While we have had many discussions in this blog about Dynamic vs. Fixed VHD’s, we still get questions about this topic from time to time. There is a misconception by some that there isn’t a significant performance difference between dynamic and fixed VHD file formats. We’ve recently put together a demonstration that shows this difference more clearly.
When talking about dynamic VHD’s, it is very important to note that Microsoft does not recommend them for production:
“We recommend that you use fixed size virtual hard disks for virtual machines that are running on a production environment.”
This is a very clear statement and there should be no further debate. However, some folks would really like to see the difference for themselves. In the demonstration below we show both the impact of on-demand file allocation and the impact to the underlying SAN of steady state random I/O: