Fun with virtual hard drives at Microsoft Tech-Ed

At Microsoft Tech-Ed 2009 in Berlin long-time Windows server expert Mark Minasi gave a session on the .vhd format used by Microsoft for virtual hard drives in Hyper-V, the virtualisation feature built into Windows server 2008.

Minasi's talk was not about Hyper-V, but rather about other things you can do with a .VHD. He even noted that in a Windows environment you can use a .VHD as a superior form of ZIP (though without the compression), if you want a single file that can contain windows files and folders while preserving NTFS security attributes, and that can be mounted - or "attached" in .VHD jargon - rather than having to be extracted elsewhere.

One neat trick is to use .VHDs for multi-boot. Multi-boot is less important now that virtualisation works so well, but can still be useful if want to test an operating system with full performance and access to hardware such as accelerated graphics. The old way to do this is with multiple partitions, but this is somewhat inconvenient. Windows 7 is able to boot from a .VHD, and you can set up mutiple VHDs so that you can choose which one to use at start-up. There are a couple of limitations. The operating system has to be Windows 7 or presumably 2008 R2 (which uses the same kernel); and sleep/hibernate does not work in this configuration.

A VHD is still just a file, so you can back it up by copying it elsewhere, provided it is not the one currently running. Note that in this configuration only the hard drive is virtual, not the computer hardware, so while you could go on to mount the VHD in Hyper-V it would be like moving Windows to a new motherboard and very likely would not boot.

Another clever tip is that Windows 7 setup support a keystroke combination, Shift F10, which gives access to the command line for MinWin, the cut-down version of Windows that runs during setup. Here you can get access to Diskpart, the command-line disk management tool, which among other things lets you create a VHD. So you can take a machine with an untouched hard drive, boot into the Windows 7 setup, shell to the command line and create a VHD, then attach and install Windows onto that new virtual drive. Setup actually states that this does not work; but it does work, and we saw it demonstrated.

There are three kinds of VHDs. A fixed VHD has the same on-disk size as its capacity. An expanding (or dynamic) VHD reports the size that you specify when it is greated, but only occupies the space on its host that is needed by the data written there. This is convenient for backup, and lets you over-commit the host drive if you choose to. The third kind is a differencing VHD - a VHD that is based on a parent and only occupies the space needed by its difference as you write to it. The GUI Windows disk management tool does not support creation of differencing VHDs: one of Minasi's points is that you should learn the command line approach using Diskpart in order to get access to all the available features. That said, differencing VHDs are supported in the Hyper-V GUI management tool.

The bottom line is that VHDs have uses that go beyond virtual machines and if you work on the Windows platform it is well worth becoming familiar with them.