VirtualServerVHDFile

Virtual Hard Disk File:

On a physical computer the disk is a disk drive, however on a virtual machine the disk can be one of a few formats:

• Dynamically Expanding Virtual Hard Disk
• Fixed Size Virtual Hard Disk
• Differencing Virtual Hard Disk
• Linked Virtual Hard Disk

The most common is the Dynamically Expanding Virtual Hard Disk.

All of the Virtual Hard Disks are stored as files accessible by the host operating system, the files can be local or anywhere the host can access (file server, SAN, etc.).

At the lower levels a hard disk is simply a bunch of sectors, the file system, files, boot records, and anything else are irrelevent to disk drive - As they are irrelevent to the .VHD Files. This is at a lower level than the file system, so the VHD format can store ANYTHING that can be stored on a disk drive, boot sectors, multiple file systems, or anything else.

The VHD file format is a standard by Microsoft which other vendors can use, so you will likely see other software support it in the future - Perhaps programs like Norton Ghost will have direct support to create VHDs in the future (I have no knowledge of Ghost, just stating a possible use of the technology).

Since the .VHD file can contain any kind of drive data it's format it used by both Dynamically Expanding and Fixed Size Virtual Hard Disks. The difference is just what the name states, one expands as it needs space, the other takes the full amount of disk space when created, even if there is no data.

A dynamically expanding VHD file, when created, takes very little space on the disk. Any sectors which don't have content will not be stored in the VHD file, instead the VHD file knows those sectors aren't used. Once sectors are used, even if the files are deleted, the space will still be used on the host unless you compress the VHD files (see links at end).

The advantage to a fixed size VHD is that the disk will be continuous on the host systems drives (assuming the file is created continuously, like any other file on windows). Even if you have to defragment your drive to get it continuous you won't have to worry about the files expanding a few sectors at a time - The continuous expansion WILL cause fragmentation over time. You likely won't want to shut down your guest for a long period of time while you defragment the VHD file(s) - So you may want to waste the space when they are initially created.

Linked VHD files are a completely different animal. You can link to a physical disk drive, for example if you are migrating a simple IDE based system to Virtual Server you can pull a disk drive, put it in a USB enclosure, plug it into the host, disable any drive letters assigned (as a safety precaution against any writes from the host), and mount it as a linked VHD - Then try booting the system. If it all works then great, you can run it like that until another maint window when you create a dynamically expanding VHD image of the linked image.

Differencing VHDs are more complicated and are explained elsewhere on the site, including the links at the end. Most basically they are the DIFFERENCES between a base image and the image for a particular machine, so for example you could have a base image of Windows 2003, then differencing images of several servers. The sectors which have never changed would be stored in the base image, while any changes would be in the differencing images for each VM.

Differencing VHDs, what they are, usage and tips?
Compressing Dynamically Expanding Virtual Hard Disks?
Removing old / unused .vhd files