Hyper-V

Microsoft Hyper-V, codenamed Viridian and formerly known as Windows Server Virtualization, is a hypervisor-based virtualization system for x86-64 systems. A beta version of Hyper-V was shipped with certain x86-64 editions of Windows Server 2008, and the finalized version (automatically updated through Windows Update) was released on June 26th 2008. Hyper-V has since been released in a free stand-alone version, and has been upgraded to Release 2 (R2) status.

Versions
Hyper-V exists in two fairly distinct versions: as a stand-alone product called Microsoft Hyper-V Server 2008 R2, and as a part of Microsoft Windows Server 2008 R2.

The stand-alone version of Hyper-V is free, and was released on October 1, 2008. It is a variant of the Windows Server 2008 "Core" that includes full Hyper-V functionality; other Windows 2008 Server Roles are disabled, and there are limited Windows Services. The free 64 bit Hyper-V "Core" version is limited to a command line interface (CLI), where configuration of the "Host" or "Parent" (Hyper-V Core Server) O/S, physical hardware and software is done using shell commands. A new menu driven CLI interface does simplify initial configuration considerably, and some freely downloadable script files extend this concept. Administration and configuration of the "Host" (Hyper-V Core Server O/S) and the "guest" or virtual O/Ses, is generally done by downloading extended Management Consoles that are installed onto a Windows Vista PC or Windows 2008 Server (32 or 64 bit). Alternatively another Windows 2008 Server, with the Hyper-V "Role" installed can be used, by redirecting the management console. Other administration / configuration of Windows Hyper-V Core Server can be done using a remote Windows Desktop RDP session (though still CLI) or redirected standard management consoles (MMC) such as "Computer Management" and "Group Policy (Local)" from a Vista PC or a full version of Windows 2008 Server. This allows much easier "point and click" configuration, and monitoring of the Hyper-V Core Server. Hyper-V Core Server Release 2 (R2) was made available in September 2009, its main feature being the inclusion of Windows PowerShell v2 for greater CLI control, and the updated Windows 2008 code base.

Architecture
Hyper-V supports isolation in terms of a partition. A partition is a logical unit of isolation, supported by the hypervisor, in which operating systems execute. A hypervisor instance has to have at least one parent partition, running Windows Server 2008. The virtualization stack runs in the parent partition and has direct access to the hardware devices. The parent partition then creates the child partitions which host the guest OSs. A parent partition creates child partitions using the hypercall API, which is the application programming interface exposed by Hyper-V.

A virtualized partition does not have access to the physical processor, nor does it handle its real interrupts. Instead, it has a virtual view of the processor and runs in Guest Virtual Address, which (depending on the configuration of the hypervisor) might not necessarily be the entire virtual address space. A hypervisor could choose to expose only a subset of the processors to each partition. The hypervisor handles the interrupts to the processor, and redirects them to the respective partition using a logical Synthetic Interrupt Controller (SynIC). Hyper-V can hardware accelerate the address translation of Guest Virtual Address-spaces by using second level address translation provided by the CPU, referred as EPT on Intel and NPT on AMD.

Child partitions do not have direct access to hardware resources, but instead have a virtual view of the resources, in terms of virtual devices. Any request to the virtual devices is redirected via the VMBus to the devices in the parent partition, which will manage the requests. The VMBus is a logical channel which enables inter-partition communication. The response is also redirected via the VMBus. If the devices in the parent partition are also virtual devices, it will be redirected further until it reaches the parent partition, where it will gain access to the physical devices. Parent partitions run a Virtualization Service Provider (VSP), which connects to the VMBus and handles device access requests from child partitions. Child partition virtual devices internally run a Virtualization Service Client (VSC), which redirect the request to VSPs in the parent partition via the VMBus. This entire process is transparent to the guest OS.

