Moving from Virtual Server 2005 R2 to Hyper-V

  • 6/10/2009

Considerations Before Migrating Virtual Machines

Hyper-V does not provide the ability to import virtual machines that exist on a Virtual Server 2005 R2 SP1 host; you must migrate the virtual machines manually. The building blocks of a virtual machine in Virtual Server 2005 R2 SP1 and Hyper-V are basically the same, a virtual hard disk and a configuration file. The virtual hard disk has not changed and can be easily migrated from Virtual Server to Hyper-V. The configuration file has completely changed, however, and Microsoft does not provide a tool to migrate the settings from the old format to the new format.

Boot Disk Configuration

Virtual Server 2005 R2 SP1 virtual machines could be attached to the virtual SCSI adapter and the virtual machine would boot in this configuration (it was a recommended configuration for best performance). This was possible because the SCSI adapter was emulated and available at boot time. Hyper-V virtual machines cannot boot from the SCSI adapter, however, because it is synthetic, and synthetic devices are not available immediately at boot time. Therefore, virtual machines that are currently configured to boot from SCSI in Virtual Server must be converted to boot from IDE as part of the migration to Hyper-V.

Virtual Machine Additions

Virtual Server 2005 R2 SP1 uses enhanced drivers to provide emulated devices and to improve performance in a virtual machine. Hyper-V uses something similar to provide synthetic devices and performance enhancements to virtual machines. The architecture and interfaces between these two technologies are not compatible; therefore, you must remove Virtual Machine Additions as part of the process of migration to Hyper-V. Although it is possible to remove the additions after you have migrated the virtual machines to Hyper-V, your ability to do this depends on the version of Virtual Machine Additions that is installed. The better approach, which will work regardless of the version installed, is to uninstall Virtual Machine Additions prior to migrating to Hyper-V.

Undo Disks

The Undo disk feature in Virtual Server 2005 R2 has been removed in Hyper-V and replaced with a more powerful feature called snapshots. You cannot migrate Undo disks to Hyper-V; they must be discarded or committed prior to migrating the virtual machine hard disk to Hyper-V.

Saved States

Virtual Server 2005 R2 SP1 and Hyper-V both have the ability to save the state of a running virtual machine to disk. The concept is similar to hibernation. In Virtual Server 2005 R2, the saved state file was a single file (.vsv) that contained the contents of memory and information on running processes, threads, and the processor stack. Hyper-V has split the save state file into two parts: the memory contents (.bin) and the stack and process information (.vsv). Because of this change, saved states cannot be migrated and must be merged or discarded before the migration can occur.

Hardware Abstraction Layer Differences

Virtual machines running in Virtual Server 2005 R2 can have only a single virtual processor. Hyper-V provides the ability for a virtual machine to be configured with up to four virtual processors. This presents an issue with hardware abstraction layer (HAL) compatibility. Hyper-V virtual machines require a multiprocessor Advanced Configuration and Power Interface (ACPI) HAL. Virtual Server 2005 R2 virtual machines can have APCI or non-ACPI HALs based on how they were created and what version of additions are loaded. Regardless of which HAL the existing virtual machine has, the HAL must be changed during the migration of the virtual machine to Hyper-V. For instructions on changing the HAL for Vista and Windows 2008 virtual machines, go to

Differencing Disks

Both Hyper-V and Virtual Server 2005 R2 support differencing disks, and the technology has not changed. Differencing disks require that the parent and the child retain the same relative path when they are moved on the same machine or between machines. The relative path to the parent VHD is stored in the disk header of the child VHD. If a child VHD is copied and the parent is not, or if the relative path is not maintained between the two files, the child VHD will not have all the information it requires and the differencing disk will not be usable.

Shared SCSI Virtual Machine Clusters

Virtual Server 2005 R2 virtual machine–emulated SCSI controllers provided a mode called shared SCSI (a parallel SCSI bus). Enabling shared SCSI on the controller allowed a Windows Server 2003 cluster to be built between virtual machines. Hyper-V has switched to a synthetic SCSI controller and removed the ability to put the SCSI controller into parallel SCSI bus mode. You can still create a cluster between virtual machines in Hyper-V, but you must use an iSCSI Initiator to attach remote iSCSI LUNs to the virtual machines. Virtual Server–based virtual machine clusters must be manually migrated to Hyper-V.