US 2016 224 572A1

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A method for converting a disk of a physical computer into a virtual disk for use by a virtual machine is described. Contents of the disk of the physical computer are copied into an image file, wherein the image file has a different sector-by-sector organization of the contents than the disk but a logically equivalent file system organization. Hardware configuration information from the image file is then extracted, wherein the hardware configuration information relates to hardware of the physical computer and, based on a comparison of the extracted hardware configuration information and a virtual hardware configuration of the virtual machine, hardware-dependent files in the image file are replaced with substitute files that are compatible with the virtual hardware configuration of the virtual machine.

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1. A method of creating an image of a source disk of a source computer, in which contents of the source disk are arranged according to a source file system, comprising:
extracting the contents of the source disk; and
storing the extracted contents of the source disk in a destination disk of a destination computer, in which contents of the destination disk are arranged according to a destination file system, wherein
the source disk and the destination disk have different sector-by-sector content, and
the source file system and the destination file system are logically equivalent.


This application is a continuation of U.S. patent application Ser. No. 14/493,680, filed Sep. 23, 2014, which is a continuation of U.S. patent application Ser. No. 13/530,780, filed Jun. 22, 2012, now U.S. Pat. No. 8,869,139, which is a continuation of U.S. patent application Ser. No. 10/611,815, filed Jun. 30, 2003, now U.S. Pat. No. 8,209,680, which claims benefit of priority of U.S. Provisional Patent Application No. 60/462,445, filed Apr. 11, 2003, all of which are incorporated herein by reference.


1. Field of the Invention

This invention relates to the creation, manipulation and deployment of computer disk images.

2. Description of the Related Art

Disks and File Systems

A computer disk can be viewed as a linear list of data blocks called sectors. Most disks are used to store files and folders. A file's content is stored in one or more sectors, called data sectors. The mapping between a file and its data sectors is stored in special sectors called metadata. Metadata also stores file attributes (such as file name and access rights) and describe the structural relationship between files and folders. A disk's data and metadata sectors form a file system.

Metadata also keeps track of the location of free sectors. A free sector is neither used as data nor metadata. Its content is undefined until the sector becomes allocated as data or metadata for a new file or folder.

The specification of the layout and interpretation of metadata for a particular type of file system is called the file system format. There exist many file system formats; each has a distinct set of characteristics and limitations.

The process of creating a file system on an uninitialized or damaged disk is called formatting. This process creates metadata defining an empty file system. Once a disk is formatted, its file system can be populated with files and folders. In general, software applications create and access files through an operating system. The operating system forwards file requests to a file system driver, which is a software module capable of manipulating a file system's metadata. A file system driver is designed for a specific file system format and can generally run only on a specific operating system. Support for a given file system format on different operating systems generally requires multiple drivers, one for each operating system.

Some file system formats such as EXT2 are public, i.e., widely published and available for free. Anyone skilled in the art can examine a public file system format, and develop a driver or software tool to decode and manipulate any file system of that format. A file system format can also be proprietary, i.e., is owned by a single vendor and not publicly shared. In order to access files residing on a proprietary file system, software generally has to use the services of a driver developed by the format's owner. Some proprietary file system drivers exist only on specific operating systems; therefore, a software application may need to run on a specific operating system in order to access a proprietary file system. For example, the NTFS file system format is proprietary, and commercial NTFS drivers exist only on certain operating systems developed by Microsoft Corp., the owner of the format.

Disk Imaging

A disk image is a file that resides on a first computer and represents a snapshot of a second computer's disk. Image capture is the process of creating an image file from a computer's disk. A common disk imaging setup is to use two computers: a first computer with the disk being captured (the source disk), and a second computer containing the generated image file. In this setup, the disk imaging system generally comprises two software programs: an imaging client, running on the first computer, and an imaging server on the second computer. During capture, disk data is transferred from the client to the server over a network or a cable.

The reverse of the capture process is called deployment. During a deployment, a third computer's disk (the destination disk) is overwritten with data from an image file residing on the second computer. The data is transferred from the imaging server to an imaging client running on the third computer. The first and third computers can be the same.

