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In Virtual Server 2005 R2 SP1, when you create a virtual machine that uses hard disks that are connected to the virtual SCSI controller you can get better performance than when using virtual IDE controller-connected hard disks. In Virtual Server, virtual SCSI controllers have major performance benefits over virtual IDE controllers, and because of that it is recommended to attach the Virtual Disks to one or more SCSI controllers to improve disk input/output (I/O) performance.

The reason for this is because IDE is limited to one transaction at a time, regardless of whether the bus is physical or virtual. This means that a virtual machine with two virtual hard disks attached to the IDE adapter is limited to a single transaction for both disks. By contrast, a SCSI controllers allows for multiple simultaneous transactions, which provides better performance than disks attached to the IDE controllers.

BTW, you might be interested in reading my "Microsoft Hyper-V will not boot virtual SCSI devices" article.

However, without manually installing the virtual SCSI controller driver during the installation of the guest operating system, under Virtual Server 2005, the performance of emulated SCSI controllers is slower than that of emulated IDE controllers.  The reason for this is that the SCSI controller is a lot more complicated to emulate than the IDE controller.  However, this changes once you have Virtual Machine Additions installed, because the Virtual Machine Additions install an accelerated SCSI driver.  Once this driver is installed the performance of the emulated SCSI controllers is significantly faster than emulated IDE controllers.

You can install the virtual SCSI controller driver during the installation of the guest operating system by performing the following steps:

Note: Description and screenshots where made while installing Windows Server 2008 on Virtual Server 2005 R2 SP1, however the same instructions apply to the installation of Windows Vista. For Windows 2000/2003/XP you will need to press the F6 key during the text phase of the installation process, then press "S" to specify additional drivers, and then provide the driver floppy image.

1. Begin the installation by inserting the appropriate Windows Server 2008 installation media into your DVD drive.

2. Continue with the installation process, until you reach the point where you're prompted for the location of the system partition. Click on the Load Driver link.

3. Now you need to load the driver files as a virtual floppy image. The image's name is "SCSI Shunt Driver.vfd", and it is located in the C:\Program Files\Microsoft Virtual Server\Virtual Machine Additions folder.  You can mount it by using VMRC Plus (read my "Manage Virtual Server Machines with VMRC Plus", or from the Virtual Server administrative website. For this example I've used VMRC Plus.

In the VMRC Plus Console Manager click on Media > Load Floppy Disk Image.

 Navigate to the C:\Program Files\Microsoft Virtual Server\Virtual Machine Additions folder and select the "SCSI Shunt Driver.vfd" driver.

 If you want, you can do the same by right-clicking on the VM's name in the main VMRC Plus window, then selecting Settings.

 Now navigate to the C:\Program Files\Microsoft Virtual Server\Virtual Machine Additions folder and select the "SCSI Shunt Driver.vfd" driver.

Click Ok.

4. Back in the installation window, click on Browse.

 5. Navigate to the virtual floppy drive's root folder, then expand the Vista subfolder.

Note: For other operating systems select the right subfolder.

Press Ok.

6. After a few seconds the driver's name will appear on the list. Click Next.

7. The driver will load.

8. In the "Where do you want to install Windows", if you're installing the server on a regular IDE hard disk, click to select the first disk, usually Disk 0, and click Next.

9. If you must, you can also click Drive Options and manually create a partition on the destination hard disk.

10. Continue the installation as usual, and when it's done the server will automatically reboot.

Don't forget to install the Virtual Machine Additions on the guest operating system right after you log on. VM Additions greatly improve the guest's performance. You can read more about it on my "Installing Virtual Server VM Additions on Windows Server 2008 & Windows Vista".