Virtual Devices can also take advantage of a Windows Server Virtualization feature, named Enlightened I/O, for storage, networking and graphics subsystems, among others. Enlightened I/O is specialized virtualization-aware implementation of high level communication protocols like SCSI to take advantage of VMBus directly, that allows bypassing any device emulation layer. This makes the communication more efficient, but requires the guest OS to support Enlightened I/O. Windows 2008, Windows Vista and SUSE Linux are currently the only operating systems that support Enlightened I/O, allowing them therefore to run faster as guest operating systems under Hyper-V than other operating systems that need to use slower emulated hardware.

System requirements / specifications

 * Host Operating system:
 * To install Hyper-v role, Windows Server 2008 or 2008 R2, Standard, Enterprise or datacenter edition. Web and foundation editions is not possible.
 * It can be installed whether on full installation or core installation
 * Processor:
 * An x86-64-capable processor running an x64 version of Windows Server 2008 Standard, Windows Server 2008 Enterprise or Windows Server 2008 Datacenter.
 * Hyper-V can not be installed on either x86 (32-bit) or ia64 (Itanium 64-bit) editions of Windows Server 2008.
 * Hardware-assisted virtualization. This is available in processors that include a virtualization option; specifically, Intel VT or AMD Virtualization (AMD-V, formerly code-named "Pacifica").
 * NX bit-compatible CPU must be available and Hardware Data Execution Prevention (DEP) must be enabled.
 * Although this is not an official requirement, Hyper-V R2 and CPU with Extended Page Table support are recommended for workstations.
 * Memory
 * minimum 2 GB. (Each virtual OS requires its own memory, and so realistically much more.)
 * Windows Server 2008 Standard (x64) Hyper-V full GUI or Core supports up to 31 GB of memory for running VMs, plus 1 GB for Hyper-V Parent OS.
 * Maximum for Windows Server 2008 R2 :32 GB (Standard) or 2 TB (Enterprise, Datacenter)
 * Guest operating Systems
 * Windows Server 2008 Standard (x64) Hyper-V full GUI or Core supports Guest operating systems with up to 4 processors ( with 1, 2 or 4 cores )
 * Windows Server 2008 Standard (x64) Hyper-V full GUI or Core supports up to 128 "Guest OSes"
 * Hyper-V supports 32-bit (x86) and 64-bit (x64) guest VMs.

Microsoft Hyper-V Server
The stand-alone Hyper-V Server does not require an existing installation of Windows Server 2008 nor Windows Server 2008 R2. The Standalone Installation is called Microsoft Hyper-V Server and Microsoft Hyper-V server R2. Microsoft Hyper-V server is a core installation of windows server 2008, with Hyper-V role installed and all other roles disabled. System Requirement of Microsoft Hyper-V server are the same for supported guest operating systems and processor, but differ in the following :


 * RAM: Minimum: 1 GB RAM; Recommended: 2 GB RAM or greater; Maximum 1 TB.
 * Available disk space: Minimum: 8 GB; Recommended: 20 GB or greater.

Supported guests
The supported/tested list of guest operating systems includes:
 * Windows Server 2008 x86/x64 SP1/SP2 & R2
 * Windows HPC Server 2008
 * Windows Server 2003 x86/x64 SP2 & R2 SP2
 * Windows 2000 Server SP4 and Advanced Server SP4
 * Windows 7 (except Home editions)
 * Windows Vista SP1/SP2 (except Home editions)
 * Windows XP Professional SP2/SP3 & x64 SP2
 * SUSE Linux Enterprise Server 10 SP1/SP2 & 11
 * Red Hat Enterprise Linux 5.2/5.3/5.4 x86/x64

Windows Server 2008 guests and Windows HPC Server 2008 can be configured for 1-, 2-, or 4-way SMP, Windows Server 2003 and Windows Vista VMs for 1- or 2-way SMP, and all other VMs as 1-way only. Other guest operating systems such as Ubuntu Linux 6.06/6.10/7.10 or Fedora 8/9 are unsupported; however, they have been reported to run.

Desktop virtualization (VDI) products are available from third-party companies, Quest Software vWorkspace, Citrix XenDesktop, Systancia AppliDis Fusion and Ericom PowerTerm WebConnect, that provide the ability to host and centrally manage desktop virtual machines in the data center while giving end users a full PC desktop experience.

