Fixed Size vs. Dynamically Expanding Virtual Hard Disks

by Chris Sanders [Published on 19 Oct. 2006 / Last Updated on 19 Oct. 2006]

Dynamically expanding virtual hard disks seem like the obvious choice when setting up a virtual machine, but when is this not such a good idea?

When you create a fixed size hard disk the space you specify is automatically allocated and reserved for that virtual machine. A dynamically expanding disk however, starts small and grows as needed until it reaches the maximum size you have allowed for it. In most cases, a dynamically expanding disk just seems like the best logical choice in order to not waste useless disk space.

There are some cases however, when a dynamically expanding disk may not be the best choice. With the size of your VHD (virtual hard disk) file constantly changing it can cause the host operating system to become increasingly fragmented. This level of fragmentation can severely degrade the performance of both the host and guest operating systems. This being said, you should carefully analyze your system resources before deciding between a fixed size or dynamically expanding hard disk.


Chris Sanders is the network administrator for one of the largest public school systems in the state of Kentucky. Chris's specialties include general network administration, windows server 2003, wireless networking, and security. You can view Chris' personal website at

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The Author — Chris Sanders

Chris Sanders is a network security analyst for EWA Government Systems Inc. Chris is the author of the book Practical Packet Analysis as well as several technical articles. His personal website at contains a great deal of information, articles, and guides related to network administration, network security, packet analysis, and general information technology.

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