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I have setup many VHD boot on my Windows 7 machine. Each of the VHD is 60GB Expandable (current size ~20GB), When I boot into the VHD, the file on the disk expands and shows up the size of VHD to be 60GB (I checked with Windows Explorer).

What if my physical disk doesn't have space to Expand it. Physical disk has space to keep it (~20GB) but to not expand it (60GB), what will happen in this case?

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Please edit your question. It isn't very clear what you're asking. –  user3463 Sep 24 '12 at 4:26
    
@RandolphWest Edited. –  Akshat Mittal Sep 24 '12 at 4:29

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Simple answer - the virtual machine software will complain when it tries to expand the VHD and can't.

You have basically promised the virtual machine software that either that extra space is available ready for when it's needed, or that you will keep an eye on the space being used and will free up space ready before it's needed.

Other than that, the only option is to prevent the virtual machine from ever needing that extra space - in which case, why not specify the smaller amount as the limit in the first place? Some software may even use more space on the virtual hard drive just because it appears to be available - obvious examples being the recycle bin and system restore if you have Windows as your guest OS, which both are often set to limit themselves to some percentage of the available disk space.

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Firstly, I am booting a physical machine with VHD as I have mentioned above. What will happen in this case? –  Akshat Mittal Sep 24 '12 at 5:18
    
Physical vs. virtual machine isn't really the issue. The issue is that hard disk space cannot be magically wished into existence. You have promised that the space will be available to expand that VHD if and when it's needed. If that's impossible, you will get some kind of failure. Depending on what you happen to be doing at the time, that could easily be a crash with data lost, and the machine may even not be bootable afterwards (e.g. because the swap file has grown to include an area of the VHD that cannot be allocated physical disk space). –  Steve314 Sep 24 '12 at 5:26

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