Mitch Tulloch's Blog

Windows tip: Running virtual machines from a compressed host drive

Compressing a hard drive will help you squeeze more space out of a hard drive, and if you use your laptop for demos using Virtual PC, Virtual Server or Hyper-V, this might seem like a good idea since virtual hard drive (.vhd) files tend to take up a lot of room. There are pros and cons you need to consider however before you do this:

Pros:

• You gain extra disk space, sometimes up to double the available space. That means you can store more virtual machines on your laptop for more demos.
• Your virtual machines will likely start up faster since it's faster to read a 10 GB file than a 20 GB one.
• Virtual machines will often perform better as well for the same reason (faster disk reads).

Cons:

• Reading from compressed drives adds processor overhead, so if your don't have a fast processor then your processor can become the bottleneck for virtual machine performance instead of your hard drive.
• If your virtual machines are performing disk-intensive operations (lots of writes) then performance may degrade instead of improving.
• Since compressed files are inherently subject to fragmentation, any performance boost you gain by compression may decrease over time as the virtual machines are used when demos are being performed.
• I've also heard some anecdotal reports that virtual machines that have very large .vhd files running on compressed drives can crash during usage and result in corruption of the .vhd file, which would mean an abrupt end to your demo.

Conclusion:

My best guess concerning this matter is that if you're just going to do relatively simple (not disk-write intensive) demos with relatively small (under 20 GB) .vhd files on a laptop with a fast dual-core processor then go ahead and compress your host drive. But don't do it on a host machine running in a production environment.

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