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I've read that you cannot boot from the VHD backups that Vista Ultimate / Server 2008 creates. However you can mount them with vhdmount.

So if you can't boot it then what good is the VHD backup over a regular compressed file backup and system restore?

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2 Answers

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Microsoft uses a technology called Volume Shadow Copy to make these backups. It's a very clever approach because it allows full volume sector-by-sector backups (like a Ghost image) that can be stored incrementally. If you backup every day, each additional copy only consumes the space required to store the changes, yet still looks like a full stand-alone image.

The combination of VHD (a convenient pre-existing disk image format) and Volume Shadow Copy makes it trivial to restore to any point in time (that you have a backup of).

Why are these image (or sector-based) backups useful?

1) Sector-based backups are orders of magnitude faster than file-based ones. In a sector-based backup the disk heads are moving sequentially whereas in a file-based backup the heads are moving randomly. In many cases this means the difference between backups being able to complete overnight or not.

2) You can do a complete system restore simply by booting from the Vista/Windows 7 DVD and going through the option of recovering your system. This covers scenarios like failed hardware or malware damaging your OS.

3) They are very space-efficient. Think of a 100 MB database file. If you do incremental file-based backups, every new backup will consume 100 MB. On the other hand, a sector-based incremental backup will only consume the space necessary to store the changes to the 100 MB file, which could be as small as a single sector.

Microsoft's use of the VHD format was a logical choice that saved them creating a new format -- a laudable approach in my opinion (the less "standards" the better). Think about it; with VHDMount I can now mount full-disk backups AND Virtual PC/Hyper-V virtual disks -- what's not to like? Nowhere in all that is the implication that an image of a computer will boot on completely different hardware (which is what Virtual PC/Hyper-V are equivalent to).

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Thanks for the explanation. All of those are very solid reasons, I have to admit however that I was disappointed when I found that I couldn't simply run the VM from the VHD. It seems at the least I ought to be able to do a system restore to VirtualPC or different hardware. Is that possible? –  Martin Murphy Nov 15 '09 at 17:17
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I think the idea behind it is that if using Hyper-V, you can quickly move it to another Hyper-V server and mount it to another Virtual server.

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I can see how that would make sense in a enterprise environment with Server 2008 machines but why on Vista? Maybe it's just one of those things that Microsoft does that doesn't make sense. But I was hoping for something more convenient for the non-enterprise user. –  Martin Murphy Oct 27 '09 at 21:15
Looks like Windows 7 support booting these and mounting them. en.wikipedia.org/wiki/… –  ITGuy24 Oct 28 '09 at 4:34
This works if you set it up that way from scratch but you cannot do this from a VHD produced from a backup of a physical machine. –  Martin Murphy Mar 17 '10 at 14:47
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