as opposed to the disk unit—the two not necessarily being the same for removable hard disks). The disk signature was introduced by Windows NT version 3.5, but it is now used by several operating systems, including the Linux kernel version 2.6 and later. Linux tools ...
A block, a contiguous number of bytes, is the minimum unit of storage that is read from and written to a disk by a disk driver.The earliest disk drives had fixed block sizes (e.g. the IBM 350 disk storage unit (of the late 1950s) block size was 100 six-bit characters) but starting with the 1301 [8] IBM marketed subsystems that featured variable block sizes: a particular track could have blocks ...
Fixed hard disk image: a file that is allocated to the size of the virtual disk. Fixed VHDs consist of a raw disk image followed by a VHD footer (512 or formerly 511 bytes). [2] Dynamic hard disk image: a file that at any given time is as large as the actual data written to it, plus the size of the header and footer.
The sector size and sectors per track depend on how the disk is initialized. For example on the GEC 4000 series minicomputers a configuration of 34 sectors of 512 data bytes each per track is used. SMD disk packs (as the Storage Module itself was most commonly called) required head alignment to assure interchangeability of media between drives.