What are the different types of VHD files in Windows 7

There are three types of VHD files that can be created using Hyper-V Manager or using Windows Disk Management Tools in Windows 7:

1. Fixed VHD. The virtual disk size to which a fixed VHD file is allocated is the virtual disk size specified when the VHD was created. For example, if you create a virtual hard disk that is 2 GB in size, the system creates a file that is approximately 2 GB in size, some of which is used for the internal VHD structure. The size of a fixed VHD does not change as data is written to the virtual hard disk.

2. Scalable or dynamic VHD. At any given time, the size of an extensible or dynamic VHD file is the same as the actual data size written to the file. As more data is written, the file is dynamically increased. For example, a file that supports a 2 GB virtual hard disk is initially about 2 MB in the host file system. As data is continuously written to this virtual hard disk, the file grows until it reaches the 2 GB capacity limit.

3. Differences VHD. The difference VHD is similar to a dynamic VHD, but only contains modified virtual disk blocks for 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; all modifications are made to the child VHD. The VHD file type of the parent VHD can be any of these three VHD types, including another differential VHD. Multiple sub-VHD files can form a differential chain.