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Bare Metal Backup - Disk Imaging

Disk imaging is often described as a "bare metal backup" because it backs up physical disks at the volume level. In other words, a true disk image is an exact copy of an entire physical disk or disk partition.

In its simplest form a disk imaging program creates a bit identical copy of a drive which is made by dumping raw data byte by byte, sector by sector from the given disk into an image file. Early disk imaging programs and image formats had a lot of limitations. Because they copied the raw blocks of data from the disk "uninterpreted" even the unused area or white space was backed up too. This led to very large image files even if the drives or partitions only contained a small amount of data. Copying every sector of a disk in this way worked quite well for "disk cloning" where everything was being moved to a replacement drive, or was being multi-cast to several computer systems with identical hardware and configurations, but it was not really a very practical solution for regular backup of a system.

Fortunately, disk imaging technologies have been developed that eliminate many of the limitations faced by early adopters of the technology. Most modern disk imaging programs have the ability to interpret the data being copied and remove or compress the empty blocks on a disk which leads to much smaller image files. The majority of programs also create compressed image formats that can be mounted and explored making it possible to retrieve individual files. Also the creation of successive incremental1 or differential2 backups is often supported, which further reduces the demands on storage. Other techniques have been developed that allow file-level operations such as file type filtering, such as the ability to exclude the large and non-essential pagefile.sys and hiberfil.sys from the image, and the ability to image a drive or partition while it is currently in use.

New technologies aside, imaging programs generally still have the ability to do the "old-fashioned" low-level raw copies of a disk which has its advantages. Full bit identical images retain deleted files or lost partitions that have not yet been over-written. All file, file-system, and partition attributes are preserved. Bit identical images facilitate the ability to restore files and partitions to the exact same sectors as the originals. This can prevent such problems as partition misalignment which can cause problems for certain operating systems.3 Since these tools can still handle un-decipherable raw data they can be used to backup any disk even if contains a foreign file system, an unknown operating system, or an encrypted partition.

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