Sysadmins can now use differencing disks to minimize the footprint of VHDs on drives and conserve bandwidth. These are special VHDs that share a special Parent/Child relationship with regular VHDs. A Parent VHD contains all the raw data (OS, software, files) that an admin wants to be “reproduced” in the differencing disk(s).

1. Carl Boisson
Mar 29 · 6 min read
What’s The Di erence? Creating Parent and
Di erencing VHDs Through Hyper-Visor
The VHD Age
Image credit: Carl Boisson

2. The VHD Age
When Microsoft created the .VHD format, they streamlined the process of
managing storage in environments that have become increasingly virtualized.
Virtual Hard Disks (VHD) are logical representations of storage that can be
manipulated in a similar fashion to real disks. Blank VHDs can be created for
use as storage spaces for Virtual Machines (VMs), or they can be preloaded
with Operating Systems (OS) and used to run those VMs themselves. VHDs
are also highly portable as admins can easily transfer them over their
networks without having to physically remove disk drives.
While VHDs allow for greater flexibility in storage management, their ease of
creation can accelerate capacity loss. For example, within minutes an
administrator can create 8 VHDs preloaded with Windows Server 2012. If
each VHD is 8GB, that would be a total of 64GB! In the real world, this
example is repeated many times over. This can greatly increase storage costs
as additional hardware would be required to meet capacity needs.
Increased portability also comes at a cost. VHDs, specifically dynamically
expanding ones, can easily swell in size, especially if they’re loaded with an
OS. Transferring these large files over a network can increase traffic and drain
bandwidth. Microsoft has found a novel way to dress both of these concerns
without affecting VHD functionality.
A Vast Di erence
Sysadmins can now use differencing disks to minimize the footprint of
VHDs on drives and conserve bandwidth. These are special VHDs that share a
special Parent/Child relationship with regular VHDs. A Parent VHD contains
all the raw data (OS, software, files) that an admin wants to be “reproduced”
in the differencing disk(s). The differencing disks “pull” the shared data from
the parent, and the only data written to the “child” VHD file are changes that
differ from the parent (hence, the name).
Because the resulting VHD file only contains differencing data, the result is a
file that’s a fraction of the size of the parent disk. Take our 8 VHDs from our
prior example, then totaling to 64GB. Now they would come in at little over
1MB if they were created as differencing disks with the same Parent.
Often, network administrators are moving VHDs across intranets and the
internet. Differencing disks are smaller files which are easier to transfer over
networks. This reduces transfer times and costs associated with mitigating
bandwidth blockages.
Do It Like Disk

3. Do It Like Disk
1 . Organization*
In order to keep track of your VHDs, it is best practice to make your folder
(directory) hierarchies and names consistent with the purpose of each disk.
For example, adding “parent” or “base” to the file name of the parent VHD. I
also recommend separating the parent VHD from the differencing disk.
**Note: If data on a parent disk is altered in any way, it will be reflected in all
the differencing disks created from that parent. This is akin to changing their
DNA as all “child” disks share the same core data (see intro image). This may
result in having to run sysprep many, many, many times.**
2. Preparing the Parent Disk
Sysprep is a command line tool which reverts a running operating system to
an out-of-box state by generalizing uniquely identifying data. This includes
user accounts and Security ID numbers (a SID can be likened to a social
security number for computers). Depending on the source of the VHD you
intend to use as the parent, this step may not be necessary. For example, if you
downloaded a Server 2012 VHD directly from Microsoft, you can move on to
the next step. However, if you’re unsure and you skip this step, you may end
up having to run sysprep many, many, many times. If performed successfully,
a VM created with a differencing disk from this parent will start with an out-
of-box experience. This ensures that identifying attributes, such as SIDs, will
not be duplicated.
Parent and di erencing disks should have their own folder
The names of the les should re ect their purpose

4. Create a new VM and use the desired parent VHD as its disk, run it and then:
Run Command Prompt
Type: cd C:Windowssystem32sysprep
(where “C:” is your OS directory)
Press Enter
Type: sysprep
Press Enter
(this will start sysprep.exe)
Select “Enter System Out-of-Box Experience (OOBE)”
Check “Generalize”
Select “Shutdown”
Click “OK”
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5. Ensure the VM is shut down.•
Make sure “Generalize” is checked
It may take a while

6. 3. Create the Di erencing Disk
Using Hyper-V (the same program used to create and manage VMs):
Click New > Select “Hard Disk…”
Select “VHD” > Click Next
(the format must match the parent)
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VM in the OFF state

7. Select “Differencing” > Click Next
Type the desired name* of the differencing disk
Browse to the desired directory* > Click Next
Browse to the Parent VHD > Click Next
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Remember, stay organized

8. Click Next > Click Finish (repeat for each disk)
4. Veri cation
You can test drive your new differencing disks by creating VMs with them as
the disk. You can also use Hyper-V to inspect the disks and verify the path to
the parent.
Click Inspect
Browse to differencing disk
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A set of di erencing disks from parent. Note the small le sizes.

9. What A Di erence A Disk Makes!
Now you’re well on your way to impressing your employers by setting up
differencing disks to save time (bandwidth) and money (storage costs). If
you’d like some learn more about differencing disks or VHDs in general, check
out the links below. Thanks for reading!
Additional Resources:
Using differencing disks | Virtual Storage Explained | Storage Pools Guide
Key Terms:
Bandwidth
Differencing disk
Dynamic disk
Logical disk
Internet
Intranet
Network traffic
Operating system
SID
Storage spaces
Sysprep
Virtualization
Windows Server 2012
Disk properties point to parent disks