I use Oracle VirtualBox for much of my testing and demos. Yesterday I figured I wanted to connect two machines in a way that enabled the VMs on one talk to the other, while at the same time keeping the traffic isolated. The easiest way I could think of was adding a second NIC in each machine (host) and connecting them with a crossover cable. One of the hosts already had a RealTek Gbps adapter and the adapter I added also used the same chip, and thus the same driver. Windows handled this fine, even though both NICs had the same device name (not connection name). Unfortunately VirtualBox did not. I couldn’t tell the two cards apart. Looking in the config files for VMs configured for bridged networking, I discovered that VirtualBox references physical NICs on the host by name, not GUID etc. So the only way I could think of for fixing this was to rename the second NIC.

Like I said, the connection name was different and easily changed, but the device name required some tweaking. Here’s how I did it:

1. Look up the Device Instance Path for the NIC in Device Manager.
   This is most easily done by opening properties for the NIC from the Network Connections window, since you will not be able to tell the NICs apart if you use Device Manager (since they have the same device name).
   The Device Instance Path will look something like this:
   PCIVEN_10EC&DEV_8168&SUBSYS_816810EC&REV_014&98ECCA6&0&00E3
2. Now open regedit as LOCAL SYSTEM
   This is most easily achieved by using PsExec.exe from Sysinternals. The command is:
   psexec.exe -i -s regedit.exe
3. Navigate to the key HKEY_LOCAL_MACHINESYSTEMCurrentControlSetEnum.
4. Under the Enum key you will find the “path” from the Device Instance Path.
5. Under the last part of the path, 4&98ECCA6&0&00E3 in this case, you will find a value called DeviceDesc. It will probably have a value something like this:
   @oem49.inf,%rtl8168.devicedesc%;Realtek PCIe GBE Family Controller
   The value first makes a reference to the driver INF file and a variable within it. I do not know how this works in detail, but you can just change it to any sting value you want. The INF file will not contain the data you choose anyway. I changed mine to the name of the device with “#2” appended.
6. Before the data in DeviceDesc can be changed you need to give the local Administrators group (or another security principal of your choice), access to the key. While the last part of Device Instance Path is selected, hit Edit and select Permissions. Add the security principal you want.
7. Change the value of DeviceDesc.
8. Exit regedit.
9. Open or refresh the Network Connections window and check that your device has the new name.
   I did not have to reboot for the change to take effect, but that may be necessary.

After this procedure my “new” NIC instantly showed up in VirtualBox and was available for Bridged Networking. I filed a bug at the VirtualBox site so maybe they will fix it. I have no idea of the consequences for making this change yet. It may break Windows networking in some subtle way, but I doubt it. You may also have trouble if you need to upgrade the device driver for the NIC, but again, I doubt it. To make sure you do not run into trouble, save the previous value of the DeviceDesc value so you can change it back later. You could also change the permissions back to what they were before. Consider this my disclaimer.

All Microsoft virtualization software allows you to manipulate virtual hard disk files (VHDs). The standard operations you can perform are listen in the table below.

* To fixed of same size as source
** To dynamically expanding of same size as source

What is missing here is the ability to shrink disks, either dynamically expanding or fixed. This is something that the built in tools cannot do. Enter the guys at vmToolkit and their VHDResizer. VHDResizer will let you shrink a fixed or dynamically expanding disk, provided you do some preparation.

Notice how VHDResizer has determined that the source VHD is a dynamically expanding disk, and that the destination VHD can be set to either fixed or dynamically expanding. This particular source VHD has a maximum size of 16 GB. Because of this the minimum size we can select for the destination VHD is 16 GB, regardless of disk type. The reason we cannot get the disk smaller than 16 GB is that the partitions or volumes in the VHD take up the entire 16 GB of space, even though the actual physical file is much smaller. This is the very basis of a dynamically expanding disk; the guest operating system sees all the space and can address it, but only blocks that have data on them are written to the VHD file. So we need to shrink the volumes inside the VHD to free up space. I find that the easiest way to do this is with Diskpart.exe and the SHRINK command. As long as there is free space at the end of the volume, SHRINK can reduce the size of the volume. You can see how much you can shrink the volume by running SHRINK QUERYMAX. Then you can use SHRINK to shrink the volume by the maximum space available.

Microsoft Windows [Version 6.0.6001]
Copyright (c) 2006 Microsoft Corporation. All rights reserved.
C:UsersAdministrator.LAB>diskpart
Microsoft DiskPart version 6.0.6001
Copyright (C) 1999-2007 Microsoft Corporation.
On computer: LAB-RODC1
DISKPART> select disk 0
Disk 0 is now the selected disk.
DISKPART> select volume 1
Volume 1 is the selected volume.
DISKPART> shrink querymax
The maximum number of reclaimable bytes is: 15 GB
DISKPART> shrink
DiskPart successfully shrunk the volume by: 15 GB
DISKPART>

But sometimes there is data at the end of the volume or very close to it, making the size you can shrink a volume by very small. To get around this you need to move the files to the beginning of the volume. The best tool I have found to do this is JKDefrag. By using JKDefrag’s action option 5 (Force together) we can force all the files on the volume together at the beginning of the volume. This will cause fragmentation, but we can deal with that by doing a normal defragmentation run when the volume has reached its desired size.

JkDefrag.exe -a 5 c:

But sometimes even this isn’t enough. However many times you run JKDefrag you will still see data at the end of your volume. Chances are that this is the NTFS Master File Table (MFT) stored in the file $Mft. The MFT also reserves a portion of the volume it calls the MFT Reserved space. This is to guarantee that the MFT has space to grow in, even when the disk is nearly full. The inability to write to or update the MFT would lead to disk corruption and orphaned files, which is the reason for this precaution. But to achieve our goal, we need to move the MFT and the MFT Reserved space blocks from the end of the volume and towards the start of the volume. (The MFT isn’t really at the end of the volume, at least it wasn’t in the beginning. Windows places the MFT around the middle of the volume by default, but since we have shrunk our volume it is now at the end.) The author of JKDefrag says that version 4 of JKDefrag will be able to move and defragment the MFT, but until that version is available we have to use another tool. The best I have found is Raxco PerfectDisk 10. PerfectDisk is commercial software, but you can download a trial version to perform your MFT defragmentation. Remember to get the correct version of PerfectDisk, the Pro version will not install on Windows Server for example. Install PerfectDisk 10 inside your virtual machine and analyze the volume you want to shrink. Select the Boot check-box next to the volume drive letter and reboot the virtual machine.

During the boot PerfectDisk will defragment your volume and move the MFT to around the middle of it. You have no control of where PerfectDisk moves the MFT, it uses its own internal logic to determine that. You probably will also see that the MFT will grow, this is also due to PerfectDisk’s internal optimization settings. After the virtual machine has booted you can again use Diskpart.exe and the SHRINK command to further reduce the size of the volume. You can repeat this process until you have reached your desired size for the volume. Then, finally, you can run VHDResizer and shrink your VHD file, and also convert it at the same time.