A common use for disk imaging is backup and restore: A first computer is backed up by capturing an image of its disk, then the image is stored on a second computer. If the first computer's disk becomes damaged for any reason, it can be restored to its original state by deploying the image from the second computer back to the first computer. Disk imaging can also be used to clone a computer; an image of a first computer can thus be deployed to other computers.

Disk Image Formats

The internal format of an image file, that is, the way in which the file represents the state of a disk, is arbitrary and generally known only to the disk imaging system's vendor. Despite this, disk image formats can generally be classified into two types: sector-based and file-based.

Sector-Based Image Formats

A sector-based image format describes the state of a disk at the sector (or block) level. The simplest of such formats, called a flat image, represents all sectors of the disk as a linear list of bytes in the image file. For example, a flat file of 512,000 bytes can represent a disk with 1000 sectors of 512 bytes.

An advantage of a sector-based image file is that it represents an exact copy of the source disk, regardless of the file system format used on the disk. When it is deployed to a destination disk, the destination disk will contain an exact copy of the original file system. A sector-based imaging system therefore guarantees faithful data recoverability and reproducibility without the need to decode any file system; in other words, it does not require a file system driver.

A first disadvantage of the sector-based approach is, when an image is deployed, the destination disk must be at least as large as the original disk since the file system metadata on the original disk may encode the disk's capacity and assume that it never changes. This metadata is captured into the image file and copied to the destination disk. If the destination disk is smaller than the source disk, some sectors that the metadata assume exist may not exist on the destination disk, resulting in an inconsistent file system. Furthermore, if the destination disk is larger than the source disk, the deployed file system may not be able to take advantage of the additional space, since its metadata would assume that the disk has a smaller capacity.

Another disadvantage of a sector-based format is its inefficiency. A disk may have a large number of free sectors, that is, sectors that are not used as data or metadata, and thus have no useful content. These sectors may be scattered all over the disk, and may be difficult to identify because they generally contain an undefined set of bytes. A free sector's content is undefined because the sector may have been used earlier as data or metadata, then released after a file or folder was deleted. Most file system drivers don't erase (i.e., fill with zeros) freed sectors. A sector-based image format is therefore inefficient because it may include a disk's unused sectors.

Sparse Files

A combination of two technologiessparse files and disk scrubbingcan solve the inefficiency problem. A sparse image file, similarly to a flat image file, is a sector-level representation of a complete disk. When a sparse image is first created, it represents a disk of a fixed capacity and filled with zeros, i.e., all sectors contain bytes with value zero (or any other predetermined null value). All sectors are said to be initially unallocated. A sparse file does not store the actual contents of unallocated sectors, since their content is known; it needs to store only information about which sectors are unallocated. For example, a sparse file may use a bit vector to keep track of which sectors are unallocated, with the bit values 0 and 1 representing the unallocated and allocated states, respectively. A newly created image file could thus represent an empty disk of 512 sectors by using 512 bits, or 512/8=64 bytes.

When a sector at a particular offset is written with non-zero contents for the first time, the image file marks the sector offset as allocated in the bit vector and creates one sector's worth of data in the file to hold the sector's new contents. This causes the image file to grow by at least one sector; it may need to grow by slightly more than one sector because additional information may be needed in order to keep track of the sector's location within the file. The actual size of a sparse image file may thus be smaller than the capacity of the disk that it represents if a large proportion of the disk's sectors remain unallocated.

When a source disk is captured into an image file, using a sparse format can greatly reduce the size of the file if free sectors in source disk were filled with zeroes, since the imaging system would only need to mark those sectors as unallocated in the file instead of copying their actual contents. As explained earlier, free sectors cannot be assumed to contain zeroes, since a free sector may previously have been allocated as a data or metadata sector, and subsequently freed but not explicitly erased with zeroes.

A common solution to this problem is to run a software tool generally known as scrubber on the source disk prior to the capture operation. The typical scrubbing tool is an application that runs on the operating system of the source computer. Its purpose is to erase free sectors with zeroes. The operating system does not usually allow applications to write directly to sectors, and even if it did, the application wouldn't know which sectors are free; only the file system driver has that knowledge.

The scrubber achieves its goal by creating a temporary file and then growing it by filling it with zeroes until the file system runs out of free disk space. The tool then deletes the file. This algorithm causes the file system driver to convert sectors that were free prior to the scrub operation to data sectors filled with zeroes. When the temporary file is deleted, the zeroed data sectors become free sectors, but their contents do not change.

