Use fixed-size virtual hard disks instead of dynamically expanding virtual hard disks

The two most common kinds of virtual hard disks are the fixed-size virtual hard disk and the dynamically expanding virtual hard disk.

When you create a fixed-size virtual hard disk, all the space that the virtual hard disk requires is reserved. Therefore, if you create a 100 gigabyte (GB) fixed-size virtual hard disk, 100 GB is reserved on the physical hard disk. The space on a fixed-size virtual hard disk is more likely to be contiguous than the space on a dynamically expanding virtual hard disk. Additionally, the file size of a fixed-size virtual hard disk does not have to expand before data is written to a file in a virtual machine. Therefore, fixed-size virtual hard disks generally provide better performance.

In a dynamically expanding virtual hard disk, the size of the .vhd file grows as data is written to the virtual hard disk. By default, Virtual Server 2005 creates this kind of virtual hard disk. When you create a dynamically expanding virtual hard disk, you specify a maximum file size. This size restricts how large the virtual hard disk file size can become. For example, if you create a 100 GB dynamically expanding virtual hard disk, the initial size of the .vhd file is about 3 megabytes (MB). As the virtual machine uses the virtual hard disk, the size of the .vhd file grows to hold the new data. Because dynamically expanding virtual hard disks grow only as required, a dynamically expanding virtual hard disk is less likely to use contiguous space on the host computer's physical hard disk. Therefore, depending on how fragmented the host computer's physical hard disk is, the performance of the virtual machine might decrease