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Hyper-V export VHD, is it really an exact copy?

I've exported a 100 Gb Hyper-V virtual to a USB drive from a Small Business Server 2011 Standard Host after stopping the virtual and when I compare the SHA-1 Hash of the original .VHD file to that of the exported .VHD file on the USB drive they are not the same, although the files are the same size.  

Is the claim that the VHD file in the export is "the same" accurate?

My choices seem to be:

1.  Have confidence that the USB export can be imported and booted in the future.  I do not have the resources to test this right now.

2.  Export the virtual again until the hashes agree.

3.  Copy the VHD file until the hashes agree.  I'm trying this but would like to know if it's a reasonable thing to do . . . it takes a while to copy and rehash.

I'm using HashTab ver 4.0.0 from Implbits (originally beeblebrox.org) and have used this and three earlier versions in the past with excellent results.

So is "the same" really identical in this case, or not?


Microsoft Server OSVirtualization

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8/22/2022 - Mon
Svet Paperov

Yes, you will be able to import the exported VM and it will be at the state you have left it on the shutdown before the export. You don't need to worry about that. I've never checked the hashes of exported VHD files but obviously VM manager does create a new VHD while exporting instead of just copying - that explains the different hashes.
Rob Williams

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I'm using hashes on these files because several months ago I stopped a virtual on the same server, hashed it, and copied (not exported) it to two separate USB portable drives.  When I attempted to boot one of the VHD's on a laptop it blue screened and wouldn't boot.  Checking the hash revealed that that particular copy was NOT identical to the VHD on the server.  The second copy had the same SHA-1 hash as the original and was bootable.

That said, I've taken a second SHA-1 hash of the original and find it different from the first hash I obtained.  The CRC32 hashes match, though, but the file is 100 Gb.  I am now not very satisfied with any of the possible courses given above and am taking a 3rd hash of the original.
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I've a chance to check the hashing on another export myself within the next few days.  That should give me an answer.
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William Peck

ArneLovius:  I agree the copy was corrupted.  It wasn't until the copy froze instead of booting that I took hashes (of the originals, not the one post-freeze, and discovered the copy had failed.

RobWill:  I agree fully on deleting(merging) the snapshots, otherwise you get very little.  Thanks for the SBS heads up.  It's not my system, so I didn't sign any agreements.  I'm just trying to provide backup capability for a client whose main IT supplier has done many "interesting" things, including selling them a "new" server with a 6 year old processor and USB1 ports.  They are far from a metropolitan area and seem to be stuck with what they can get.  The corrupted copy, by the way, was done with a USB2 card, a factor of 40 faster than the stock ports.
Rob Williams

>>" It's not my system, so I didn't sign any agreements.
It's not just the licensing but it is not possible to run a fully functional SBS with hyper-v running.  Chances are you have not running some of the wizards and are not running a default configuration.  Hyper-v will break various SBS functions such as DHCP, some network functions, cause backup failures, and wizards and BPA fail. The fact that you did not sign the agreements will not carry much weight with Microsoft, if you are the admin :-)

No problem running SBS as a Hyper-v VM, just not as a host.

Rob -

I am not the admin.  I didn't set it up.  I didn't advise or consult with the client over the setup or participate in any way.  If I thought they'd understand I'd be glad to pass your advice along to them.  The system does appear to be running at this time.  I'm just trying to get them some workable disaster recovery on a system which is sub-optimal in more than one way.

- Chuck
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My thanks to both experts.  Time to resolve this one.