Subsequently, during the image capture operation, the disk imaging system discards sectors filled with zeroes and does not store them in the sparse image file. Only the useful data is copied, thus keeping the image file's size to a minimum.

In practice, however, few imaging systems employ the combination of scrubbing and sparse files, because it is generally unreasonable to require a user to run the scrubbing tool on the source computer. First, the tool must generally be run manually, making the overall disk imaging process difficult to automate from start to finish. Second, by using up all free disk space, the tool may negatively interfere with other applications running on the operating system.

In summary, sector-based disk image formats are subject to two main limitations: the capacity matching problem, where the destination disk of deploy operation must be as large or larger than the source disk used to create the image, and the efficiency problem, where the image file may contain useless sectors, which unnecessarily increases its size and the time it takes to capture or deploy.

File-Based Image Formats

Unlike sector-based disk image formats, file-based formats store only file and folder information, not sectors. During a capture operation, the imaging system uses a file system driver to decode a source disk's file system. This allows the imaging system to enumerate all existing files and folders, and then read their attributes and contents. All of this information is copied and stored into a single image file using an internal layout that is either publicly known, such as the ZIP or TAR format, or proprietary and thus only known to a particular imaging system vendor.

To deploy a file-based image to an uninitialized or damaged destination disk, a file system driver is first used to format the disk in order to create an empty file system on it. The imaging system then reads the file and folder information from the image and uses the file system driver to re-create those files and folders in the destination file system.

The file-based approach does not have the weaknesses affecting the sector-based approach. First, the source and destination disks can have different capacities, as long as the destination disk has enough capacity to hold all the file and folder content encoded in the image file. For example, if the source disk has a capacity of 10 Gigabytes, but only 4 Gigabytes worth of files and folders are stored on it, the image could be deployed to a 5 Gigabyte destination disk. Second, file-based images are efficient since, by definition, they store only useful information.

The biggest issue with the file-based approach is its reliance on a file system driver, both during capture and deployment operations. A challenge in designing a file-based imaging system is deciding which file system driver to use and how to integrate it into the imaging process. Furthermore, many file system formats exist, so that an imaging system may need to interoperate with more than one file system driver.

One natural choice is to use the file system driver included with the source computer's operating system. A computer's disk generally contains an operating system. Without an operating system, the computer could not function correctly. An operating system is a collection of programs and software modules that exist as files in a file system on the disk. One of those modules is a file system driver capable of decoding the disk's file system. When an operating system startsa process called bootingthe operating system generally loads the file system driver into memory before most other drivers and modules. The file system driver is critical because it allows the operating system to load other modules from the file system, and to expose files to software applications, which are generally loaded last.

Since the file system driver itself is a file on the file system, one may wonder how it could be extracted from the file system in the first place, when no driver is loaded. Every type of operating system has a different way of addressing this issue. One possible solution is to store the sector offset corresponding to the beginning of the contents of driver file in a special sector not used by the file system, such as a master boot record (MBR). When the operating system first loads, it could use the services of the computer's BIOS (basic input/output system) to read the sector offset from the special sector, then load the driver file's contents into memory, and then execute the driver's code in order to decode the entire file system.

In order to take advantage of the operating system's file system driver to perform a capture operation, the imaging client can be implemented as an application running on the source computer. When the source computer is powered on and its operating system has finished loading, the imaging system initiates the image capture operation by starting the imaging client. The client first connects to the imaging server over the network, and then uses the operating system's file API (application programming interface) to enumerate and read all existing files and folders, streaming their content over to the imaging server.

The Issue of Open Files

An issue that arises when running the imaging client on the operating system is that some files, such as operating system files, may be locked, i.e., inaccessible to applications, including the imaging client. Other files may be accessible but open by other applications, meaning their contents may be cached in memory and may change while the imaging client copies the files. The imaging system thus faces the risk of capturing an incomplete or corrupt set of files.

