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Windows 7: Virtual PC - Differencing disk bloat

16 Apr 2013   #1

Windows 7 Professional 64 bit
Virtual PC - Differencing disk bloat


I've installed Windows XP mode on Win7 x64, loaded the VM, run all the Windows Updates on it, which bloated the vhd file to over 3GB. I then merged the Windows XP Mode and Windows XP Mode Base vhds to create a new parent disk.

I then created a new differencing disk based on the new parent vhd. At first the file was very small, which I expected. However I left it running overnight (did not do any updates, software installs, anything) and now the differencing disk is 1667 MB!

Is this normal? I want to set up a test/development environment. Am I better off using a dynamic disk rather than differencing ones?

Thanks for any help!


My System SpecsSystem Spec
16 Apr 2013   #2


Hi Jim, welcome to the Seven forums.

For me that sounds quite normal. If your base is about 3 GB and now the user specific part, differencing vhd a bit over 1.5 GB after all Windows Updates, your XP seems to be about OK.

You of course know the idea of differencing disks? Using them lets every user of your Windows 7 host have their own individual XM Mode vm. The base vhd contains the core of XP, all parts needed for the XP to run remaining write protected, and the individual differencing vhd then the user specific part of XP and all changes this user makes to it. The base is fixed size, the differencing vhd can be fixed size or dynamically expanding. Each individual XP can then have multiple user accounts.

That being said, it comes to this: there are no benefits in running merged or differencing XP Mode, both need the same amount of HD space. The only deciding factor should be the planned usage: if two or more user profiles of the host computer need an individual and independent XP Mode, you need to use base and differencing disks. If all users may use the same XP Mode then it's up to you if you want to merge the two disks. In this case host user A's installed apps on XP Mode can be used by host user B and each XP Mode admin user can see other users documents in XP Mode.

My System SpecsSystem Spec
17 Apr 2013   #3

Windows 7 Professional 64 bit

Hi Kari,

Thanks for your reply and for putting my mind at ease. I think what threw me was the the differencing file was growing, even though there were no writes to the disk.

I am the only user, but the VM is for a database development environment. The data files themselves are not especially big, but there are a large number of disk writes (record additions, deletions etc.) every day so I wondered if I am better with dynamically expanding disks or differencing ones. My understanding is that each disk write increases the size of a diff disk.

My concern is that as I backup regularly (more than once a day) backing up vhd files which are 10's of GB in size will be a problem.

I've now set up the environment (installed all the software etc.) to a dynamic disk, compacted it (6GB), set it read-only & added a differencing disk (so the base vhd won't change), and I have also added a second dynamic vhd which contains JUST the database itself (it's less than 200MB.)

Is that a strategy you would recommend?

Many thanks for your help,

My System SpecsSystem Spec

17 Apr 2013   #4


Please allow me first clarify this with dynamic and differencing as it might get confusing.

A Virtual PC vhd can be of fixed size or dynamic. XP Mode can use a merged vhd or differencing disks. Hence dynamic and differencing are not two equal alternatives; a differencing XP Mode setup can use (and usually does use) dynamically expanding disks.

Back when Microsoft first published XP Mode, copying and backing it up was a bit more complicated. I even wrote a tutorial back then about copying the XP Mode vhd. Today this tutorial is close to useless.

I would keep using differencing disks if I were you. The base is always there, as a backup you could simply copy the differencing part, your personal XP vhd to an external drive every now and then. Worst case scenario: XP Mode becomes useless, not responding > Create a new vm selecting Use an existing vhd option using your backup copy of the differencing individual vhd. Virtual PC would then automatically attach this differecing vhd to the base vhd and create a new XP Mode vm.

My System SpecsSystem Spec

 Virtual PC - Differencing disk bloat

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