Vista - How to discard changes?

I have the Undo disk option enabled. I want to use the guest OS as a
test platform for unknown and untrusted software. When I'm done
trialing the software, I want to discard any changes it made. In VMWare
Server, I simply reverted to a baseline snapshot of the guest VM. In
VirtualPC 2007, the only thing that I see that comes close to is close
the VM using the "Turn off and discard changes" action (which also
disabled the "Commit changes to the virtual disk" option). Yet, I
recall seeing in its help somewhere a mention that turning off the VM
was like yanking the power cord to the host and could cause file system
corruption. So once I'm done playing with some software in the VM, how
to I wipe the virtual hard disk back to the state it was in before I
installed that experimental software?

On Tue, 23 Dec 2008 05:06:23 -0600, VanguardLH <V@xxxxxx> wrote:
When you close the machine normally, at the end there will be a
dialogue asking you if you want to:
- Commit changes to the hard disk
- Save the changes for later
- Discard the changes

Select Discard and you will lose all changes since you enabled the
undo disk.

--

Bo Berglund (Sweden)

"VanguardLH" <V@xxxxxx> wrote in message
news:giqgn7$id0$1@xxxxxx
As long as you enabled undo disks, "Turn off and delete changes" is OK. I
use it all the time and never had any problem.

Yes, some virtual hd could have file system problems but that's the undo
disk. This undo disk is a file on the host separate from the original *.VHD,
and the "Turn off and delete changes" does delete it completely. The
original *.VHD was not touched at all, and this is the only one that remains
and will be used the next time you start the virtual machine.

PS: Sorry, I think I clicked a wrong button and so did not reply to the
group.

Bo Berglund wrote:
The close options presented are:

What do you want the virtual machine to do?
- Save state and save changes.
- Shutdown Windows XP and save changes.
- Turn off and save changes.
- Turn off and delete changes. (*)

and a separation option:

Commit changes to the virtual hard disk.

(*) This choice deselects and disables the "Commit changes to the
virtual hard disk" option.

So I can stop a VM session and save changes so they are there for the
next VM session; i.e., when I need to get back to trialing the software,
it will still be there. However, I must deselect the "Commit changes"
option to ensure that the undo disk changes are not applied to the
virtual hard disk. That way, eventually I can toss the undo disk's
changes so remove all changes that were made to the VM.

Eventually I want to discard all changes and get back to my baseline VM.
The only option that seems appropriate is "Turn off and delete changes".
This deselects and disables the "Commit changes" option. So apparently
I am expected to power off the VM (instead of shutting it down) and not
commit any changes (and not save state as per the other choices).

My concern is that the help mentions somewhere that "turning off" could
result in file system corruption in the VM yet that is the only close
option that I can see that will not save state and discard any changes.


As a side issue, I sure wish that I could default the "Commit changes"
option to off (deselected) instead of enabled (selected) for the first
three close options. I don't want to remember on every close to NOT
commit the changes because eventually I'll be discarding them.

On Tue, 23 Dec 2008 16:13:14 -0600, VanguardLH <V@xxxxxx> wrote:
Correct.
In this case your concern is unfounded. the changes occur when you
*start* the virtual machine. When you shut off the VM, the only thing
that is discarded are those changes after you started the VM. As far
as the base VHD is concerned, it was never turned on.
You can. Once you select this and turn off the VM, the next time you
do this, it will remember the last state.

--
Cheers,
Steve Jain, Virtual Machine MVP
http://vpc.essjae.com/

SaGS wrote:
That's what I concluded but wanted some validation that I made the right
choice. The "Commit changes" option can be disabled for the first three
close options so that I'm only saving the VM's state between VM sessions
but not altering the state of the .vhd file (the changes stay on the
undo disk). The last close option of "Turn off and discard changes" was
the only one that looked like it would discard the undo disk and the
changes recorded there. Thanks for the feedback. Looks like I'm doing
it the right way. Just wish the options were more clearly written.

As a side issue, I sure wish that I could default the "Commit changes"
option to off (deselected) instead of enabled (selected) for the first
three close options. I don't want to remember on every close to NOT
commit the changes because eventually I'll be discarding them. While
testing software in the VM, I need to save state between VM sessions
until I get done trialing the software. Everytime I close and save
state, I have to remember to deselect the "Commit changes" option. If I
forget just once, I'm screwed because my .vhd will then get polluted
with the test software.

The best that I've come up with to emulate VMWare's snapshot feature is
to copy the folder containing the VM's files (with no changes so no undo
disk recordings) but occupies a LOT of disk space. I elected to create
a fixed-size VHD for performance reasons instead of a dynamically
growing VHD. The VHD is 16GB in size, so copying the folder with the
VM's files as a backup to provide an emulated snapshot (by copying it
back) would occupy another 16GB. I don't even want to waste the disk
space to backup the VHDs (if I lose them, I'll rebuild the guest OS from
scratch).

The defaults has Microsoft assuming that you usually want to move your
VM forward with additional changes but that's contradictory to having
undo disks. The undo disks only work if you do NOT commit changes, so
the "Commit changes" option should be off be default. Then when the
user actually decides to move forward and keep the new software would
they then commit the changes.

I've hunted around in the registry but haven't yet found a setting that
configures the default state for the "Commit changes" option.

On Tue, 23 Dec 2008 16:22:09 -0600, VanguardLH <V@xxxxxx> wrote:
alternately, you could use differencing disks instead then. Make your
base VHD read-only and then create as many child VMs and VHDs off it
as you like. The base never changes, and you can do whatever you want
in the children without affecting the base image.

--
Cheers,
Steve Jain, Virtual Machine MVP
http://vpc.essjae.com/

On Tue, 23 Dec 2008 16:22:09 -0600, VanguardLH <V@xxxxxx> wrote:

This is in the .vmc file, not the Registry. Look for the section:

<shutdown>
<prompt type="boolean">true</prompt>
<quit>
<action type="integer">4</action>
<was_running
type="boolean">false</was_running>
</quit>
<save>
<enable type="boolean">true</enable>
</save>
<shutdown>
<enable type="boolean">true</enable>
</shutdown>
<turn_off>
<enable type="boolean">true</enable>
</turn_off>
<last_shutdown>
<choice type="integer">3</choice>
<commit type="boolean">false</commit>
</last_shutdown>
</shutdown>
****
what you want is the last couple lines, last_shutdown, choice 3, false
commit, etc

--
Cheers,
Steve Jain, Virtual Machine MVP
http://vpc.essjae.com/

On Tue, 23 Dec 2008 16:22:09 -0600, VanguardLH <V@xxxxxx> wrote:
A suggestion:
- Enable undo disks (you have)
- Set the VHD disk file as *readonly* in the host file system

Now I think that even if you forget to uncheck the commit checkbox the
commit will fail because the VHD file is readonly.
With undo disks enabled the VHD file is *only* written to if you have
selected to commit changes, never otherwise. So running with a
readonly VHD file should work just fine.
It is very similar to using differencing disks in several guests all
using the original (readonly) VHD as the parent.

--

Bo Berglund (Sweden)

Steve Jain [MVP] wrote:
Thanks for the heads up. I tested this and it does appear to be a
sticky setting. Great! I just know that one day I'll get busy or
interrupted and forget to turn off that option and end up polluting my
baseline VHD.

By the way, in the VPC console when I right-click on a VM, one of the
entries is "Delete saved state". I'm guessing that also gets rid of the
changes that got recorded on the undo disk and I'd be back to the
baseline VHD (and without have to startup the VHD to then close it to
get the close options to discard changes).