It is thus difficult to image, or backup, a disk's files while active programs are accessing a subset of those files. One existing solution to the open files problem is to make an application provide an API to the imaging or backup system. The imaging system would use the special API to copy files opened by the application, instead of using the operating system's standard file access API. The special API would be responsible for exposing the correct and up-to-date contents of open files to the imaging system. This solution has been commonly implemented for database applications. The main drawback of the solution is that it is not general: files opened by applications that do not expose a special backup API cannot be reliably copied.

Deploying to a New or Damaged Disk

The file-based imaging approach faces another issue. In a deployment operation, the destination computer's disk's content may be uninitialized or damaged. An existing operating system may thus not exist on the destination computer, which means the imaging client cannot run on it. Even if the destination computer had a functional operating system, the disk imaging software user may want to overwrite it with the operating system and files from the image; however, the existing operating system would not allow any application to overwrite existing operating system files.

Offline Disk Imaging

Offline disk imaging is a solution to the open files and the deployment issues described earlier. The idea is to run a secondary operating system on the source computer or destination computer during imaging operations. Before a capture operation, the imaging system shuts down the source computer, causing all software from its disk, including applications and the primary operating system, to unload from memory. The imaging system then reboots the source computer from the secondary operating system, which can be loaded from a floppy disk, a CD-ROM, or from the network using a protocol such as PXE (Preboot Execution Environment).

The secondary operating system is self-sufficient, i.e., it does not need to read any files from the disk attached to the computer, and operates using only the computer's memory and processor. The secondary operating system includes and loads the imaging client, which can then access the disk safely because no other programs are accessing it.

If the secondary operating system includes a driver capable of decoding the source disk's file system, the imaging client can use the operating system's file API to read the disk's files. Otherwise, the client itself must include its own driver or software module in order to access the file system.

In a deployment operation, the destination computer is shut down, and then rebooted from the secondary operating system, which includes the imaging client. The client then uses the secondary operating system's file system driver, or its own driver, to format the destination disk, thereby creating an empty file system. The client then reads the image file from the imaging server, and re-creates the appropriate files and folders on the destination file system.

When the deployment operation finishes, the secondary operating system shuts down the destination computer and reboots it from its disk. This time, the computer loads the operating system that was restored from the image.

Choice of Secondary Operating System

The secondary operating system chosen by an imaging system vendor has to meet strict size requirements, since it cannot rely on the computer's disk for storageit must be capable of functioning using only the computer's memory. It must also be small enough to fit on the boot medium, i.e., a floppy disk, a CD, or a memory image downloaded from the network.

Another requirement the secondary operating system must generally meet is low licensing cost, since it is an additional software component that contributes to the overall cost of the product. Consequently, disk imaging system vendors tend to choose a low-cost or free (in terms of software licensing cost) operating system for the task. Typical choices include DOS (disk operating system) and Linux.

For these reasons, the chosen secondary operating system is usually not a general-purpose operating system, and is likely to be different from the operating system residing on the source computer's disk.

The Issue of Proprietary File System Formats

Offline disk imaging requires the secondary operating system or imaging client to supply a file system driver compatible with the source disk's file system format.

Proprietary file system formats pose a challenge to imaging system designers, since drivers compatible with a particular proprietary format may exist only on a limited set of operating systems and tend to be supplied by few vendors, generally one. If the source computer's disk is formatted with a proprietary file system, the secondary operating system may not have a compatible driver, making the capture operation impossible.

A disk imaging system vendor has three choices for solving this problem. The first choice is to license a special-purpose operating system from the owner of the file system format, assuming that such an operating system exists and it meets other requirements, such as footprint. The drawback of this approach is the imaging system vendor may have to pay a higher license cost for this operating system compared to other choices for operating system.

The second choice is to license the specification to the proprietary format from the owner, and then develop a custom driver for the chosen secondary operating system, or a driver to be embedded in the imaging client itself. This approach is also costly, since it includes both the cost of the license, and the cost of developing new software. The file system format owner may also choose not to allow any company to license the format, which would make this approach impossible.

The third choice is to attempt to reverse-engineer the proprietary format, or to use a free file system driver that is based on reverse engineering. For instance, the NTFS format is proprietary and NTFS drivers are commercially available only on operating systems made by Microsoft. An NTFS driver exists on Linux, a free operating system, and was developed by using both publicly available information and information collected from reverse engineering. Unfortunately, reverse engineering is inherently risky and unreliable, which explains why the Linux NTFS driver is still at an experimental stage and known to be unstable for certain file operations, such as writes.

