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Differencing disk configuration handling?

Source: microsoft.public.virtualpc
Sent: 09/07/2008
From: Bo Berglund <(email address - cut out)>
Message:

In addition to my portability question yet unanswered I would like to
ask specifically about the handling of differencing disks:

When a diff didk is created it is about 30-40 kbytes in size. There
seems to be no other additional file concerning the differencing disk.

My question now is how the diff disk is handled if it is moved to
another location and this other location is not on the same drive as
before. Will a guest based on this diff disk be able to start at all?

I have looked in the vmc file of a guest created with the diff disk
and there is no mention in it that the VHD drive is a differencing
disk. So where is this information stored so it can be handled if the
guest is moved?

Or maybe such a guest is not movable at all?

--

Bo Berglund (Sweden)

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Related Messages

Re: Differencing disk configuration handling?

Sent: 09/07/2008
From: "Tim Walsh" <(email address - cut out)>
Message:

A diff disk is linked to an original disk, only the differences between the
original are stored in the diff disk. The diff disk needs to be able to
access the linked original disk in order to function. The use of a diff disk
wouldn't be a good idea if your looking to move the VM from machine to
machine on your flash drive, unless the linked file is also brought along on
the flash drive.

You might be better off using a dynamically growing disk, these also start
out small but grow as needed up to your defined limit. You'll need a big
flashdrive to use it.

"Bo Berglund" <(email address - cut out)> wrote in message
news:(email address - cut out)...

Show quoted text

> In addition to my portability question yet unanswered I would like to
> ask specifically about the handling of differencing disks:
>
> When a diff didk is created it is about 30-40 kbytes in size. There
> seems to be no other additional file concerning the differencing disk.
>
> My question now is how the diff disk is handled if it is moved to
> another location and this other location is not on the same drive as
> before. Will a guest based on this diff disk be able to start at all?
>
> I have looked in the vmc file of a guest created with the diff disk
> and there is no mention in it that the VHD drive is a differencing
> disk. So where is this information stored so it can be handled if the
> guest is moved?
>
> Or maybe such a guest is not movable at all?
>
> --
>
> Bo Berglund (Sweden)

Re: Differencing disk configuration handling?

Sent: 09/07/2008
From: Steve Jain <(email address - cut out)>
Message:

On Sun, 07 Sep 2008 10:48:20 +0200, Bo Berglund <(email address - cut out)>
wrote:

When you start the VM, if the parent VHD can't be found, you will be
prompted for the location of it.

--
Cheers,
Steve Jain, Virtual Machine MVP
http://vpc.essjae.com/
I do not work for Microsoft.

Show quoted text

>In addition to my portability question yet unanswered I would like to
>ask specifically about the handling of differencing disks:
>
>When a diff didk is created it is about 30-40 kbytes in size. There
>seems to be no other additional file concerning the differencing disk.
>
>My question now is how the diff disk is handled if it is moved to
>another location and this other location is not on the same drive as
>before. Will a guest based on this diff disk be able to start at all?
>
>I have looked in the vmc file of a guest created with the diff disk
>and there is no mention in it that the VHD drive is a differencing
>disk. So where is this information stored so it can be handled if the
>guest is moved?
>
>Or maybe such a guest is not movable at all?

Re: Differencing disk configuration handling?

Sent: 09/07/2008
From: Bo Berglund <(email address - cut out)>
Message:

On Sun, 7 Sep 2008 08:22:28 -0400, "Tim Walsh" <(email address - cut out)>
wrote:

The reason I am looking at diff disks is that I am creating a large
number of different test machines with XP-Pro on all of them. Using a
diff disk where the operating system files are the same makes for a
smaller disk footprint for the total exercise....
The system files need not be duplicated on all different guests, only
the different files I am testing will get to the individual diff disk.

I *am* keeping all files for my guests on the same removable drive,
basically I thought of this structure:

\VirtualPC\DiffPC <---- The original XP-SP2 parent disk here
\VirtualPC\DiffPC\XP1 <--- diff disk and vmc file for PC #1
\VirtualPC\DiffPC\XP2 <--- diff disk and vmc file for PC #2
\VirtualPC\DiffPC\XP3 <--- diff disk and vmc file for PC #3
\VirtualPC\DiffPC\XP4 <--- diff disk and vmc file for PC #4
etc

I have looked in the diff disk file with a hex editor directly after
it is created (size 37 kb) and I found that there is stored both a
relative and an absolute path to the parent disk inside.
So I am wondering what will happen if the removable disk with all
these files is plugged into another PC where it will undoubtedly get a
different drive letter than the original?
Will the diff disk use the relative path or fail?

--

Bo Berglund (Sweden)

Show quoted text

>"Bo Berglund" <(email address - cut out)> wrote in message
>news:(email address - cut out)...
>> In addition to my portability question yet unanswered I would like to
>> ask specifically about the handling of differencing disks:
>>
>> When a diff didk is created it is about 30-40 kbytes in size. There
>> seems to be no other additional file concerning the differencing disk.
>>
>> My question now is how the diff disk is handled if it is moved to
>> another location and this other location is not on the same drive as
>> before. Will a guest based on this diff disk be able to start at all?
>>
>> I have looked in the vmc file of a guest created with the diff disk
>> and there is no mention in it that the VHD drive is a differencing
>> disk. So where is this information stored so it can be handled if the
>> guest is moved?
>>
>> Or maybe such a guest is not movable at all?
>>
>A diff disk is linked to an original disk, only the differences between the
>original are stored in the diff disk. The diff disk needs to be able to
>access the linked original disk in order to function. The use of a diff disk
>wouldn't be a good idea if your looking to move the VM from machine to
>machine on your flash drive, unless the linked file is also brought along on
>the flash drive.
>
>You might be better off using a dynamically growing disk, these also start
>out small but grow as needed up to your defined limit. You'll need a big
>flashdrive to use it.
>

Re: Differencing disk configuration handling?

Sent: 09/07/2008
From: Bo Berglund <(email address - cut out)>
Message:

On Sun, 07 Sep 2008 12:23:06 -0700, Steve Jain
<(email address - cut out)> wrote:

Thanks,
what will happen if there is a parent disk file *both* in the relative
path (one directory above) *and* on the full path (on a different
drive)??
Which path has precedence in this situation?

While setting up the differencing disk machines I did it on my fixed
disk, but then I copied the files over to the removable drive. Then I
started the guest on the removable drive and now I am not sure *which*
parent it will be using.....
This is of course a theoretical question since once the removable
drive is on the other PC there will be no parent drive in the full
path anymore.

--

Bo Berglund (Sweden)

Show quoted text

>On Sun, 07 Sep 2008 10:48:20 +0200, Bo Berglund <(email address - cut out)>
>wrote:
>
>>My question now is how the diff disk is handled if it is moved to
>>another location and this other location is not on the same drive as
>>before. Will a guest based on this diff disk be able to start at all?
>>
>>I have looked in the vmc file of a guest created with the diff disk
>>and there is no mention in it that the VHD drive is a differencing
>>disk. So where is this information stored so it can be handled if the
>>guest is moved?
>>
>When you start the VM, if the parent VHD can't be found, you will be
>prompted for the location of it.

Post Reply | This is Spam! | Mark as Spam

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