VHD files under the Win7 system and their different types

One, what is VHD file

First of all, Microsoft's VHD file format is a virtual machine hard disk, and can be compressed into a single file stored on the host machine's filesystem, mainly including the system files required for virtual machine startup.

About the application of the VHD: Virtual PC is a kind of Windows virtual machine, it can be virtualized various versions of Windows, a Windows should be installed on a hard disk partition, and it is virtual windows, it is impossible to separate a hard disk partition to install it, So it starts the required system files that are compressed into a VHD format file on the hard drive.

The VHD format will also be used for Microsoft Windows Server 2008 R2 and Microsoft Windows 7, including hypervisor based virtualization technology-hyper-V. Hyper-V can operate offline vhd-allows administrators to access the system securely through a VHD file, allowing administrators to visit virtual files (VHD) and perform some offline administrative tasks.

The VHD format is also applied to Windows Vista Business, Enterprise and Ultimate, for complete system backups.

　Two, multiple types of VHD files

1, fixed VHD

The fixed VHD file is assigned a virtual disk size that is the size of the virtual disk specified when the VHD was created. For example, if you create a virtual hard disk size of 2 GB, you will create a file size of approximately 2 GB, where part of the space is used for the internal VHD structure. The size of the fixed VHD does not change as the data is written to the virtual hard disk.

2, scalable or dynamic VHD

At any given time, the size of the extensible or dynamic VHD file is the same size as the actual data that was written to the file. The file also increases dynamically as more data is written. For example, files that support a 2 GB virtual hard disk are initially about 2 MB in size on the host file system. As the data is being written to this virtual hard disk, the file grows until it reaches a capacity limit of 2 GB.

3, the difference VHD

The

diff VHD is similar to a dynamic VHD, but contains only the modified virtual disk blocks of the associated parent virtual disk. The differential VHD is linked to a specific parent VHD, called a child VHD. The parent VHD file is read-only, and all modifications are made against the child VHD. The VHD file type of the parent VHD can be any of these three VHD types, including another differential VHD. Multiple child VHD files can form a chain of differences.