Learn more about the VHD technology for Windows 7

Understanding VHD Format

The vast majority of people initially contacted files in the VHD format, when using Virtual PC or VM server, the VHD is a disk of VMS that exist as files. Microsoft released full details of the VHD format file in 2005 and can download Virtual Hard Disk Image format specification from Microsoft's TechNet website. This shows that Microsoft will not only use the VHD in the Virtual machine field, but also hope that the VHD can become a new disk format and carrier for more support in the industry.

Let's put the virtual machine aside and focus on the VHD file itself. A VHD is a virtual hard disk, unlike the Platters, heads, and tracks of a traditional hard disk, and the carrier of the VHD hard disk is a VHD file on the file system. If you read the technical standard of the VHD file carefully, you will find that the standard defines a number of hard disk-specific terms such as cylinder, heads, and sectors to simulate I/O to the hard disk. Since the VHD is a hard drive, it can be partitioned, formatted, read-write, and so on as a physical hard disk.

We can think of it this way, the VHD is also a hard disk, like 2.5-inch, 3.5-inch, SCSI, SATA, IDE, and other different sizes of hard disks, the VHD is a hard disk in a file.

How Windows 7 supports the VHD technology implementation

Before we discuss specific technology implementations, we first need to understand how the operating system manages disk and file. The operating system has a disk system and a file system that handles I/O read and write instructions to the hard disk (the disk system), and manages the partitions, data, and file formats (file systems) on top of the hard disk. A VHD is a disk that exists on a file, and it is conceivable that its technical implementation must span both the disk system and the file system. If you are not very familiar with this part of the operating system, listen to the Windows Storage Technology Series webcast on the Microsoft Web site.

In short, to read and write a VHD file as a hard disk, you must have the appropriate driver. In Virtual PC and virtual Server, Microsoft is the driver that adds a VHD to the VM, allowing the virtual machine to boot from the VHD and perform subsequent operations.

In Windows 7, Microsoft built the VHD driver into the operating system, also included in the Windows 7 boot program. This means that when using Windows 7, we can directly access the contents of the VHD file (the hard disk of the VHD is mapped to a letter, which is described later), or you can launch another operating system on the VHD disk through the Windows 7 bootstrapper. Understanding the details of the VHD-driven work requires a lot of windows underlying knowledge, and the following figure is a simpler and more abstract structure that can be found that the VHD driver is closely tied to disk drives and file systems.