Contemporary Disk Imaging Systems

Products such as Symantec Ghost and Powerquest DriveImage represent the current state of the art in disk imaging systems. They employ a file-based image format, allowing them not only to copy only the useful contents of disks but also to capture from and deploy to disks of different sizes. In order to work around the problem of open files, these systems use the offline imaging method. The secondary operating system used tends to be based on DOS or Linux, since those operating systems tend to be lightweight, low cost, and easily customizable for disk imaging tasks. The imaging client used is generally a custom-developed program designed to run on the chosen secondary operating system. For instance, Symantec Ghost uses DOS as the secondary operating system, and its imaging client, called GHOST.EXE, is a DOS program.

Modern disk imaging systems generally support multiple file system formats. For example, Symantec Ghost supports EXT2, FAT, FAT32, and NTFS, the latter two of which are proprietary. In order to access proprietary file systems, existing disk imaging systems include their own file system driver, or build the functionality into the imaging client itself. For instance, the GHOST.EXE client contains code to decode the four different types of file system formats supported by the product, including the proprietary ones.

Whether or not the code to access proprietary file systems was developed based on reverse engineering, or from information licensed from other companies, is information not publicly known. One fact is certain: supporting proprietary file system formats increases the cost of the developing disk imaging products and thus the cost of the product to end customers.

Disk Image Editing

Contemporary disk imaging software sometimes includes a tool to browse the file and folders contained within an image file. Symantec's Ghost Explorer application, for example, allows a user to view files in an image through a graphical user interface; the user can also extract a file from the image and copy it onto the computer's native file system, or take an existing file from the file system and insert it into the image.

The file-based image format used by the majority of contemporary imaging systems does not lend itself well to internal modifications after an image has been created. The reason for this is that most image formats used today favor compactness over flexibility by tightly packing file and folder contents from the source disk into the image file. Sections of the image file may also be compressed to reduce the file's size even further. Modifying the contents of a file-based image may involve deleting files and adding new ones, potentially creating holes in the file. This phenomenon is called fragmentation.

Fragmentation increases file size and potentially reduces image deployment performance, since the imaging system may need to read multiple, non-contiguous areas of the image file in order to extract the correct sequence of files to expand onto the destination disk. To address this issue, a disk imaging product, such as Symantec Ghost, may provide a program to create a new image file from a modified and therefore fragmented image. Symantec Ghost calls this process image recompilation. Once an image is recompiled from a modified one, the modified one can be discarded.

In summary, existing file-based disk image formats are not well suited for content editing. Contemporary imaging software products provide tools for casual editing of a small number of files. More substantial modifications may reduce an image's efficiency or performance, a problem sometimes alleviated by recompiling the image.

Disk Image Identification and Tagging

When a disk image is created, a user is required to give it a file name. This name usually identifies the contents of the source disk from which the image was captured. For example, it may contain words indicating the type of operating system, the computer name, etc. Multiple disk images are sometimes archived together on a disk or other storage medium, so that they can be used later for deployment to new or existing computers. When the number of images grows, managing this image library can become challenging. In particular, before a deployment operation, a user may want to search the image library for a disk image that satisfies specific requirements, such as an operating system type, an operating system version, and possibly a number of software applications.

If a disk imaging system included a program for assisting a user to search an image library, this program would have a difficult time performing an accurate search based solely on file names. The first reason is that file names are usually created by humans, and may be ambiguous or not accurately reflect the contents of an image. For example, an imaging containing a Windows 2000 operating system may be named Bob's Windows 2000, or Alice's Win2K computer.

Second, file names are inherently restricted in length and cannot convey much information beyond basic computer identification. A disk imaging system could easily augment images with a set of standard attributes, such as computer name, network address, and operating system type. However, these attributes would still need to be manually entered by a user, and are thus subject to human error.

Most importantly, there is an abundance of intricate information contained in a disk image that is not commonly exposed by contemporary disk imaging systems. Since a disk image's ultimate purpose is to be deployed to a computer, it is important for a disk imaging system administrator to reliably query the software configuration encapsulated in an image, in order to determine whether an image is the appropriate one for a specific deployment operation.

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