Disk cloning

Disk cloning is a category of software which copies the contents of one computer hard disk to another. Often, the contents of the first disk are written to an image file as an intermediate step, and the second disk is loaded from the image.

A disk cloning program is most commonly used by large companies to provision new computers, by a computer manufacturer to install the initial package of the operating system and applications for sale, or by a training or educational institute to refresh the software on the computers used by a class. An individual user may use disk cloning to upgrade to a new hard disk, or to create a comprehensive backup of their operating system and installed software.

This article is specific to disk cloning on the x86 platform; specific details may not apply to other platforms.

To provision the hard disk of a computer without using disk cloning software, the following steps are generally required for each computer:

Create one or more partitions on the disk
Format each partition to create a file system on it
Install the operating system
Install device drivers for the particular hardware
Install application software

With disk cloning, this is simplified to:

Install the first computer, as above.
Create an image of the hard disk (optional)
Clone the first disk, or its image, to the remaining computers

History

Before Windows 95, some computer manufacturers used hardware disk copying machines to copy software. This had the disadvantages of copying not just the used data on the disk, but also unused sectors, as the hardware used was not aware of the structures on the disks, and usually required the disks to be copied before being inserted into the machines. A larger hard disk could not be copied to a smaller one, and copying a smaller one to a larger left the remaining space on the new disk unused. The two disks required identical geometries.

Other manufacturers and companies partitioned and formatted disks manually, then used file copy utilities or archiving utilities, such as tar or zip to copy files. It is not sufficient simply to copy all files from one disk to another, because there are special boot files or boot tracks which must be specifically placed for an operating system to run, so additional manual steps were required.

Windows 95 compounded the problems because it was larger than earlier popular operating systems, and thus took more time to install. The long filenames added to the FAT filesystem by Microsoft in Windows 95 were not supported by most copy programs, and the introduction of the FAT32 filesystem in 1997 caused problems for others. The growth of the personal computer market at this time also made a more efficient solution desirable.

Ghost was introduced in 1996 by Binary Research. It initially supported only FAT filesystems directly, but it could copy but not resize other filesystems by performing a sector copy on them. Ghost added support for the NTFS filesystem later that year, and also provided a program to change the Security Id (SID) which made Windows NT systems distinguishable from each other. Support for the ext2 filesystem was added in 1999.

Competitors to Ghost soon arose, and a features war has carried on to the present day. Many disk cloning programs now offer features which go beyond simple disk cloning, such as asset management and user settings migration.

On UNIX based computer systems, dd was more commonplace due to the lack of filesystem support in Ghost.

Post-cloning operations

Two machines with identical names are generally not allowed on the same network, and, for Windows NT and its successors, two machines with identical security IDs (SIDs) are not allowed on the same Active Directory domain. A disk cloning program must change these as part of copying the disk or restoring the image. Some operating systems are also (by design) not well suited to changes in hardware, so that a clone of Windows XP for example may object to being booted on a machine with a different motherboard, graphics card and network card, especially if non-generic drivers are used. Microsoft's solution to this is Sysprep, a utility which runs hardware detection scans and sets the SID and computer name freshly when the machine boots. Microsoft recommends that Sysprep be set up on all machines before cloning, rather than allow third party programs to configure them. By contrast, Linux systems simply require the necessary kernel modules to be available (or compiled directly into the kernel), which will automatically load on the new hardware.

There are files in some Microsoft operating systems (called BOOTSECT.*) which are copies of the Boot Partition Block (BPB). They must be altered if partition sizes are changed during the clone.

Linux systems usually boot using either the LILO or grub bootloaders. These contain lists of absolute disk sectors in their MBR, which must be altered by the cloning program as the files they refer to are likely not to be in the same location of the destination disk.

Operating environment

A disk cloning program needs to be able to read even protected operating system files on the source disk, and must guarantee that the system is in a consistent state at the time of reading. It must also overwrite any operating system already present on the destination disk. To simplify these tasks, most disk cloning programs can run under an operating system different from the native operating system of the host computer, for example, MS-DOS or an equivalent such as PC-DOS or DR-DOS, or Linux. The computer is booted from this operating system, the cloning program is loaded and copies the Windows file system. Many programs (e.g. PowerQuest's DriveImage, Acronis True Image) can clone a disk, or make an image, from within Windows, with special provision for copying open files; but an image cannot be restored onto the Windows System Drive under Windows.

Disc cloning program running under non-Windows operating systems must have device drivers or equivalent for all devices used. The manufacturers of some devices do not provide suitable drivers, so the manufacturers of disk cloning software must write their own drivers, or include device access functionality in some other way. This applies to tape drives, CD and DVD readers and writers, and USB and FireWire drives. Cloning software contains its own TCP/IP stack for multicast transfer of data where required.

Image transfer

The simplest method of cloning a disk is to have both the source and destination disks present in the same machine, but this is too restrictive. Disk cloning programs can link two computers by a parallel cable, or save and load images to a network drive. As disk images tend to be very large (usually at least several hundred MB), performing several clones at a time puts excessive stress on a network. The solution is to use multicast technology. This allows a single image to be sent simultaneously to many machines without putting greater stress on the network than sending an image to a single machine.

Image manipulation

Although disk cloning programs are not primarily backup programs, they are sometimes used as such. A key feature of a backup program is to allow the retrieval of individual files without needing to restore the entire backup. Disk cloning programs either provide a Windows Explorer-like program to browse image files and extract individual files from them, or allow an image file to be mounted as a read-only filesystem within Windows Explorer.

Some such programs allow deletion of files from images, and addition of new files.

Software

External links