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Why does it take so long to create a fixed size virtual hard disk?

Why does it take so long to create a fixed size virtual hard disk?

Postby Ernie » Fri Sep 16, 2011 12:09 pm

If you have ever created a fixed-size virtual hard disk that was larger than, oh - 2GB, you probably noticed that it takes quite a while to create. The reason why this takes so long is that when we create a new fixed-size virtual hard disk we take the time to explicitly zero-out all of the disk space that is being assigned to the new file.

Now - we could do this practically instantaneously by not zeroing out the data - but this has an interesting potential security problem.

Imagine the following situation:

    • You have a virtual machine with a bunch of confidential data running on a central server (e.g. your company payroll).
    • This virtual machine gets moved to a new physical server in response to increased work load.
    • You create a new virtual machine which is given to someone on from the in-house dev team - but the virtual hard disk data was not zeroed out.
    • Developer then runs data recovery tools on his new, blank virtual machine and is able to recover data from the old payroll server (yikes!)

You see - data is never actually deleted from a disk when a file is moved or deleted (it is just dereferenced) so to avoid the above scenario - we must take the time to "do the right thing" and zero out the VHD contents.

Here is a tool to create quick, but not secure, fixed virtual hard disks. Details Here

http://blogs.msdn.com/b/virtual_pc_guy/archive/2008/12/10/why-does-it-take-so-long-to-create-a-fixed-size-virtual-hard-disk.aspx
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