Guest operating systems with Enlightened I/O and a hypervisor-aware kernel such as Windows Server 2008, Windows Vista SP1 and planned offerings from Citrix XenServer and Novell will be able to use the host resources better since VSC drivers in these guests communicate directly with the VSPs directly over VMbus. Non-enlightened operating systems will run with emulated I/O; however, integration components (which include the VSC drivers) are available for Windows Server 2003 SP2, Windows XP SP3, Windows Vista SP1 and Linux to achieve better performance. Xen-enabled Linux guest distributions can also be paravirtualized in Hyper-V. Currently, only SUSE Linux Enterprise Server 10 SP1/SP2 x86 and x64 Editions are officially supported by Microsoft in this way, though any Xen-enabled Linux should be able to run. In February 2008, Red Hat and Microsoft signed a virtualization pact for hypervisor interoperability with their respective server operating systems, to enable Red Hat Enterprise Linux 5 to be officially supported on Hyper-V.

Linux support
Hyper-V only provides basic virtualization support for Linux guests out of the box. Paravirtualization support is, however, available by installing the Linux Integration Components or Satori InputVSC drivers. On July 20, 2009, Microsoft submitted these drivers for inclusion in the Linux kernel under GPL, so that kernels from 2.6.32 may include inbuilt Hyper-V paravirtualization support. However, they were in danger of being dropped beginning 2.6.33 due to lack of support from Microsoft. This threat made Microsoft developers step up to maintain the code, but they soon "disappeared" again, resulting in the drivers being slated for removal for 2.6.35.

VHD Compatibility with Virtual Server 2005 and Virtual PC 2004/2007
Hyper-V, like Virtual Server 2005 and Virtual PC 2004/2007, saves each Guest OS to a single file with the extension .VHD. 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 Windows 2008 Hyper-V Server, but the old "Virtual Machine add-ons" require removing prior to migration. After the migrated Guest O/S is configured and started using Hyper-V, the Guest O/S will detect changes to the (virtual) hardware. Installing "Hyper-V Integration Services" (similar in function to Virtual Machine Add-ons) installs five services to improve performance, at the same time adding the new Guest Video and Network Card drivers. Consequently later versions of Windows may require re-activation. Virtual Server 2005 R2 SP1 supports guest operating systems OS/2 4.5 and Solaris 10 while Hyper-V does not.

USB pass-through
As of November 2009, Hyper-V does not support access to USB devices or support sound in guest VMs. However, 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. Sound can be enabled through a similar workaround.

Graphics issues on the host
When manufacturer-supplied Vista-compatible (WDDM) display drivers are installed on the host OS, most servers and PCs will experience a dramatic drop in graphic performance, including 'page flipping'-like effects when viewing high definition content or scrolling in applications and system hangs when switching between applications on the host; guest systems performance is unaffected. Users may install XP/2003-compatible (Non-WDDM) display drivers but may have compatibility problems and will be unable to use Aero. This occurs regardless if a VM actually exists or whether any Hyper-V services are running.

Microsoft recommends to use default VGA drivers (shipped with Windows Server 2008) on the host systems instead of manufacturer- supplied ones, however, such drivers, in turn, do not support multiple displays and high resolution configurations, as well as Aero, DirectX, Hardware-accelerated video decoding and other graphic accelerated features, thereby limiting its use on workstations (default VGA drivers do not support e.g. resolutions higher than 1280*1024).

As of November 2009, the issue in Windows Server 2008 (not R2) is still not solved. According to a Microsoft specialist's statement, systems with Windows Server 2008 R2 and processors supporting Extended Page Table are unaffected ; however, at least some users experience an even more significant problem: Windows Server 2008 R2 with Hyper-V and manufacturer-supplied graphic drivers installed won't boot at all. This issue is fixed in SP1.

Live migration
Hyper-V in Windows Server 2008 (not R2) 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. A third-party product called Melio FS allows live migration by allowing all of the virtual servers to use a single LUN concurrently.

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