HyperV tips

• Never save state on a Domain Controller doing so can cause domain synchronisation issues.
• Never pause a Domain Controller, this can cause replication issues.
• Do not take Snapshots of Domain Controllers.
• Use Fixed size disks, this will improve performance, and help reduce disk fragmentation.
• Always defragment a physical disk before creating a virtual hard disk.
• Be mindful of the integration services affect on a Domain Controller. If you provide the time synchronisation service to a Domain Controller, you can cause time synchronisation issues in your domain.
• Don't expand the Virtual Hard Disk if you have snapshots which haven't merged with the main VHD. This will make it impossible to remerge them.
• If you cannot merge your snapshot (avhd) files back into the main VHD. You can attempt to use WinImage (http://www.winimage.com) to retrieve the data from within the snapshot file. To do this simply rename the snapshot file from string.avhd to string.vhd then open with winimage.
• Remember to uninstall the VM Additions before migrating Virtual Machines from Virtual PC or Virtual Server 2005 R2, otherwise you will receive the following error when you try to uninstall them from inside Hyper-V:
• You can install Virtual Machine Additions only on a virtual machine that is running a supported guest operating system

Merge avhd to vhd in HyperV by manual

Hyper-V brings to life several additional disaster recovery scenarios that can be leveraged to our advantage when the need arises. One of the features that really comes into play are VM snapshots. A Snapshot is basically a spot in time where the current running configuration of the Virtual Machines is saved to a Snapshot Differencing Disk file (AVHD), from which you can return to from the future. This tip will show you how to manually merge hyper-v snapshots into a single VM for point in time restores.

When you create a differencing disk the original VHD is no longer modified and the snapshots are merged with the original vhd only when it is powered off. In disaster recovery scenarios, There may be cases where we want to manually merge snapshots ( avhd )

In order to do this, You must first change the extension of the Youngest AVHD file to VHD. 
Any VHD differencing disk(avhd) will always go to its parent, not the root parent.

So if you have a bunch of AVHDs, Each depends on the one before it - like the rungs of a ladder - they are sequential.  Most folks have a very simple linear chain of snapshots. Example : VHD - AVHD1 - AVHD2 - AVHD3 - AVHD4. ( Here AVHD4 is the Youngest and AVHD1 is the Oldest)

The parent of AVHD4 is AVHD3.  The parent of AVHD3 is AVHD2  The parent of AVHDn would simply be the one before it.

You need to Start Merge From Young AVHD to Old AVHD to complete the Merge Operation.

To Start Merge follow the below steps

• First Identify the Youngest AVHD ( In Our Ex : AVHD4)

1. Rename the Youngest AVHD (AVHD4)  to VHD ( Should not rename all the AVHDs at a time, Need to Rename only Youngest Avhd First) 
2. You can choose the Edit Disk option from the Actions menu in the Hyper-V Management Console.
3. Click Next through the first screen and select the snapshot file on the following screen.
4. Select Merge on the next screen and choose the To parent virtual disk option and click Finish.
5. After Successfully Completing Merge, Now Automatically AVHD4 will be deleted.

• Now Rename the Second Youngest Avhd (AVHD3) to Vhd. and Repeats the Steps from 1 to 4 until Oldest AVHD ( AVHD1) Get Merge with Parent VHD.

Once this has completed you should create a new virtual machine with the default options you would normally use, selecting to use an existing virtual disk on the hard disk screen. You should select the newly merged VHD file.

Now you have Successfully Completed Manually Merge avhd to vhd in Hyper-V

http://social.technet.microsoft.com/wiki/contents/articles/6257.manually-merge-avhd-to-vhd-in-hyper-v.aspx