Haven’t you heard about chimney which work as an exhaust in traditional Kitchens and factories?

Now one of the virtual networking features of Hyper-V in Windows Server 2008 R2 is using this word as “VM Chimney” - may be MS wants to emphasize that you got an easy path to flow. Anyway, VM Chimney in Hyper-V allows a VM to offload its network processing load onto the NIC of the host computer. This is something similar to TCP offload scenario, Hyper-V simply extends this functionality into the virtual world. This benefits both CPU and overall network throughput performance, and it is fully supported by Live Migration.

VM Chimney is disabled by default in Windows Server 2008 R2, primarily for compatibility reasons. Vendors like Intel, supports the compatibility for VM Chimney and if you are sure that the hardware you purchased support this – TURN IT ON for a better host system performance and a simultaneous boost to VM network throughput.

Support for Jumbo Frames was first introduced in initial release of Windows Server 2008 to support the hardware and now the same has been extended to support VMs. So, just like improving performance on hardware level, Jumbo Frame capability for VMs now can increase the performance of virtual networking and provides 6 times larger payloads per packet, which improves not only overall throughput but also reduces CPU utilization for large file transfers.

Microsoft's Virtualization Software Hyper-V has finally been RTM'd and available for download from here

I pointed out about device access in my earlier post, differentiating between emulated device and non-emulated device in Hyper-V. Ever wondered how non-emulating devices work internally in Hyper-V? If not, then read on...

Device access virtualization is a method for sharing hardware efficiently without any emulation. All physical devices are controlled by existing device drivers without creating new ones.

VSP's or Virtualization Service Providers runs within parent partition. It talks to the respective device driver and acts as a multiplexer and offers hardware services to guests by establishing a communication with VSC's or Virtualization Server Clients over VMBUS (A communication channel for VSP's and VSC's).

VSC's runs within the child partition and acts as a service. For every VSC, there will a corresponding VSP in parent partition. It's the VSP/VSC pair which makes a device type within a guest. Microsoft provides VSP/VSC pairs for storage, networking, video, input, USB.

Ok. This is what I've learned so far about device access in Hyper-V.

For device access in Hyper-V, there are two underlying methods:- Emulated and VSP/VSC devices.

Emulated devices are just like the devices you have seen or used in Virtual Server 2005 R2. Whenever the virtual machine or VM attempts to access I/O, the hypervisor traps that request and the I/O gets emulated in the parent partition. This approach is highly compatible across most of the OS, but it has its own overhead and slow.

To overcome the above mentioned I/O performance issues, VSP/VSC devices have been developed in Windows Server 2008 Hyper-V. The hidden caveat is, these are supported only on specific OS. Of course, Windows 2003 and above is supported. This approach requires VSC drivers in the guest OS which looks like normal drivers in windows. With VSC drivers, I/O is not trapped by the hypervisor as it does in emulated one - rather the VSC drivers directly communicates with VSP's over VMBUS and operates much faster than emulated devices.

Ok, now we know the two underlying methods available for device access. Let's take a look at the type of disk contoller Hyper-V provides for storage. The excerpt below has shamelessly been copied from here.

There are two types of disk controllers that Hyper-V supports: SCSI and IDE. There are two IDE controllers and four SCSI controllers available.

Each IDE controller can have two devices. You can't boot from a SCSI controller. This means an IDE disk will be required. The boot disk will be IDE controller 0 Device 0. If you want a CDROM it will consume an IDE device slot.

EACH SCSI controller can support up to 255 devices.

Both SCSI and IDE can support pass-through, Fixed, Dynamic, Sparse, and Delta drives (see http://blogs.msdn.com/tvoellm/archive/2007/10/13/what-windows-server-virtualization-aka-viridian-storage-is-best-for-you.aspx). The difference lies in how the controllers are actually implemented. The IDE controller is emulated where as the SCSI controller is synthetic. So what does this mean?

The IDE controller implements a well-known IDE controller and this means there is extra processing before the I/O is sent to the disk. This processing occurs in vmwp.exe (a user mode process that exists for each started VM. More on this in a later post). Once the IDE emulation is complete the I/O is sent into the Root Partition's I/O Stack. I/O completion requires a trip back to vmwp.exe.

The SCSI controller is not emulated. The SCSI controller uses VMBUS (Virtual Machine BUS. More on this in a later post). The I/O's pass from the Child (aka Guest) Partition to the root over VMBUS and enter the I/O stack. You can already see one less process/machine context switch is required because vmwp.exe does not get invoked. Once and I/O completes its completion is sent over VMBUS.

There is a lot more to how both the IDE and SCSI controllers work however the descriptions below should help you to understand why SCSI controllers are the right choice for the best performance.

One of the main reasons you can't boot off of SCSI device is because it is implemented as a synthetic device in Hyper-V and the VMBUS is not available during the boot. Rest of the devices such as CD, IDE, Legacy Network Adapter and Floppy is supported though.

Note: Since Hyper-V is not RTM'ed all numbers are subject to change as are the behaviors.

I've been testing Hyper-V lately and thought of compiling a list of things which are either incompatibilities, issues or "bugs" with Hyper-V.
Note: This is what I have experienced personally or from the field and is never a complete one. I'll be updating it as and when new information is available.

1. All roles except DC (Domain controller) role is supported
2. If you are using virtual switches in a VM cluster then make sure they are identically named on both nodes. Make sure you have installed integration services before attempting a failover. I also noticed that creating virtual switches using the GUI during installation doesn't cut it and you need to delete it and recreate it again to make it properly work inside a cluster
3. Since, we are dealing with pre-release material here and one thumb rule is, don't expect everything to work straight forward. For instance, upgrading a VM configured on Hyper-V pre-beta code (RC0) to Hyper-V Beta RC1 may not be supported and may offer un-predicatable results.
4. Always install the virtualization integration components for Linux inside Virtual Guests. Otherwise, you won't see the devices. The Linux downloads are available from Connect Website
5. While enabling Hyper-V role, make sure you keep (do not change) EN-US as your default language otherwise you may get VMM service failures and Hyper-V wouldn't start
6. Wireless network adapters on laptops are not supported by Hyper-V.