VHD Compatibility With Virtual Server 2005 And
Virtual PC 2004/2007.
Hyper-V, like Microsoft Virtual Server and Windows
Virtual PC, saves each guest OS
to a single virtual hard disk file with the extension .VHD, except in Windows 8 and Windows Server 2012 where it can be
the newer .vhdx. This file contains the entire guest OS, though other files can
also be configured to allow "undo information" etc. Older .vhd files from Virtual Server 2005 and Virtual PC
2004/2007 can be copied and used by Hyper-V, but any old virtual machine
integration software (equivalents of Hyper-V Integration Services for other
virtualization software) must be removed from the virtual machine. After the
migrated guest OS is configured and started using Hyper-V, the guest OS will
detect changes to the (virtual) hardware. Installing "Hyper-V Integration
Services" installs five services to improve performance, at the same time
adding the new guest video and network card drivers. Consequently, Windows
guests may require re-activation.
Limitations.
USB
Pass-through.
Hyper-V supports USB devices in
Hyper-V guest VMs with a new feature called Virtual Machine Connection-
Enhanced Session Mode. This fact
makes it very inconvenient to run software protected by dongles in the guest. A workaround to access
USB drives in Windows guest VMs involves using the Microsoft Remote Desktop
Client to "share" host drives with guests over a Remote Desktop Connection.
Audio.
Hyper-V does not virtualize
audio hardware. Before Windows
8.1 and Windows Server 2012 R2, it was
possible to work around this issue by connecting to the virtual machine with
Remote Desktop Connection over a network connection and use its audio
redirection feature. Windows 8.1
and Windows Server 2012 R2 add the enhanced session mode which provides
redirection without a network connection.
Optical Drives
Pass-through.
Optical drives virtualized in
the guest VM are read-only. Hyper-V
does not support the host/root operating system's optical drives to
pass-through in guest VMs. As a result, burning to discs, audio CDs, video
CD/DVD-Video playback are not supported. However a workaround exists using the iSCSI protocol. Setting up an iSCSI target
on the host machine with the optical drive can then be talked to by the
standard Microsoft iSCSI initiator. Microsoft produces their own iSCSI Target
software or alternative third party products can be used.
Graphics Issues
On The Host.
On CPUs without Second Level Address Translation,
installation of most WDDM accelerated graphics drivers on the
primary OS will cause a dramatic drop in graphic performance. This occurs
because the graphics drivers access memory in a pattern that causes the Translation look aside buffer to be flushed frequently. In Windows Server 2008,
Microsoft officially supported Hyper-V only with the default VGA drivers, which do not support Windows Aero,
higher resolutions, rotation, or multi-monitor display. However, unofficial
workarounds were available in certain cases. Older non-WDDM graphics drivers
sometimes did not cause performance issues, though these drivers did not always
install smoothly on Windows Server. Intel integrated graphics cards did not
cause TLB flushing even with WDDM drivers. Some
NVidia graphics drivers did not experience problems so long as Windows Aero was
turned off and no 3D applications were running. In Windows Server 2008 R2,
Microsoft added support for Second
Level Address Translation to
Hyper-V. Since SLAT is not required to run Hyper-V with Windows Server, the problem will continue to occur if
a non-SLAT CPU is used with accelerated graphics drivers. However, SLAT is
required to run Hyper-V on client versions of Windows 8.
Live Migration.
Hyper-V
in Windows Server 2008 does not support "live
migration" of guest VMs (where "live migration" is defined as
maintaining network connections and uninterrupted services during VM migration
between physical hosts). Instead, Hyper-V on Server 2008 Enterprise and Datacenter
Editions supports "quick migration", where a guest VM is suspended on
one host and resumed on another host. This operation happens in the time it
takes to transfer the active memory of the guest VM over the network from the
first host to the second host. However, with the
release of Windows Server 2008 R2, live migration is supported with the use of Cluster
Shared Volumes (CSVs). This allows for failover of an individual VM as
opposed to the entire host having to failover (it seems that when a node
(Hyper-V server, not a VM) fails then each "VM running on the failed
node" may migrate to other live nodes independently of "other VMs on
the same LUN running on other nodes that share the LUN with the failed
node". In Hyper-V we are clustering the Hyper-V nodes not the VMs.). See
also Cluster
Shared Volumes. Windows
Server 2012's implementation of Hyper-V (Version 3.0)
introduced many new features to increase VM mobility, including the ability to
execute simultaneous live migrations (Windows Server 2008 R2 only supported
live migrating a single VM at a time, significantly increasing the time
required to carry administrative tasks, such as draining a node for scheduled
maintenance). The only real limiting factor here is hardware and network
bandwidth available. Windows Server 2012 also supports a new "shared
nothing live migration" option, where no traditional shared storage is
required in order to complete a migration. Also referred to as “Live System
Migration”, a shared nothing live migration will move a running VM and its
storage from one Hyper-V host to another without any perceived downtime. Live
Migration between different host OS versions is not possible, although this is
soon to be addressed in Windows Server 2012 R2.Windows
Server 2012 also introduced the ability to use simple SMB shares as a shared
storage option (in conjunction with the new Scale out File Services role in
Server 2012 for highly available environments), alleviating the need for
expensive SANs. This is particularly useful for low budget environments,
without the need to sacrifice performance due to the many new improvements to
the SMB3 stack. Windows Server 2012 will fully support the live migration of
VMs running on SMB shares, whether it be a live or live system migration. Hyper-V
under Windows Server 2012 also supports the ability to migrate a running VM's
storage, whereby an active Virtual Machines storage can be moved from one
infrastructure to another without the VM's workload being affected, further
reducing the limitations associated with VM mobility.
Degraded
Performance For Windows XP VMs.
Windows XP frequently accesses CPU's APIC task-priority register (TPR) when interrupt request level changes, causing a performance
degradation when running as guests on Hyper-V. Microsoft has fixed this
problem in Windows Server 2003 and later.Intel adds TPR virtualization (Flex Priority) to VT-x on Intel
Core 2 step E onwards
to alleviate this problem. AMD has a similar feature on AMD-V but uses a new register for the purpose. This however
means that the guest has to use different instructions to access this new
register. AMD provides a driver called "AMD-V Optimization Driver"
that has to be installed on the guest to do that.