It is a common question we hear a lot times. Should we be using IDE or SCSI under Virtual Server 2005?

By default you might have noticed that when creating a new virtual machine in Virtual Server 2005 the bus is set to IDE, however you can change it to SCSI (see image below).

This blog entry will explain the difference between using an IDE and SCSI bus in Virtual Server 2005 and why you would want to choose one or the other. This blog entry is based on a post by Ben from Virtual PC Guy’s Weblog who has answered it here, but we will recap it in a bit more detail once more.

Please note that this is for Virtual Server 2005 ONLY!, when you are using Hyper-V other rules apply which we will cover in another blog post. (also note that the SCSI bus is not supported in Virtual PC)

There are a number of factors to consider:

• The emulated IDE can only support 4 disks, while the emulated SCSI supports up to 28 disks;
• The emulated IDE can only support up to 128GB disks, while the emulated SCSI can support up to 2TB disks;
• The emulated IDE controller has a higher level of driver support for various operating systems than our emulated SCSI controller.

The other, and most important item is performance. IDE will outperform SCSI when virtual machine additions are NOT installed.

When you install the Virtual Machine Additions an accelerated SCSI driver gets installed as well.  Once this driver is installed the performance of the emulated SCSI controller is significantly faster than the emulated IDE controller.

So with this in mind, it is recommended to do the following on Virtual Server 2005:

If you are running Windows NT, 2000, XP, Vista, 2003, 2008 with Virtual Machine Additions installed; use SCSI.
If you are running any other configuration; use IDE unless you specifically use more than 4 disks, or larger than 128gb disks.

So where can you choose what bus to use?

When you create a new virtual machine in Virtual Server 2005 you will find this option right there on the create virtual machine configuration page under Virtual Hard Disk. When you choose to create a “new virtual hard disk” or ” use an existing virtual hard disk” you will find the option in a drop down box. (see the image below)

Also read our blog post about switching between IDE and SCSI bus on Virtual Server 2005.